Does weight = mass?
Gene Nygaard
What is weight? Is it different from mass?
Physicists often define weight as a particular kind of force,
resulting from gravity.
Those who've had a smattering of physics lessons are also often
critical of the general public, saying that they are confusing the
concepts of "weight" and "mass." Others often claim that the English
customary system measures "weight," and that this weight is something
different from the "mass" which is generally measured in the metric
system (often ignoring the facts that the International System of
Units and other obsolete metric systems have perfectly good units of
force, and that the English units are more often used for mass than
for force, and that these mass definitions are the primary definitions
of the English units.)
But the original meaning of weight, still in general use today, is
that quantity measured with a balance. Balances measure mass, not
force. Weight is equivalent to mass, in this definition, and this
definition is a perfectly valid one.
If you have any opinions on this matter, or would like to know more
about it, check out my new web page:
"Weight vs. Mass: What's the Difference"
http://ourworld.compuserve.com/homepages/Gene_Nygaard/weight.htm
This is a work in progress. I'd appreciate it if some of you would
review this for me and send me any corrections, suggestions,
additions, or other comments, or post them in response to this
message.
Gene Nygaard
gnyg...@crosby.ndak.net
Gene_N...@compuserve.com
++++++++++++++++++++++++++++++
There is another type of measure that is also very
ancient and that is the measure of mass. (Actually,
the average man, in speaking of mass, usually calls
it weight. But mass and weight are different . . .)
As time passed, each nation and region developed
its own standard masses against which unknown masses
could be compared. The chief such unit is called
pound in English, from a Latin word meaning "a weight."
Isaac Asimov
Realm of Measure, 1960
DB
If not its WEIGHT, what is the "force on [said object] due to the [acting
body]'s gravitational pull"? Most UK science teachers use the word
"weight." Fair enough, they go wrong in not calling it the object's "Earth
weight," but give them a break...
"force on [said object] due to the [acting body]'s gravitational
pull"=="[acting body] weight"
NB I'm not referring to any dictionaries here, which are _mostly_ written by
English, not Science graduates.
Jeff
A 1 kg mass is always a 1 kg mass. That same item (by definition) happens
to weigh in at 1 kg on earth at sea level. Take it to the moon and it still
has a mass of 1 kg, but its weight has changed to 1/7kg. That is why it is
so important to differentiate between weight and mass,
Jeff
DB
>"force on [said object] due to the [acting body]'s gravitational
>pull"=="[acting body] weight"
Self-correction
"force on [said object] due to the [acting body]'s gravitational
pull at [position]"=="[acting body at position] weight"
Gene Nygaard
Paul Skoczylas <P.Sko...@cfer.ualberta.ca> wrote:
>NOOO!!!!
>A 1 kg mass does NOT weigh 1 kg on earth at sea level. It weighs 9.81
>N. A kilogram is not a unit of force in the SI system (although it may
>have been used unofficially for that in the past).
Wake up, both of you! (I don't have Jeff's article yet on my server,
so don't know if he said any more).
"Weight" is either too broad or too narrow a concept to be set up
against "mass" in this manner.
The comments by Paul Skoczylas make sense if you substitute "force"
for "weight." Kilograms do measure weight, but not in the sense used
here. Newtons measure force, a couple of particular kinds of which
are also called weight, but also other forces which are not generally
called weight. You'd know that if you'd visited my web page at
http://ourworld.compuserve.com/homepages/Gene_Nygaard/weight.htm
Consider what the U.S. national standards laboratory, the National
Institute of Standards and Technology (NIST), says about this in
Special Publication 811 (1995):
In commercial and everyday use, and especially in common
parlance, weight is usually used as a synonym for mass. Thus
the SI unit of the quantity weight used in this sense is the
kilogram (kg) and the verb "to weigh" means "to determine the
mass of" or "to have a mass of."
>By having completely different units for mass and weight, we avoid the
>confusions that exist in the English system.
>Newton's law can be written F = m * a, with F in newtons, m in kilograms
>and a in m/s^2, with no other conversion factors. In the case of
>gravity, a is 9.81 m/s^2 on earth at sea level. If you use pounds-mass
>and pounds-force, you meed a conversion factor. Likewise if you use the
>pseudo-unit kilogram-force.
>-Paul
Gene Nygaard
'Taint what a man don't know that hurts him;
it's what he knows that just ain't so.
--Frank McKinney "Kin" Hubbard
Julio VANIA
In article <608a12$ddl$1...@news.ispn.net>, gnyg...@crosby.ndak.net (Gene Nygaard)
writes:
|> What is weight? Is it different from mass?
|>
|> Physicists often define weight as a particular kind of force,
|> resulting from gravity.
|>
|> Those who've had a smattering of physics lessons are also often
|> critical of the general public, saying that they are confusing the
|> concepts of "weight" and "mass." Others often claim that the English
|> customary system measures "weight," and that this weight is something
|> different from the "mass" which is generally measured in the metric
|> system (often ignoring the facts that the International System of
|> Units and other obsolete metric systems have perfectly good units of
|> force, and that the English units are more often used for mass than
|> for force, and that these mass definitions are the primary definitions
|> of the English units.)
|>
|> But the original meaning of weight, still in general use today, is
|> that quantity measured with a balance. Balances measure mass, not
|> force. Weight is equivalent to mass, in this definition, and this
|> definition is a perfectly valid one.
|
Balances measure weight not mass.
Mass is an intrinsic property of matter. It doesn't change. 1 kg of mass is
always 1 kg of mass.
The weight is a force P=mg. It dependes on mass and on g. If g varies the weight
will cahnge but the mass is still the same.
You can measure the weight in a balance.
To measure mass, you have to make a comparison between to weigths, or know the
volume of your sample (knowing the density you can calculate the mass).
It is common to say that the mass is equal to the weigth in kgf.
The only different are the unities:
mass - kg
weigth -kgf or N
JCV
Ronald D. Cuthbertson
Gene Nygaard wrote:
>
> What is weight? Is it different from mass?
>
> Physicists often define weight as a particular kind of force,
> resulting from gravity.
[cut]
From AHD3 --
mass n. 6. Abbr. m Physics. The measure of the quantity of matter that a
body or an object contains. The mass of the body is not dependent on
gravity and therefore is different from but proportional to its weight.
Hence, mass is different from weight.
Regards,
Ron
Rodger Whitlock
An easily understood physical distinction between mass and weight is this:
a 50-kg mass is hard to pick up on Earth but easy on the moon because its weight
depends on its mass times the acceleration of gravity
a rolling automobile on the level would just as hard to stop on the moon as on the
earth because its momentum depends on its mass times its (horizontal) velocity.
----
Rodger Whitlock
Victoria, British Columbia, Canada
on beautiful Vancouver Island
evbill@gte.net Bill Baldwin
Julio VANIA thinks:
> Balances measure weight not mass.
> Mass is an intrinsic property of matter. It doesn't change. 1 kg of mass
is
> always 1 kg of mass.
> The weight is a force P=mg. It dependes on mass and on g. If g varies the
weight
> will cahnge but the mass is still the same.
> You can measure the weight in a balance.
> To measure mass, you have to make a comparison between to weigths...
Huh? We're clearly not using the same definition of the word "balance"
here. A balance, by definition I would have thought, compares two weights
by BALANCING them against each other. So a one kilogram object will ALWAYS
exactly balance another one kilogram object here, on the moon, on Mars...
anywhere. However, on a non-balance type scale, i.e. a spring scale such as
many bathrooms sport, a one kilogram object will register one kilogram on
earth but considerably less (1/6?) on the moon. That is because it is
exerting less force against the springs while the springs maintain a
constant force in the reverse direction regardless of gravity. Clear as
mud.
Anyway, a balance is a thing that looks like this, right (Apologies if the
ASCII art doesn't line up on your screen)?
!
! !
_______________ _______________
! !
! ! !! !
!
! 1 KG ! !! ! 1 KG !
!!!!!!!!!!!! !!
!!!!!!!!!!!!
!!
!!
!!!!!
------------
Paul Skoczylas
creynold wrote:
> A balance measures force - not mass. Don't believe me? Put a mass on one
> side and see if you can 'balance' it with your finger. No matter what mass
> you put on the scale (within reason for the context following) you can
> balance it with your finger. Yet you finger didn't change mass did it?
But a balance is virtually always used to compare two objects. Usually
one of these objects has an accurately known mass. Since the two
objects on the balance are subject to the same gravitational field, you
actually compare mass. (Take the same balance and accurately known mass
to the moon, and balance will indicate the same mass for the unknown
object.)
While the balance itself actually measures force, its common use
actually measures mass, regardless of what planet it's used on. A
spring scale can only measure force. It can have a scale that is
calibrated in a mass unit, but that scale will be invalid if the scale
is moved to a very different altitude on earth, or to the moon.
-Paul
Larry Krakauer
Paul Skoczylas wrote:
> Jeff wrote:
> > A 1 kg mass is always a 1 kg mass. That same item (by definition) happens
> > to weigh in at 1 kg on earth at sea level. Take it to the moon and it still
> > has a mass of 1 kg, but its weight has changed to 1/7kg. That is why it is
> > so important to differentiate between weight and mass,
> NOOO!!!!
> A 1 kg mass does NOT weigh 1 kg on earth at sea level. It weighs 9.81
> N. A kilogram is not a unit of force in the SI system (although it may
> have been used unofficially for that in the past).
>
> By having completely different units for mass and weight, we avoid the
> confusions that exist in the English system.
You're basically on target here, but your last sentence is nonsense.
I don't want to defend the *use* of the cumbersome English
system, but it does take elementary physics into account. It certainly
*does* have separate units for mass and weight, just like the metric
system.
The English unit of mass is the "slug", defined as "The unit of mass
that
is accellerated at the rate of one foot per second per second when
acted upon by a force of one pound weight." Look it up in any
dictionary or American physics text.
> Newton's law can be written F = m * a, with F in newtons, m in kilograms
> and a in m/s^2, with no other conversion factors. In the case of
> gravity, a is 9.81 m/s^2 on earth at sea level. If you use pounds-mass
> and pounds-force, you meed a conversion factor. Likewise if you use the
> pseudo-unit kilogram-force.
Substitute "slug" for "pounds-mass", and the above is correct.
The web site that started this thread is really kind of bizarre. This
matter is one of technical definitions, and is completely understood
by anyone who has taken a first year physics course. The author of the
site seems to be confusing people's everyday usage with the technically
correct scientific definitions, and making a big deal of it.
Everything he lists as a "myth" on the page is, using the proper
scientific definitions, quite correct. For example, the site
says:
"MYTH: The metric system measures mass; the English system measures
weight (meaning force)."
Using the technical definitions used in any first year physics course,
the statement that follows the word "MYTH:" is precisely correct.
Since on the surface of the earth, weight (a force towards the earth)
and mass are related by a constant, the so-called "accelleration of
gravity", for our everyday lives, it is not important to distinguish
them clearly (as it is for, say, a physics student). Thus, it happens
that the Metric system uses the unit of Mass (grams) for the everyday
measurement of weight, while the English system uses the unit of force
(pounds). In everyday usage, it's no big deal which you use, but
if you want to be accurate, indeed the gram (or kilogram, of course)
is a unit of mass, and the pound is a unit of force (weight).
I won't bother discussing the other so-called "myths"; the author needs
to study his physics harder, and he should stop expecting the everyday
language of non-physics-aware people to be precise. He should also stop
making a big deal of it when our daily language turns out to *not* be
precise, but instead is found to take some convenient shortcuts
(like the technically innacurate use of the "kilogram" as a "weight"
in the Metric system).
--
Larry Krakauer (lar...@kronos.com)
William L. Bahn
Please note that NIST's SP811 is merely an acknowledgment that the COMMON
usage of weight and mass are often confused and that when someone says to
"weigh something" they are typically wanting to know the mass of that
object as opposed to its weight - i.e., they want to know how much of it
they have as opposed to how much effort it takes to hold it off the ground.
This document does not say that mass and weight ARE the same, nor is it
saying that the verb "to weigh" is defined as meaning mass. The key words
are "used in this sense". The purpose of the publication is to facilitate
communication given the realities that most people will never consistently
make a clear distinction between mass and weight in common usage. If the
Earth's gravitational field varied by a factor of two or three over the
populated surface we wouldn't have this confusion, but since nearly all of
human COMMON experience involves the condition that mass and weight are
directly proportional to each other and are independent of position and
time then it is not surprising that the "conversion" between mass and
weight is commonly seen as being no different than the conversion between
inches and feet - just different units for the same thing.
If this is not the case, i.e., if "to weigh" is defined to mean "to have a
mass of", then what does it mean to be "weightless" when in orbit or in
freefall or on a parabolic trajectory. Does a person "weigh" 1/6 of their
Earth weight when on the Moon?
All SP811 is really saying is that we can't be to nit-picky about semantics
when dealing with common parlance. That when told that something weighs so
many pounds or so many kilograms or so many Newtons that we should
interpret that as being an indicator of the object's mass if it makes sense
to do so.
It's like saying you need to get some gas even though you are driving a
diesel. In most situations, it is fully understood that you mean "fuel" and
not "gasoline". Used in that sense it is perfectly fine to "gas up" a
diesel even though it is not perfectly fine to put gasoline into the tank.
Would it be better if the correct terms were always used according to their
literal and precise meanings? Probably.
Is it possible for dangerous, even fatal, situations to arise because of
this sloppy usage? Yes, it has happened on more than one occasion.
Is it reasonable to expect people to adhere to precise and correct usage in
the common parlance? Not at all.
Is the most reasonable approach therefore to make people aware of the
typically intended meaning that should be assumed in most situations? Sure.
Gene Nygaard <gnyg...@crosby.ndak.net> wrote in article
<608mqv$f73$1...@news.ispn.net>...
> Paul Skoczylas <P.Sko...@cfer.ualberta.ca> wrote:
>
> >Jeff wrote:
> >>
> >> A 1 kg mass is always a 1 kg mass. That same item (by definition)
happens
> >> to weigh in at 1 kg on earth at sea level. Take it to the moon and it
still
> >> has a mass of 1 kg, but its weight has changed to 1/7kg. That is why
it is
> >> so important to differentiate between weight and mass,
>
>
> >NOOO!!!!
>
> >A 1 kg mass does NOT weigh 1 kg on earth at sea level. It weighs 9.81
> >N. A kilogram is not a unit of force in the SI system (although it may
> >have been used unofficially for that in the past).
>
> Wake up, both of you! (I don't have Jeff's article yet on my server,
> so don't know if he said any more).
>
> "Weight" is either too broad or too narrow a concept to be set up
> against "mass" in this manner.
>
> The comments by Paul Skoczylas make sense if you substitute "force"
> for "weight." Kilograms do measure weight, but not in the sense used
> here. Newtons measure force, a couple of particular kinds of which
> are also called weight, but also other forces which are not generally
> called weight. You'd know that if you'd visited my web page at
> http://ourworld.compuserve.com/homepages/Gene_Nygaard/weight.htm
>
> Consider what the U.S. national standards laboratory, the National
> Institute of Standards and Technology (NIST), says about this in
> Special Publication 811 (1995):
>
> In commercial and everyday use, and especially in common
> parlance, weight is usually used as a synonym for mass. Thus
> the SI unit of the quantity weight used in this sense is the
> kilogram (kg) and the verb "to weigh" means "to determine the
> mass of" or "to have a mass of."
>
> >By having completely different units for mass and weight, we avoid the
> >confusions that exist in the English system.
>
> >Newton's law can be written F = m * a, with F in newtons, m in kilograms
> >and a in m/s^2, with no other conversion factors. In the case of
> >gravity, a is 9.81 m/s^2 on earth at sea level. If you use pounds-mass
> >and pounds-force, you meed a conversion factor. Likewise if you use the
> >pseudo-unit kilogram-force.
>
Chad English
Gene Nygaard wrote:
>
> What is weight? Is it different from mass?
>
> Physicists often define weight as a particular kind of force,
> resulting from gravity.
>
> critical of the general public, saying that they are confusing the
> concepts of "weight" and "mass." Others often claim that the English
> customary system measures "weight," and that this weight is something
> different from the "mass" which is generally measured in the metric
> system (often ignoring the facts that the International System of
> Units and other obsolete metric systems have perfectly good units of
> force, and that the English units are more often used for mass than
> for force, and that these mass definitions are the primary definitions
> of the English units.)
>
> But the original meaning of weight, still in general use today, is
> that quantity measured with a balance. Balances measure mass, not
> force. Weight is equivalent to mass, in this definition, and this
> definition is a perfectly valid one.
>
> about it, check out my new web page:
>
> "Weight vs. Mass: What's the Difference"
> http://ourworld.compuserve.com/homepages/Gene_Nygaard/weight.htm
>
> This is a work in progress. I'd appreciate it if some of you would
> review this for me and send me any corrections, suggestions,
> additions, or other comments, or post them in response to this
> message.
>
> Gene Nygaard
> gnyg...@crosby.ndak.net
> Gene_N...@compuserve.com
>
> ++++++++++++++++++++++++++++++
> There is another type of measure that is also very
> ancient and that is the measure of mass. (Actually,
> the average man, in speaking of mass, usually calls
> it weight. But mass and weight are different . . .)
> As time passed, each nation and region developed
> its own standard masses against which unknown masses
> could be compared. The chief such unit is called
> pound in English, from a Latin word meaning "a weight."
> Isaac Asimov
> Realm of Measure, 1960
Admittidly, I only skimmed over the page and most of it seems ok, though
I don't really know the origin of the terms "weigh" and "weight". I had
always learned that weight is *supposed* to be the force due to gravity
on an object, hence terms like "weightless" in space and saying you
"weigh less on the moon", even though you still have the same mass.
(Technically speaking, you aren't really weightless in space even using
the force definition since gravity still applies a force on you, you are
just in freefall.)
The only problem I have is this idea that balances measure mass whereas
spring-scales measure force. I'll probably get some headed responses on
this because I've seen it argued here before, but I can see where these
arguments are incorrect and have yet to see anything wrong in my
arguments (which I'm willing to accept if someone can point them out).
Here's the jist of it:
When you use a balance scale you are comparing moments about the balance
point. Do a free body diagram and you will see this. You don't even
need the concept of a mass to realize this. When the objects balance,
they are applying the same moment about the focal point. The moment is
the gravitational force (or really any external force, e.g. pushing on
it) multiplied by the moment arm of the balance. Thus, you are
comparing graviational forces (usually referred to as "weights", but I'm
hesitant to use that term so as not to confuse issues as described on
the web page).
Now, assuming that the objects are in the same gravitational field,
their masses will be the same proportion as their gravitional forces
(weights), hence you *can* use a balance to compare masses, but only by
deduction. That is not the physical process that is taking place.
There is the traditional argument, as on the web page, that balances
compare masses because they would stay balanced on the moon, for
instance. The problem with this argument is that all you've done is
changed the gravitational field on both objects, so that the
gravitational forces, and hence moments, though reduced in magnitude are
still in equilibrium. The way to prove that it doesn't compare masses
is imagine the two objects (two ends of the balance scale) in two
*different* gravitational fields. The objects, with identical masses,
would *not* balance because the gravitational forces, hence moments, are
no longer the same and no longer in equilibrium.
Perhaps a more accurate description of the differences between scales is
this:
1. A balance compares gravitational forces (weights) with a
"known/standard" gravitational force.
2. A spring scale compares gravitational force with a "known" spring
force (or stiffness, or calibration, or something like that).
The only way I can think to actual directly compare masses (regardless
of gravitational field) is to apply the same *net* force to the objects
and measure their acceleration. (Net force includes gravitational force
components.)
I hope this makes sense. Feel free to poke holes in this argument, but
I can't see any.
--
Chad English
ceng...@mae.carleton.ca
http://www.mae.carleton.ca/~cenglish
creynold
Gene Nygaard <gnyg...@crosby.ndak.net> wrote in article
much snippage
<608a12$ddl$1...@news.ispn.net>...
> But the original meaning of weight, still in general use today, is
> that quantity measured with a balance. Balances measure mass, not
> force. Weight is equivalent to mass, in this definition, and this
> definition is a perfectly valid one.
NOT SO
Dario Alejandro Alpern
Bill Baldwin wrote:
>
> Julio VANIA thinks:
>
> > Balances measure weight not mass.
> > Mass is an intrinsic property of matter. It doesn't change. 1 kg of mass
> is
> > always 1 kg of mass.
> > The weight is a force P=mg. It dependes on mass and on g. If g varies the
> weight
> > will cahnge but the mass is still the same.
> > You can measure the weight in a balance.
> > To measure mass, you have to make a comparison between to weigths...
>
> Huh? We're clearly not using the same definition of the word "balance"
> here. A balance, by definition I would have thought, compares two weights
> by BALANCING them against each other. So a one kilogram object will ALWAYS
> exactly balance another one kilogram object here, on the moon, on Mars...
> anywhere.
Not anywhere. You need a gravitational force (or acceleration) to use a
balance.
--
Dario Alejandro Alpern
Buenos Aires - Argentina
http://members.tripod.com/~alpertron (en castellano)
http://members.tripod.com/~alpertron/ENGLISH.HTM (english)
Si su navegador no soporta JavaScript:
http://members.tripod.com/~alpertron/INDEX2.HTM
If your browser does not support JavaScript:
http://members.tripod.com/~alpertron/ENGLISH2.HTM
Antes era fanfarron... Ahora soy perfecto!!
Paul Skoczylas
Jeff wrote:
>
> A 1 kg mass is always a 1 kg mass. That same item (by definition) happens
> to weigh in at 1 kg on earth at sea level. Take it to the moon and it still
> has a mass of 1 kg, but its weight has changed to 1/7kg. That is why it is
> so important to differentiate between weight and mass,
NOOO!!!!
A 1 kg mass does NOT weigh 1 kg on earth at sea level. It weighs 9.81
N. A kilogram is not a unit of force in the SI system (although it may
have been used unofficially for that in the past).
By having completely different units for mass and weight, we avoid the
Markus Laker
Rodger Whitlock <toto...@mail.pacificcoast.net>:
> a rolling automobile on the level would just as hard to stop on the moon as on the
> earth because its momentum depends on its mass times its (horizontal) velocity.
Much harder, in fact. For a given speed, the car would have just as
much momentum on the moon as on earth, but you would only have one-sixth
as much friction between yourself and ground with which to stop the car.
I'm removing a.u.e from the follow-ups.
Markus Laker.
--
My real address doesn't include a Christian name.
Emailed copies of responses are very much appreciated.
Paul Skoczylas
Gene Nygaard wrote:
> "Weight vs. Mass: What's the Difference"
> http://ourworld.compuserve.com/homepages/Gene_Nygaard/weight.htm
>
> This is a work in progress. I'd appreciate it if some of you would
> review this for me and send me any corrections, suggestions,
> additions, or other comments, or post them in response to this
> message.
One thing I noticed was the quote:
"The pounds used in the grocery store are always units of mass; they are
never units of force."
This may be strictly true in terms of what is being sold, but NOT it
terms of how it is measured. I've never been in a grocery store that
measured the mass of a bag of apples. All the stores I've been in use a
spring scale, which can only measure force. (I think there may some
quaint old general stores around that use balances, however.) The force
measurement is converted to a mass measurement based on the assumption
that the scale is being used on Earth. (Actually, many of the more
sensitve and accurate spring scales that give an output in a mass unit
can be calibrated by placing a known mass on them. In this way they can
be accurate to the same degree at sea level or on a mountain top, where
the difference in gravity is greater than the accuracy of the scale.)
-Paul
marisal
Gene Nygaard wrote:
> What is weight? Is it different from mass?
> Physicists often define weight as a particular kind of force,
> resulting from gravity.
> that quantity measured with a balance. Balances measure mass, not
> force. Weight is equivalent to mass, in this definition, and this
> definition is a perfectly valid one.
> about it, check out my new web page:
> http://ourworld.compuserve.com/homepages/Gene_Nygaard/weight.htm
> This is a work in progress. I'd appreciate it if some of you would
> review this for me and send me any corrections, suggestions,
> additions, or other comments, or post them in response to this
> message.
if one mass M1 is double than another mass M2, then you need to exert a
force to lift M1 twice as big as to lift M2. This force is what is
called weight. However, weight is not intrinsic to matter: it depends on
where you are standing. For example, if you were on the Moon or in the
MIR station (technical problems apart) you would have little or no
weight. If you jumped on the Moon you would get much higher than on
Earth, because your weight depends on the gravitational force of our
planet.
In fact, the whole MIR station is weightless in space. But now here's
the difference with mass: if you collide against the MIR station at, say
50Km/h (a speed with which a feather with little mass wouldn't harm you)
be sure that it will crush you as if a whole building fell on you.
So even if a certain object has no weight, it still has a mass (which is
intrinsic to it).
I hope you see the difference.
Jim Carr
Paul Skoczylas <P.Sko...@cfer.ualberta.ca> writes:
>
>While the balance itself actually measures force, its common use
>actually measures mass, regardless of what planet it's used on.
A balance _compares_ forces (or torques), it does not measure them.
>A spring scale can only measure force.
A spring scale and similar devices measure forces.
--
James A. Carr <j...@scri.fsu.edu> | Commercial e-mail is _NOT_
http://www.scri.fsu.edu/~jac/ | desired to this or any address
Supercomputer Computations Res. Inst. | that resolves to my account
Florida State, Tallahassee FL 32306 | for any reason at any time.
Tak To
Larry Krakauer wrote:
>
> [...]
> The English unit of mass is the "slug", [...]
Note that there are other systems of terms. In UK, for example,
the unit of mass is "pound" and the unit of force is "poundal"
or "pound weight". (I am referring to what is taught in physics
classes, not the general everyday usage.)
----------
This thread reminds me of an anecdote from Paul R Halmos (the famous
mathematician). He once had a student in a linear algebra class
who complained that Halmos' definition of "vector" was narrow-
minded; and she submitted the definition from the Encyclopedia of
Britanica as the right definition. (This is from Halmos'
autobiography, "I want to be a Mathematician".)
Tak
----------------------------------------------------------------------
Tak To (617) 949-1377
Aspen Technology, Inc Fax: (617) 949-1030
10 Canal Park, Cambridge, Ma 02141. tak...@aspentech.com.-
----------------------------------------------------------------------
Disclaimer: I do no speak for Aspen Technology. [taode takto ~{LU5B~}]
Paul Skoczylas
Chad English wrote:
> The only way I can think to actual directly compare masses (regardless
> of gravitational field) is to apply the same *net* force to the objects
> and measure their acceleration. (Net force includes gravitational force
> components.)
A better way may be to build a vibrating system--if you know the spring
stiffnes of a spring attached to the unknown object, you can calculate
its mass from the period of virbation. That's much easier to measure
accurately than acceleration.
> I hope this makes sense. Feel free to poke holes in this argument, but
> I can't see any.
Your argument is very good. (Especially the bit about moments about the
balance point, since many balances we use these days don't have two
trays, but only one, balanced against a mass on a sliding scale.
Instead of increasing the mass, the mass is slid further from the
balance point to offset an increased mass on the tray.) But it doesn't
change the fact that the traditional use of the balance indirectly
calculates the mass, and that the same balance can be used to measure
the same mass on any planet. (Just not in free-fall, as in the space
shuttle or Mir. The poster who said that a balance has to be used in a
gravitational field is not strictly correct, since if you're in orbit
around the earth, you are most definitely subject to its gravity, but
you're balance still won't work.)
-Paul
evbill@gte.net Bill Baldwin
Chad English suggested:
> When you use a balance scale you are comparing moments about the balance
> point. Do a free body diagram and you will see this. You don't even
> need the concept of a mass to realize this. When the objects balance,
> they are applying the same moment about the focal point. The moment is
> the gravitational force (or really any external force, e.g. pushing on
> it) multiplied by the moment arm of the balance. Thus, you are
> comparing graviational forces (usually referred to as "weights", but I'm
> hesitant to use that term so as not to confuse issues as described on
> the web page).
>
> Now, assuming that the objects are in the same gravitational field,
> their masses will be the same proportion as their gravitional forces
> (weights), hence you *can* use a balance to compare masses, but only by
> deduction. That is not the physical process that is taking place.
True enough. A balance actually compares "weights" or downward forces
against one another. But it would be quite difficult to have a balance in
which the left and right trays are subjected to different gravitational
fields. (Or if someone can think of an easy way, still one would have to go
out of one's way to provide these conditions.) So, as you note, the balance
becomes a quick and dirty way of measuring the relative mass of objects.
And this is why the popularized version suggests that balances actually
measure mass. This would make an interesting question to put on a Physics
test.
Jim Carr
Chad English <ceng...@mae.carleton.ca.NOSPAM> writes:
>
>When you use a balance scale you are comparing moments about the balance
Correct, but it need not be the force of gravity.
One can also use torsional balances.
>There is the traditional argument, as on the web page, that balances
>compare masses because they would stay balanced on the moon, for
>instance. The problem with this argument is that all you've done is
>changed the gravitational field on both objects, so that the
>gravitational forces, and hence moments, though reduced in magnitude are
>still in equilibrium. The way to prove that it doesn't compare masses
>is imagine the two objects (two ends of the balance scale) in two
>*different* gravitational fields. ...
Correct. Thus there is an approximation in using a pan balance.
But a torsional balance would work in space, if you account for the
mass of the thing it is attached to, as will a pan balance if you
swing it in a circle around you.
Benoit Evans
Chad English wrote:
> [..]
> Now, assuming that the objects are in the same gravitational field,
> their masses will be the same proportion as their gravitional forces
> (weights), hence you *can* use a balance to compare masses, but only by
> deduction. That is not the physical process that is taking place.
>
Would the reasoning being used in this seemingly endless argument be analogous
to what happens when we use a mercury column thermometer to measure
temperature?
We are actually measuring the height of a column of liquid under different
conditions of heat and cold. However, since changes in temperature are
proportional to changes in the height of the column, we say that a thermometer
measures temperature when in fact it measures linear expansion.
The distinction may be critically important to scientists in some situations,
but is of no consequence to most people in everyday life.
Regards,
K.-Benoit Evans
Quebec, Canada
Paul Skoczylas
Larry Krakauer wrote (with snippage):
>The English unit of mass is the "slug", defined as "The unit of mass
>that
>is accellerated at the rate of one foot per second per second when
>acted upon by a force of one pound weight." Look it up in any
>dictionary or American physics text.
But there are more than one version of the English system. I've seen at
least three. lbf/slug; lbf/lbm (with a non-unity constant in Newton's
second law); and poundal/lbm.
>Substitute "slug" for "pounds-mass", and the above is correct.
Indeed. But most people (i.e. most engineers) don't use slugs, they use
pounds-mass and pounds-force, and insert conversion factors where
necessary.
The use of a kilogram to measure force is one of my pet peeves. I won't
argue that SI is "better" than the English systems; I find both useful
for certain things. However, seeing "kilogram-force" really irritates
me. (Actually, any incorrect use of SI irritates me.)
An object with a mass of 1 kilogram has that mass anywhere you go. It
is _not_ correct to say it weighs 0.17 kg on the moon. If, as Gene
suggests, that in this context "weight" means "mass", than it still
weighs 1 kg. Of course, I don't agree, because nobody uses weight in
that context. If you put it on a spring scale, it would "weigh" 0.17 of
whatever it weighed on earth. Hence, it is incorrect to label a spring
scale with kilograms unless it is to be used only on earth.
"An object with a mass of 1 kilogram weighs 9.8 N on earth and 1.7 N on
the moon," is a correct statement. In the duality of the English
system, "An object with a mass of 1 pound weighs 1 pound on earth and
0.17 pounds on the moon," is also a correct statement, recognizing that
the mass is constant at one pound-mass, but the weight (meaning the
gravitational force exerted on the object, measured in pounds-force)
changes as you go from earth to the moon.
-Paul
Paul Skoczylas
Julio VANIA wrote (with much snippage):
> Balances measure weight not mass.
> To measure mass, you have to make a comparison between to weigths
But a balance _does_ compare the weight of two objects, and since they
are presumably in the same gravitational field (it would be difficult to
build a balance that did otherwise), it therefore compares their masses
as well. If you have a balance, and a series of objects whose masses
are accurately known, you can measure the mass of another object on
earth, the moon, or anywhere, and get the same answer.
A spring scale (sometimes incorrectly called a balance) measures
gravitational force (in other words, weight) and will get different
results on earth and the moon.
-Paul
P.S. I still maintain that it is incorrect to use kilogram-force to
measure force. Gene quotes some US lab as saying weight can be used as
a synonym for mass, but in this case, the unit is just a kilogram, a
unit of mass, not force.
Jim Carr
Gene Nygaard wrote:
}
} What is weight? Is it different from mass?
}
} Physicists often define weight as a particular kind of force,
} resulting from gravity.
[cut]
"Ronald D. Cuthbertson" <inm...@flash.net> writes:
>
>From AHD3 --
>mass n. 6. Abbr. m Physics. The measure of the quantity of matter that a
>body or an object contains. The mass of the body is not dependent on
>gravity and therefore is different from but proportional to its weight.
>
>Hence, mass is different from weight.
The best I can do right now is refer you to DejaNews for an
earlier incarnation of this discussion in sci.physics where
I quoted at length from what one learns in the OED. I note
that you used the _Physics_ definition up above, which is
not the only one and certainly not the oldest one in this
context, an important distinction.
It is clear to me that the meaning of weight in "Weights and
Measures" concerns a mass measurement, and (as the OED documents)
that the usage of weight to mean force is a relatively recent
meaning introduced in the physical sciences. However, since
this new term is the only one taught in school, particularly
in physics classes, the fact that the legal meaning of "net wt."
concerns a mass is often unclear to most people.
Gene does need to make this distinction clear on his page.
evbill@gte.net Bill Baldwin
Bill Baldwin wrote:
> > So a one kilogram object will ALWAYS
> > exactly balance another one kilogram object here, on the moon, on
Mars...
> > anywhere.
Dario Alejandro Alpern shot back:
> Not anywhere. You need a gravitational force (or acceleration) to use a
> balance.
Uh huh. A balance doesn't work in a box too small to hold all the
components either. So obviously I should have specified the balance works
in spaces big enough to contain it.
And the balance doesn't work when it's in mud or concrete. So I should have
specified that it works only in atmospheric materials that allow for
sufficient free movement of its arms (and if there is any inhibition to
that movement, it must not completely keep the arms from moving and it must
inhibit both arms equally).
And it doesn't work in high winds either. So I should have mentioned the
atmospheric conditions must be sufficiently calm.
And the balance doesn't work when it's not on a level surface. So I
neglected to mention that you shouldn't take one on your Mount Everest Hike
unless you have a level and some pretty steady hands.
And it doesn't work on the freakin' island of the tiny balance-tampering
fairies either.
Obviously, Dario my good man, when I said "anywhere" I meant "anywhere in
which a balance might be said to work at all." And by that I mean work for
its intended purpose. So don't be e-mailing me about how in space you can
still use it to poke somebody's bleedin' eyeballs out.
--
A Testy Bill Baldwin
Southern California
Gary Williams, Business Services Accounting
In article <3427F5...@rrq.gouv.qc.ca>,
Benoit Evans <benoit...@rrq.gouv.qc.ca> writes:
> The distinction may be critically important to scientists in some situations,
> but is of no consequence to most people in everyday life.
Because, as I understand it (but I was known to fall asleep in high school
physics class), the gravitational attraction of two bodies to one another is a
function of the mass of each and the distance between them.
But since most of us experience gravity, for non-scientific purposes, at the
surface of the earth, two of the function's variables (mass of the earth and
distance from the earth) become constants. So, in the limited case in which
non-scientists work, the only variable is the mass of the object being
attracted by the earth. And that means that, in this limited case, mass and
weight are interchangeable.
Gary Williams
WILL...@AHECAS.AHEC.EDU
Jim Carr
Incorrect. Use your imagination or a torsional balance.
Jim Carr
Incorrect. The English do not use this unit for mass.
There was an extremely short period of time when it was used
by aeronautical engineers in Great Britain, as documented in
the OED and posted in a fairly recent thread in sci.physics.
I believe that is where American physics teachers picked up
the idea of using the slug to make teaching SI mass units
simpler by adopting the conventional meaning of pound as a
force as the physics teaching definition of the pound.
The legal definition of the pound in the United States
is that of a mass, defined by the International kilogram.
>defined as "The unit of mass that
>is accellerated at the rate of one foot per second per second when
>acted upon by a force of one pound weight." Look it up in any
>dictionary or American physics text.
If you look it up in a good dictionary, such as the OED, you
will see that "slug" has not been used that way until recently.
If you look in circa 1900 physics books, some of which I cited
in a lengthy article on the subject, you will not see slug used.
It was not used in engineering when my Dad was in college (1950).
>The web site that started this thread is really kind of bizarre. This
>matter is one of technical definitions, and is completely understood
>by anyone who has taken a first year physics course.
Incorrect. The technical knowledge of anyone who has only had
a first year physics course in the US is _zero_ regarding this
distinction and particularly as regards the three very different
'english' systems that were used in the U.S. in recent decades.
>The author of the
>site seems to be confusing people's everyday usage with the technically
>correct scientific definitions, and making a big deal of it.
The big deal is that he is correct.
>Everything he lists as a "myth" on the page is, using the proper
>scientific definitions, quite correct. For example, the site
>says:
>
>"MYTH: The metric system measures mass; the English system measures
>weight (meaning force)."
>
>Using the technical definitions used in any first year physics course,
>the statement that follows the word "MYTH:" is precisely correct.
That is because the first year course teaches a myth.
I first learned of the slug myth last spring (?) when my Dad e-mailed
me about a letter to the editor in their paper that took them to task
for something about mass, and repeated the canonical Halliday and
Resnick version. My Dad, educated before H&R and a long time
practitioner of civil engineering in the US 'english' system, said
"What?" and told me about pounds-mass and pounds-force (lb and glb
in the system he was taught). An hour in the library showed he was
correct and that what is taught in US physics classes changed circa
the 1960 arrival of H&R.
Given the wide readership this time, perhaps others will chew over
their library collections of circa 1950 - 1960 physics textbooks
and see when this crept into American usage.
Mark Barton
On Tue, 23 Sep 1997 13:13, Jeff <mailto:Har...@btinternet.com> wrote:
>A 1 kg mass is always a 1 kg mass. That same item (by definition) happens
still
>has a mass of 1 kg, but its weight has changed to 1/7kg. That is why it is
>so important to differentiate between weight and mass,
Arrgh, if you're going to be pedantic, get it right! The weight can't
change to 1/7 kg because you can't measure weight in kilograms. It could
change to 1/7 kgf (kilograms force), but then it's weight on earth probably
wasn't 1 kgf in the first place, because local gravity isn't the 9.80665
m/s^2 everywhere, which the definition of kgf assumes.
Cheers,
Mark B.
----------------
Please remove the spam filter from my address before replying.
Mark Barton
On Tue, 23 Sep 1997 16:06, Julio VANIA <mailto:va...@flore.cma.fr> wrote:
>Balances measure weight not mass.
What type of balance are you talking about? It matters. Ignoring the
buoyancy due to air, pan balances measure mass and spring balances measure
weight. If you take a pan balance to the moon, together with its reference
masses and the mass to be tested, it will give you the same reading. A
spring balance won't, by a large factor.
Mark Barton
On Tue, 23 Sep 1997 13:15, DB <mailto:no...@nk.you> wrote:
>If not its WEIGHT, what is the "force on [said object] due to the [acting
>body]'s gravitational pull"? Most UK science teachers use the word
>"weight." Fair enough, they go wrong in not calling it the object's
"Earth
>weight," but give them a break...
>
>"force on [said object] due to the [acting body]'s gravitational
>pull"=="[acting body] weight"
Most professional physicists I know avoid the word "weight" entirely and
say "the gravitational force on the object". "Weight" is just hopelessly,
irreparably imprecise.
BrockBadge
>creynold wrote:
>
>> A balance measures force - not mass. Don't believe me? Put a mass on one
>> side and see if you can 'balance' it with your finger. No matter what mass
>> you put on the scale (within reason for the context following) you can
>> balance it with your finger. Yet you finger didn't change mass did it?
For me, the above ^ version is more used than the below v ... anybody else?
Paul Skoczylas <P.Sko...@cfer.ualberta.ca> writes:
>But a balance is virtually always used to compare two objects. Usually
[-snip-]
>While the balance itself actually measures force, its common use
>actually measures mass, regardless of what planet it's used on. A
[-snip-]
One of the things I do most often upon seeing a scale/balance/whatever of
any kind is just to push down on it (or whatever direction it's set to
measure from) to see how much I can make it go...
...I'm just simple-minded, I guess, but I'm pretty sure that scales are
often used to measure force, rather than the "compare two obects" ideal...
In any event, I don't see that you justify saying "virtually always" is a
scale used this way. Could you elaborate a little?
>>BKNambo
H newsm...@earthling.net -Spam me, I dare ya ____ __
=@==== http://members.aol.com/brockbadge/index.html /_ \ / /
H H H "World Domination Through Trivia" -S3Kitties / /\ \/ /
H H H Marcher -- Just my imagination -- Nightwatch /_/ \__/
Paul Skoczylas
Jim Carr wrote:
> It is clear to me that the meaning of weight in "Weights and
> Measures" concerns a mass measurement, and (as the OED documents)
> that the usage of weight to mean force is a relatively recent
> meaning introduced in the physical sciences. However, since
> this new term is the only one taught in school, particularly
> in physics classes, the fact that the legal meaning of "net wt."
> concerns a mass is often unclear to most people.
>
> Gene does need to make this distinction clear on his page.
I think the one thing that is painfully clear right now is that when one
uses the word "weight", one should specify if one means mass, or
gravitational force on an object, or whatever other obscure meaning.
"net wt." in packaging is really irrelevant if weight or mass is meant,
because the difference is very small as one moves to different places on
earth. Maybe when we get moon colonies, they'll be more specific. Here
in Canada, however, virtually everything is already sold by the kilogram
(even a pound of butter is labelled 454 g), and the kilogram is clearly
(IMHO) _only_ a unit of mass. So 454 g (0.454 kg) of butter is still
454 g of butter on the moon. (While a pound-mass of butter is still a
pound-mass of butter, only now it "weighs" only a sixth of a
pound-force.)
-Paul
Gene Nygaard
In article <01bcc844$83cd9170$0400a8c0@BAHN>,
"William L. Bahn" <ba...@pcisys.net> wrote:
>
> Please note that NIST's SP811 is merely an acknowledgment that the COMMON
> usage of weight and mass are often confused and that when someone says to
> "weigh something" they are typically wanting to know the mass of that
> object as opposed to its weight - i.e., they want to know how much of it
> they have as opposed to how much effort it takes to hold it off the ground.
It's much clearer than that. NIST SP811 is saying that the term "weight"
is ambiguous. While this ambiguity may lead to confusion, SP811 is not
saying that there is anything particularly "confused" about using weight
to mean mass, in the ancient longstanding meaning of weight.
The Swedish metrologists make it very clear on their web pages, quoted
with a link at my web page
http://ourworld.compuserve.com/homepages/Gene_Nygaard/weight.htm
Mass is a measure of the amount of matter in a body. The term
"weight" is sometimes used as a synonym for mass. However, this
term has also several other meanings and should therefore be avoided.
> This document does not say that mass and weight ARE the same, <snip>
Nor am I saying that they are the same. I'm merely saying that weight
often does mean mass, and that it has other meanings as well. I didn't
think it was necessary to explain all the ramifications in my subject
heading, since my intentions should be clear in my newsgroups postings,
and especially on my web pages.
> ... then it is not surprising that the "conversion" between mass and
> weight is commonly seen as being no different than the conversion between
> inches and feet - just different units for the same thing.
>
The problem is that many people see a "conversion between mass and
weight" when there is none. For example, in an American grocery story
where a can of beans might be labeled "net wt. 1 lb (454 g)" both the
pounds and the grams are measures of mass, and "net weight" means the
mass of the beans, excluding the mass of the can.
<snip>
>
> Is it reasonable to expect people to adhere to precise and correct usage in
> the common parlance? Not at all.
It's time for physicists to stop badmouthing the general public on this
issue. Not only does weight often mean mass, but this is a very
legitimate use of the word, well supported by history. To the extent
that there is validity to a stereotypical division into two groups of
people, history and linguistics are on the side of the general public on
this issue, not on the side of the physicists who insist that only the
meaning as a particular force is legitimate.
Physicists need to realize not only that weight often has meanings other
than the force due to the acceleration of gravity, but also that those
meanings are quite proper uses of the word "weight."
>
> Is the most reasonable approach therefore to make people aware of the
> typically intended meaning that should be assumed in most situations? Sure.
Right. The typically intended meaning is mass. That is what should be
assumed unless it is otherwise stated. Of course, it is reasonable to
hold physicists, etc., to a higher standard in regard to the
identification of the meaning intended. But in most cases, it is a
sufficient identification of the meaning if newtons (or poundals) are
used as the units in which the "weight" is measured. It's when pounds or
kilograms are used for "weight" or when no specific measurements are used
that its especially important to use some other method to identify which
meaning is intended.
Gene Nygaard
-------------------==== Posted via Deja News ====-----------------------
http://www.dejanews.com/ Search, Read, Post to Usenet
Andy Bell
> >The English unit of mass is the "slug",
>
> Incorrect. The English do not use this unit for mass.
snip...
> If you look it up in a good dictionary, such as the OED, you
> will see that "slug" has not been used that way until recently.
Does it matter? It's being used now.
snip...
> It was not used in engineering when my Dad was in college (1950).
I am in engineering now (1997), and that is what I, and my
fellowclassmates, have been taught.snip...
> >Using the technical definitions used in any first year physics
> course,
> >the statement that follows the word "MYTH:" is precisely correct.
>
> That is because the first year course teaches a myth.
What??? Are you telling me that you want us to believe that
everyphysics course on Earth that teaches the unit for mass in english
is the
slug is part of some conspiracy?
My point: So what if 100 years ago (or 30 for that matter) people did
or
did not use slug for the unit for mass. We do now. Your argument, or
your Dad's to be precise, is like saying that the Earth really isn't
round,
because at one time it was "thought" to be flat.
Regards,
Andy Bell
Mick Lord
Not quite correct:
1 kg = 1 kg, anywhere!
At sea level on earth, 1 kg will weigh F = mg = 9.8 newtons (N).
On the moon, the same 1 kg will weigh F = mg which is appr. 1.6 N but still
a mass of 1 kg. That is because g on the Moon is about 1/6th of what it is
on Earth.
But I believe the real question is what is the English unit for mass
without getting mixed up with force, as the pound does very well. I am not
sure. It looks to me, in glancing through an encyclopedia, that the writer
must specify "pound mass" or "pound force" to make the distinction.
Mick
----------
From: Jeff <Har...@btinternet.com>
Newsgroups: sci.physics; alt.usage.english; sci.engr; sci.math; sci.misc;
sci.lang
Subject: Re: Does weight = mass?
Date: September 23, 1997 9:13 AM
A 1 kg mass is always a 1 kg mass. That same item (by definition) happens
to weigh in at 1 kg on earth at sea level. Take it to the moon and it still
has a mass of 1 kg, but its weight has changed to 1/7kg. That is why it is
so important to differentiate between weight and mass,
Jeff
Jim Carr
brock...@aol.com (BrockBadge) writes:
>
>One of the things I do most often upon seeing a scale/balance/whatever of
The distinction is important. What will happen when you push
down on a spring scale on the moon or in orbit will be different
from what happens if you push down on the pan of a typical school
balance (the one with weights on a calibrated arm) in those two
conditions compared to the earth.
>...I'm just simple-minded, I guess, but I'm pretty sure that scales are
>often used to measure force, rather than the "compare two obects" ideal...
How would you use a two-pan balance "scale" (of the type that
Justice uses) to measure a force on the moon?
Jim Carr
Paul Skoczylas <P.Sko...@cfer.ualberta.ca> writes:
>
>"net wt." in packaging is really irrelevant if weight or mass is meant,
>because the difference is very small as one moves to different places on
>earth.
No it is not.
What error would you tolerate? Can they sell 453 grams and label
it as 454 grams? I actually don't know the law on it, but if
you consistently shorted a product like that you would have a
built in profit advantage.
Now compare that to the variation with elevation if the
force of gravity were the reference rather than mass.
>Maybe when we get moon colonies, they'll be more specific. Here
>in Canada, however, virtually everything is already sold by the kilogram
>(even a pound of butter is labelled 454 g), and the kilogram is clearly
>(IMHO) _only_ a unit of mass. So 454 g (0.454 kg) of butter is still
>454 g of butter on the moon.
And that is the way it is sold in the US, because a pound is
defined in terms of the kilogram in the US as well.
Julio VANIA
In article <3427FB...@mae.carleton.ca.NOSPAM>, Chad English
<ceng...@mae.carleton.ca.NOSPAM> writes:
|
|>
|> Perhaps a more accurate description of the differences between scales is
|> this:
|> 1. A balance compares gravitational forces (weights) with a
|> "known/standard" gravitational force.
|> 2. A spring scale compares gravitational force with a "known" spring
|> force (or stiffness, or calibration, or something like that).
I completely agree with you.
|>
|> The only way I can think to actual directly compare masses (regardless
|> of gravitational field) is to apply the same *net* force to the objects
|> and measure their acceleration. (Net force includes gravitational force
|> components.)
|>
|> I hope this makes sense. Feel free to poke holes in this argument, but
|> I can't see any.
|>
This depedends on accurancy.
Because the mass is dependent upon the velocity. even though that a slow
velocities the variations of mass are negligeble.
This seems a very interesting theme. How can we measure the mass of a body?
Anyway,
There some confusions in the English system and the KMS system.
English Systeme mass - pound-mass (lb)
weitgh (force) - pound-force (lbf)
KMS Systeme mass - kilogram (kg)
weitgh - kilogram-force - kgf
Why does everybody just use the SI system. There is clearly a differente between
a kilogram (kg) of mass and a Newtow (N) of weitgh.
Jim Carr
jac wrote:
|
| >The English unit of mass is the "slug",
|
| Incorrect. The English do not use this unit for mass.
| If you look it up in a good dictionary, such as the OED, you
| will see that "slug" has not been used that way until recently.
Andy Bell <gt5...@prism.gatech.edu> writes:
>
>Does it matter? It's being used now.
It matters if you think there is something fundamental about
that usage, such as that it has legal meaning in the US system
of weights and measures.
| It was not used in engineering when my Dad was in college (1950).
|
>I am in engineering now (1997), and that is what I, and my
>fellowclassmates, have been taught.snip...
Taught in physics class, or taught in engineering? Which
engineering class? Which textbook? This is not a rhetorical
question, because tracing this usage is an interesting
challenge. It appeared from nowhere.
Ask a professor who has worked as an engineer if they use slugs
when doing real engineering design today. I would like to know
the answer to this. I only know that I have taught slugs for
decades in physics and only recently noticed the fine print in
the earlier H&R and what was done prior to 1960.
When it was first taught in physics it was not used anywhere
else in the world as a mass unit. Nature had already pointed
out that it was out of favor in the one community (British
aeronautics) that had tried it.
| >Using the technical definitions used in any first year physics
| >course, the statement that follows the word "MYTH:" is precisely
| >correct.
|
| That is because the first year course teaches a myth.
>What??? Are you telling me that you want us to believe that
>everyphysics course on Earth that teaches the unit for mass in english
>is the slug is part of some conspiracy?
Yes. The passive conspiracy of cribbing from good books. Taught
with authority and believed on that basis.
Textbooks have awesome power. They moved p out of Hamiltonian
mechanics of p's and q's into p = mv in less than a decade after
centuries of following Newton's style. The second American
revolution. The slug was part of that.
>My point: So what if 100 years ago (or 30 for that matter) people did
>or did not use slug for the unit for mass. We do now.
No we do not. We use the kilogram. Legally we use the pound.
We teach the slug in physics because (my hypothesis) the glb
and the poundal blew away too many future physicists and both
were fading from favor with the adoption of SI.
Chris Dearlove
Andy Bell (gt5...@prism.gatech.edu) wrote:
: > >The English unit of mass is the "slug",
: >
: > Incorrect. The English do not use this unit for mass.
Jim Carr, who wrote the line above, is as far as I can tell, spot
on in everything he says. Certainly I'd never heard of a slug
before Usenet, and I'm old enough to have used pounds and
poundals. If you must refer to a slug call it an American
unit not and English one. Anyway the modern English (*) unit is
the kilogram (with a few pounds left over). The whole discussion
makes me think of Feynman's piece where he describes textbooks
with disgust.
Incidentally whilst I might agree in theory with people who
say balances compare forces, the point is that this is equivalent
to comparing masses (assuming virtually constant local
gravitation) and so the important question is what you put on
them. This is usually unknown object on one side, standard
calibrated known masses on the other. When you've finished
you know the unknown object's mass, but not the force it
exerts.
(*) I say English here because that's what's used above. British
is more to the point. If you want to refer to languages not
countries then replace American by American English, English by
British English (or, as we say, English).
--
Christopher Dearlove Personal comments only.
Chad English
Bill Baldwin (rev...@gte.net) wrote:
: Huh? We're clearly not using the same definition of the word "balance"
: here. A balance, by definition I would have thought, compares two weights
: by BALANCING them against each other. So a one kilogram object will ALWAYS
: exactly balance another one kilogram object here, on the moon, on Mars...
: anywhere. However, on a non-balance type scale, i.e. a spring scale such as
: many bathrooms sport, a one kilogram object will register one kilogram on
: earth but considerably less (1/6?) on the moon. That is because it is
: exerting less force against the springs while the springs maintain a
: constant force in the reverse direction regardless of gravity. Clear as
: mud.
A balance actually compares the forces on the two ends, regardless of the
masses (if there are any). It can usually be assumed that the masses are
also equal if other external forces (including buoyancy) can be neglected.
--
--
Chad English
ceng...@mae.carleton.ca
http://www.mae.carleton.ca/~cenglish
Chad English
Paul Skoczylas (P.Sko...@cfer.ualberta.ca) wrote:
: Chad English wrote:
: > The only way I can think to actual directly compare masses (regardless
: > of gravitational field) is to apply the same *net* force to the objects
: > and measure their acceleration. (Net force includes gravitational force
: > components.)
: A better way may be to build a vibrating system--if you know the spring
: stiffnes of a spring attached to the unknown object, you can calculate
: its mass from the period of virbation. That's much easier to measure
: accurately than acceleration.
Agreed. I don't know why I didn't think of that since I've done it
before. (^=*
: > I hope this makes sense. Feel free to poke holes in this argument, but
: > I can't see any.
: Your argument is very good. (Especially the bit about moments about the
: balance point, since many balances we use these days don't have two
: trays, but only one, balanced against a mass on a sliding scale.
: Instead of increasing the mass, the mass is slid further from the
: balance point to offset an increased mass on the tray.) But it doesn't
: change the fact that the traditional use of the balance indirectly
: calculates the mass, and that the same balance can be used to measure
: the same mass on any planet. (Just not in free-fall, as in the space
: shuttle or Mir. The poster who said that a balance has to be used in a
: gravitational field is not strictly correct, since if you're in orbit
: around the earth, you are most definitely subject to its gravity, but
: you're balance still won't work.)
Yes, I though about those sliding arm balances afterwards, which makes it
quite clear that you are comparing moments (changing moment arm, but not
mass or even gravitational force). However, you are also quite correct
that in common usage that you can deduce the equivalence of masses on a
balance scale. Only in special situations would this not be so. However,
it is commonly stated (as on the web page) that balance scales measure
(compare) mass, *not force*, which is incorrect.
jbveale@remove
Actually, to say the 1 kg mass has a weight of "1/7 kg" on the moon is
incorrect. On the earth, the 1 kg mass has a weight of 9.8 N. On the moon, the
1 kg mass will have a weight of 9/7 N.
However, I will cheerfully agree that for the majority of users, no one uses
newtons as a measure of weight. Bad enough being asked to pick up 454 grams of
butter, 4.45 N would be worse.
Gene Nygaard
In article <34290fd3....@news.nas.com>,
jbveale@[remove]memes.com wrote:
>
> Actually, to say the 1 kg mass has a weight of "1/7 kg" on the moon is
> incorrect. On the earth, the 1 kg mass has a weight of 9.8 N. On the moon,
the
> 1 kg mass will have a weight of 9/7 N.
>
> However, I will cheerfully agree that for the majority of users, no one uses
> newtons as a measure of weight. Bad enough being asked to pick up 454 grams
of
> butter, 4.45 N would be worse.
>
That's not because no one uses newtons as a measure of weight meaning
force. It's because the "weight" in your second paragraph is something
different from the "weight" in your first paragraph. The weight in a
pound of butter is a measure of mass, and that's what it should be. To
measure it in newtons would be incorrect.
This is covered several times on my web page. The meaning of "weight"
which is equivalent to mass is the older, original meaning of weight. It
is just as valid and legitimate as the "weight" meaning a particular kind
of force. So if you use the word weight, make sure it is clear which
definition you are using.
Gene Nygaard
http://ourworld.compuserve.com/homepages/Gene_Nygaard/weight.htm
Jim Carr
Mass does not depend on velocity, but that is a different thread,
and an idea the originates in a different sequence of cribbing
from one textbook to another.
>There some confusions in the English system and the KMS system.
>
>English Systeme mass - pound-mass (lb)
> weitgh (force) - pound-force (lbf)
or lb and glb, or the very different system of pound and poundal,
both used in the US engineering community for a century, or the US
Physics teaching system of slug and pound.
>KMS Systeme mass - kilogram (kg)
> weitgh - kilogram-force - kgf
Using this should be a criminal act.
>Why does everybody just use the SI system. There is clearly a differente
>between a kilogram (kg) of mass and a Newtow (N) of weitgh.
I assume you meant "doesn't"
Everyone in physics (worldwide) uses that system. Even the United
States uses that system to define its own commercial system of
weights and measures.
--
Andy Bell
Jim Carr wrote:
> >My point: So what if 100 years ago (or 30 for that matter) people
> did
> >or did not use slug for the unit for mass. We do now.
>
> No we do not. We use the kilogram. Legally we use the pound.
> We teach the slug in physics because (my hypothesis) the glb
> and the poundal blew away too many future physicists and both
> were fading from favor with the adoption of SI.
This is an engineering newsgroup, not a business/legal/whatever
newsgroup.In engineering, the slug and the kilogram are the units for
mass. I am not
a historian about these issues, however, I do know my above statement
to be true today.
If you like, I can drudge through all my textbooks and cite each and
every
one. For now, this the definition for mass in
Shigley & Mische's "Mechanical Engineering Design," 5th edition.
M = (F*T^2)/L = (pound-force)*(second)^2 / foot = lbf * s^2 / ft = slug
> Taught in physics class, or taught in engineering? Which
> engineering class? Which textbook? This is not a rhetorical
> question, because tracing this usage is an interesting
> challenge.
Physics and engineering. Which classes? Try them all. Dynamics,
Def Bods, System Dynamics, Statics, Fluid Dynamics....blah, blah.
And what does it matter physics or engineering anyway? Disciplines
do not change or manufacture units of measurement to suit their needs.
A system of measurement is the same across the board, regardless if you
are an engineer,doctor, or ditch digger.
I do not wish to debate the origins of the slug, I have no need to know
that.
However, I do have an issue with your statement that the slug IS NOT
a unit for mass. I am having a difficult time understanding this whole
discussion. Simple: Force is pound/newton....Mass is slug/kilogram.
Has been taught for years, and will be taught for time to come.
Look in any book regarding mechanics. Because the U.S. is in
a transition between English and metric, around 50% of problems
are given in SI, and the other 50% in english. If mass is given in
these
questions, it is given in kilograms or slugs, always.
Regards,
Andy Bell
William L. Bahn
Julio VANIA <va...@flore.cma.fr> wrote in article
<60ago9$svp$1...@bego.cma.fr>...
<snip>
> |>
> |> The only way I can think to actual directly compare masses (regardless
> |> of gravitational field) is to apply the same *net* force to the
objects
> |> and measure their acceleration. (Net force includes gravitational
force
> |> components.)
> |>
> |> I hope this makes sense. Feel free to poke holes in this argument,
but
> |> I can't see any.
> |>
>
> This depedends on accurancy.
> Because the mass is dependent upon the velocity. even though that a slow
> velocities the variations of mass are negligeble.
> This seems a very interesting theme. How can we measure the mass of a
body?
>
>
One way that is used is to measure the frequency of a spring-mass based
simple harmonic oscillator. The "weight" of the object due to gravity
manifests itself as a spatial offset but the frequency if dependent only
upon the mass and the spring constant. The force due to gravity (g)
disappears from the expression for the frequency of oscillation. Note that
you can't play this same trick with a pendulum because (g) does NOT vanish
from the solution because the restoring force is provided by gravity in
that case.
Mark Barton
On Wed, 24 Sep 1997 13:37, Chad English <mailto:ceng...@mae.carleton.ca>
wrote:
This is a matter of semantics, but I don't think equating "compare" and
"measure" is a good idea, or fair to Mr Nygaard. First, note that using a
pan balance by placing it in the gravitational field of a large mass gives
a value for the "passive gravitational mass" of the object, whether you
choose to call it a measurement or not. Similarly using it on a platform
accelerated by rockets gives a value for the "inertial mass". Although
these are conceptually different in Newtonian physics, according to our
best theory, General Relativity, they are fundamentally the same thing. By
Einstein's equivalence principle, the only possible way of measuring mass
is to look at the force required to cause an object to move non-inertially.
The suggestion of mounting the mass on the end of spring and looking at the
period does not escape this. If you define "measure" in such a way as to
disqualify all procedures which involve forces as an intermediate you
produce the nonsensical result that a measurement of mass is impossible in
principle - nothing but nothing is a measurement of mass. The important
feature of the pan balance (as opposed to a simple spring balance) is that
it is automatically self-calibrating, so that the numbers that come out are
automatically values of mass.
Gene Nygaard
In article <60b4be$1...@bertrand.ccs.carleton.ca>,
But the balance cannot MEASURE the magnitude of those forces. If the
balance is moved to a place with a different acceleration of gravity, the
force on the side with the unknown masses is greater, but so is the force
on the side with the known masses. The same mass that will balance a
known mass on Earth (weighed in a vacuum, or with both the known and
unknown masses of the same density), will also balance on the Moon. Even
though the magnitude of the force on Earth is about 6 times greater than
that on the Moon, a balance cannot measure that difference. To determine
forces with a balance, you also need an independent measure of the
acceleration of gravity.
A balance does MEASURE the magnitude of the mass.
Gene Nygaard
Gene Nygaard
In article <60b524$1...@bertrand.ccs.carleton.ca>,
ceng...@mae.carleton.ca (Chad English) wrote:
>
> Yes, I though about those sliding arm balances afterwards, which makes it
> quite clear that you are comparing moments (changing moment arm, but not
> mass or even gravitational force). However, you are also quite correct
> that in common usage that you can deduce the equivalence of masses on a
> balance scale. Only in special situations would this not be so. However,
> it is commonly stated (as on the web page) that balance scales measure
> (compare) mass, *not force*, which is incorrect.
>
You must have a strange definition of "to measure."
William L. Bahn
Andy Bell <gt5...@prism.gatech.edu> wrote in article
<3429686F...@prism.gatech.edu>...
> Jim Carr wrote:
>
> > >My point: So what if 100 years ago (or 30 for that matter) people
> > did
> > >or did not use slug for the unit for mass. We do now.
> >
> > No we do not. We use the kilogram. Legally we use the pound.
> > We teach the slug in physics because (my hypothesis) the glb
> > and the poundal blew away too many future physicists and both
> > were fading from favor with the adoption of SI.
>
> This is an engineering newsgroup, not a business/legal/whatever
> newsgroup.In engineering, the slug and the kilogram are the units for
> mass. I am not
> a historian about these issues, however, I do know my above statement
> to be true today.
>
> If you like, I can drudge through all my textbooks and cite each and
> every
> one. For now, this the definition for mass in
<snip>
But be aware that other people can fish out references, including the CRC
Handbook of Chemistry and Physics, that define pound as a unit of mass
(given in terms of kilograms). They generally give several definitions for
pound including what is more explicitly called pound-mass, pound-force. I
am not fond of this duality at all - but it exists whether I like it or
not. I think it is simply a recognition of the fact that so much
engineering data is given in terms of pounds when mass is what is being
referred to. Consider all of the thermodynamic data that is given in terms
of BTU/(lb*degF). The main reason for this is that a lot of engineering
data was developed prior to the distinction between mass and gravitational
force being pushed by the scientific community. So we have the MKS and SI
systems that supposedly cleanly divorce these two concepts - except how
many pressure gauges are calibrated in kg/m^s and other improper uses of kg
as a force instead of a mass? I don't think there will ever be a clean
separation of these two concepts as long as the distinction between them in
the common experience of most people involves only a "units conversion".
Jim Carr
Jim Carr wrote:
|
| >My point: So what if 100 years ago (or 30 for that matter) people
| >did or did not use slug for the unit for mass. We do now.
|
| No we do not. We use the kilogram. Legally we use the pound.
| We teach the slug in physics because (my hypothesis) the glb
| and the poundal blew away too many future physicists and both
| were fading from favor with the adoption of SI.
Andy Bell <gt5...@prism.gatech.edu> writes:
>
>This is an engineering newsgroup, not a business/legal/whatever
>newsgroup.
This discussion is cross-posted between sci.physics, alt.usage.english,
sci.engr, sci.math, sci.misc, and sci.lang because the discussion
transcends specific newsgroups. It is also not specific to the US.
It was interesting to hear the comment by a British engineer and
your observations about current US textbooks; I am also interested
to know if slugs are used in engineering practice in the US.
>In engineering, the slug and the kilogram are the units for mass.
However, by law, the pound is a unit of mass in the United States
when used in commerce. I know that the US DOT has shifted to the
use of metric for construction, but it would be interesting to
know how specs are written in other areas. Do slugs or poundals
ever appear in specs?
>I am not
>a historian about these issues, however, I do know my above statement
>to be true today.
>
>If you like, I can drudge through all my textbooks and cite each and
>every one. For now, this the definition for mass in
>
>Shigley & Mische's "Mechanical Engineering Design," 5th edition.
>
>M = (F*T^2)/L = (pound-force)*(second)^2 / foot = lbf * s^2 / ft = slug
Thanks for that cite. It is interesting that they find it
necessary to use the pound-force. Intro physics texts, the
only place slugs are used in that field, do not. Do they
say anything at all about pound-mass or poundals?
>Physics and engineering. Which classes? Try them all. Dynamics,
>Def Bods, System Dynamics, Statics, Fluid Dynamics....blah, blah.
>
>And what does it matter physics or engineering anyway? Disciplines
>do not change or manufacture units of measurement to suit their needs.
To the contrary, the slug is an example of just that:
My earlier comments drawn from the OED:
----- start insert -----
An important detail is that the 'mass' meaning of "slug" does not
appear in the original part of the OED or the original supplement
(vols. I and II of the so-called Compact Edition). It appears only
in vol. III, the Supplement assembled in the 1930's. Also note that
the * in the listing means the sense does not fit into the pattern
of the others in that list, but is placed there following the
organizational plan of the dictionary. The other senses in the
second substantive set are all "lump" related, like a nuclear fuel
slug, a slug of liquor, a mass added to an electrical device, a
piece of lead from a linotype, etc.
slug, {\it sb.}^2 Add: ... ...
5*. {\it Engin.} A unit of mass equal to 32.1740 lb., being
the mass of a body which accelerates at one foot per second
when acted on by a one pound force.
Aside: note that a slug is equal to a mass given in *pounds*
and that 'pound force' is used to make that usage distinct
from the pound mass, which carries no modifier.
1902. A.M.WORTHINGTON {\it Dynamics of Rotation} (ed. 4)
p. viii, I have ventured to give the name of 'slug' to the
British Engineer's Unit of Mass, i.e. to the mass in which
an acceleration of one foot-per-sec.-per-sec. is produced
by a force of one pound.
Note that by implication, it is not in the 3rd edition of this text.
So now we know who to blame.
1923 A.R.LOW in W.L.Marsh {\it Re. Internat. Air Congr.} 62
The 'slug' of 32.2 pounds avoirdupois mass, which has actually
been imposed on British aeronautics by the Advisory Committee.
Again note it is equated to the pound as a unit of mass.
1936 F.W.LANCHESTER {\it Theory of Dimensions & its Applicatons
for Engineers} v. 37 Even amongst the advocates Perry's system ..,
the slug has never taken shape except on paper; it has, and has
had, no real material existence.
skip 1944 (simple usage of the unit in an Aerodynamics text)
1973 {\it Nature} 30 July 184/3 The statement that the unit
of mass in the British system is the slug is several years
out of date.
Succinctly put. Yet several years earlier is when it was gaining
widespread use in the Halliday and Resnick physics textbook. The
only missing research now is to sort out when the 'slug' crawled
into U.S. physics education. It was clearly limited to use in
Britain in the aeronautics community when first introduced at the
start of this century. There is no historical precendent for its
use in physics outside first-year textbooks.
----- end insert ----
I should add that later searches have not located a US text
using slug before 1960. My guess is that it migrated to the
US from Britain during WW II and worked its way into textbooks
when they were rewritten in the late 50s or 60s.
We have also seen an article from a British engineer saying
they used pounds/poundals rather than slug/pound, consistent
with some of the remarks quoted above.
>However, I do have an issue with your statement that the slug IS NOT
>a unit for mass. I am having a difficult time understanding this whole
>discussion. Simple: Force is pound/newton....Mass is slug/kilogram.
>Has been taught for years, and will be taught for time to come.
It may have been taught that way, but the pound is a unit of
mass by law in the United States. That is why your book found
it necessary to add the "force" adjective to it, a distinction
dropped from physics textbooks like Halliday and Resnick (1960).
Paul Skoczylas
Gene Nygaard wrote:
> But the balance cannot MEASURE the magnitude of those forces. If the
> balance is moved to a place with a different acceleration of gravity, the
> force on the side with the unknown masses is greater, but so is the force
> on the side with the known masses. The same mass that will balance a
> known mass on Earth (weighed in a vacuum, or with both the known and
> unknown masses of the same density), will also balance on the Moon. Even
> though the magnitude of the force on Earth is about 6 times greater than
> that on the Moon, a balance cannot measure that difference. To determine
> forces with a balance, you also need an independent measure of the
> acceleration of gravity.
>
> A balance does MEASURE the magnitude of the mass.
This is getting rather pedantic, but ...
A balance (by which I mean the simple two-pan balance) COMPARES the
relative magnitude of the gravitational forces exerted on the two pans
and the objects held by the two pans. It doesn't MEASURE anything.
(Similarly, all a thermometer "measures" is the expansion of mercury or
alcohol with temperature--it's how we interpret that measurement that
gives us temperature.) If we get the balance into equilibrium then we
know that the two gravitational forces are equal. Since both pans are
in the same gravitational field, they must therefore have equal mass.
If we know the mass of one object accurately, we therefore know the mass
of the other. We haven't DIRECTLY MEASURED anything, only COMPAREED two
gravitational forces.
However, we use the two-pan balance to INDIRECTLY MEASURE mass, but this
is done by COMPARING forces.
Clear as mud?
-Paul
Dick Brewster
In <8751324...@dejanews.com>, Gene Nygaard of
gnyg...@crosby.ndak.net wrote:
>
> A balance does MEASURE the magnitude of the mass.
Unless, of course you are using a balance to measure a force.
>
> Gene Nygaard
>
> -------------------==== Posted via Deja News ====-----------------------
> http://www.dejanews.com/ Search, Read, Post to Usenet
>
--
Dick Brewster dbrewste@
` ix.netcom.com
Suzuki GSF1200S Honda CB700SC
Gene Nygaard
In article <342984...@cfer.ualberta.ca>,
Paul Skoczylas <P.Sko...@cfer.ualberta.ca> wrote:
>
>
> However, we use the two-pan balance to INDIRECTLY MEASURE mass, but this
> is done by COMPARING forces.
>
> Clear as mud?
>
Exactly. It's what we can MEASURE that matters, not the mechanism behind
it. No balance (in the classic meaning, not including spring scales and
the sort), whether a double pan balance or a steelyard or a beam balance)
can MEASURE those forces at all, without an independent measurement of
acceleration to go with the measurement of mass the balance gives you.
Albert Marshall
Chris Dearlove <c...@gmrc.gecm.com> wrote
>Andy Bell (gt5...@prism.gatech.edu) wrote:
>: > >The English unit of mass is the "slug",
>: >
>: > Incorrect. The English do not use this unit for mass.
>
>Jim Carr, who wrote the line above, is as far as I can tell, spot
>on in everything he says. Certainly I'd never heard of a slug
>before Usenet, and I'm old enough to have used pounds and
>poundals. If you must refer to a slug call it an American
>unit not and English one.
The only time I ever came across the "slug" was in my physics "O" Level
(public exam at age 16) paper. I had never been taught that system of
units and I remember being most irate that a fairly simple calculation
question had been put beyond my reach by my lack of knowledge of the
relationships between the units.
However, based on this episode I can confirm that the slug was
officially recognised in UK use in the mid 1960s.
--
Albert Marshall
Executive French
Language Training for Businesses in Kent
01634 400902
Dick Brewster
In <60c0nd$h6h$1...@news.fsu.edu>, Jim Carr of
j...@ibms48.scri.fsu.edu wrote:
>
>
>
> I should add that later searches have not located a US text
> using slug before 1960. My guess is that it migrated to the
> US from Britain during WW II and worked its way into textbooks
> when they were rewritten in the late 50s or 60s.
My high school physics class in Fairfield California used slugs
in 1959.
My older brother was in engineering school in the same area before
that (~1957) and used slugs.
I coined the term "Hut" for mass in the g = 386.4 in^2/s world. I
was working a lot with inertial properties in an area where
everything was dimensioned in inches and using Huts saved using a
lot of constants.
Jaba the Hut was the big slimy slug like creature in one of the
Star Wars episodes. And since a natural unit of mass in the inch
lb second system is much larger than in the ft lb second system,
what better name?
I used the term for several weeks before anyone actually asked me
what it was. Some SI purist twit that I worked with finally asked
me, when he found out he went whining to our boss. Our boss told
him get lost because his SI results were frequently wrong and my
made up unit of mass results were always correct.
My observation has been that the SI purists for the most part are
uncreative people who don't have much to contribute so turn to
criticism of other systems. Why else would a person pride
themself on not understanding another person?
Paul Skoczylas
Jim Carr wrote:
>
> It may have been taught that way, but the pound is a unit of
> mass by law in the United States. That is why your book found
> it necessary to add the "force" adjective to it, a distinction
> dropped from physics textbooks like Halliday and Resnick (1960).
It is interesting to note that my 1988 copy of Halliday and Resnick
barely even acknowledges the existence of unit systems other than SI,
and certainly never _uses_ them.
Clearly, there are several variations on the system known as "English".
One can use pounds for force and mass, one can use poundals for force
and pounds for mass, or one can use slugs for mass and pounds for
force. These have all been used in the past, and there are probably
more. That doesn't make any of them incorrect or invalid. They are all
valid when used properly. The "English" system is confused further by
the fact that many of the units have different definitions depending on
how or where they are used.
When and where these systems were popular, and who teaches them, is
absolutely irrelevant. The fact that US law says that the pound is a
unit of mass for commerce is irrelevant to most of the world, and
irrelevant to scientists and enginners within the US, who will either
use SI, or use pounds for force and/or mass, as appropriate. The
"pound" in the omnipresent unit "psi" is virtually always a force, after
all. It is especially interesting that the legal US unit of mass pound
is actually _defined_ by US law using the kilogram!
-Paul
Mark Baker
In article <34281F...@aspentech.com.->,
Tak To <tak...@aspentech.com.-> writes:
>> The English unit of mass is the "slug", [...]
>
> Note that there are other systems of terms. In UK, for example,
> the unit of mass is "pound" and the unit of force is "poundal"
> or "pound weight". (I am referring to what is taught in physics
> classes, not the general everyday usage.)
I think it _very_ unlikely that you'll find a physics class in the UK that
teaches about any of these.
To ordinary people here, mass and weight are the same thing and are measured
in pounds or kilograms; the only people who care about the distinction never
use anything other than SI.
Jim Carr
"goldbach" <gold...@idcnet.com> writes:
>
>My 1960 edition of H&R, "Physics for Students of Science and
>Engineering" uses slug. No foot note about it.
I have the 1966 revision, and the commentary I alluded to
is on page 91 (second page of section 5-6, right after the
second displayed equation F[lb] = m[slugs] x a [ft/sec^2])
in slightly smaller print than the rest. I have always
assumed this is a slightly longer version of what was there
in the first edition, set that way so the rest of the book
did not have to be reset in type. "In this book forces will
only be measured in pounds ...." starts the next paragraph.
Can you verify that this is a change?
Gene Nygaard
In article <01bcc927$9ab6f3c0$0400a8c0@BAHN>,
"William L. Bahn" <ba...@pcisys.net> wrote:
>
> >
> But be aware that other people can fish out references, including the CRC
> Handbook of Chemistry and Physics, that define pound as a unit of mass
> (given in terms of kilograms). They generally give several definitions for
> pound including what is more explicitly called pound-mass, pound-force. I
> am not fond of this duality at all - but it exists whether I like it or
> not. I think it is simply a recognition of the fact that so much
> engineering data is given in terms of pounds when mass is what is being
> referred to. Consider all of the thermodynamic data that is given in terms
> of BTU/(lb*degF).
Better yet, consider the American "rocket scientists" who measure
specific impulse in seconds. Those look to be perfectly good units in
any of the English systems or in SI or in any of the older cgs or mks
systems. But they are not. They are actually thrust in pounds force
divided by fuel consumption in pounds mass per second, using the pounds
mass to cancel out the pounds force.
>The main reason for this is that a lot of engineering
> data was developed prior to the distinction between mass and gravitational
> force being pushed by the scientific community. So we have the MKS and SI
> systems that supposedly cleanly divorce these two concepts - except how
> many pressure gauges are calibrated in kg/m^s and other improper uses of kg
> as a force instead of a mass? I don't think there will ever be a clean
> separation of these two concepts as long as the distinction between them in
> the common experience of most people involves only a "units conversion".
I think that is changing. It is kilopascals that is embossed into the
side of most automobile tires to indicate maximum pressure, and tire
pressure gauges in kilopascals are readily available in the United
States. I also know that the oil pressure gauges on things such as
Belarus tractors have changed in recent years from kg/cm^2 to kPa, and
the old kg/cm^2 were much more established in the former Soviet Union
than they ever were in the United States.
Gene Nygaard
http://ourworld.compuserve.com/homepages/Gene_Nygaard/weight.htm
Terry Moore
In article <3429686F...@prism.gatech.edu>, Andy Bell
<gt5...@prism.gatech.edu> wrote:
> This is an engineering newsgroup, not a business/legal/whatever
That's strange, I'm reading it in sci.math :-)
> newsgroup.In engineering, the slug and the kilogram are the units for
> mass. I am not
> a historian about these issues, however, I do know my above statement
> to be true today.
Inconsitency alert! See below.
> And what does it matter physics or engineering anyway? Disciplines
> do not change or manufacture units of measurement to suit their needs.
You just hinted that systems may change, now you say they don't.
When I learned mechanics around 1960-1967, as well as two systems
of metric units, cgs and mks, we were taught that mass is measured
in pounds and force in pounds weight, the latter being the force of
the Earth's gravity on a pound mass at a specific place on Earth.
Later we were taught that a more modern unit of force was the
poundal, being the force needed to accelerate a pound mass by
1 ft per sec^2. We routinely used both units and converted
between them. A few years ago I heard about the slug (on this
newsgroup, I think). So it is clear that systems of measurement
have changed at least twice, contradicting what you said before.
> A system of measurement is the same across the board, regardless if you
> are an engineer,doctor, or ditch digger.
Then why are the Troy, Apothecaries' and the Avoir dupois weights all
different?
I bet you've never even heard of these. Troy weight is used by
jewellers, and there are 12 ounces in a pound, Apothecaries' weight
is used by Pharmacists, and Avoir dupois is used by everyone else so
long as they are stupid enough not to have changed to SI units :-)
> I do not wish to debate the origins of the slug, I have no need to know
> that.
> However, I do have an issue with your statement that the slug IS NOT
> a unit for mass. I am having a difficult time understanding this whole
> discussion. Simple: Force is pound/newton....Mass is slug/kilogram.
> Has been taught for years, and will be taught for time to come.
See Synge & Griffiths, Principles of Mechanics, 3rd ed,
McGraw-Hill, 1959. This describes the pound/poundal
system as well as cgs. Funnily enough they don't describe
mks which I thought was already used then, but I may be
wrong. I don't think the SI version of rationalised mks
came in until 1963. (Oops, that's a few more changes of
units, cgs, mks, rationalised mks, SI. Do you still believe
they don't change?).
Clearly the slug is a unit of mass and nothing else. What you have
difficulty with is the following. In one system the pound is a
unit of mass (actually, in at least three different systems), whereas,
in another, the pound is a unit of force. The latter is an incredibly
stupid idea, IMO, because it gives a different meaning to a term that
is well established in the supermarket to mean something else.
--
Terry Moore, Statistics Department, Massey University, New Zealand.
Theorems! I need theorems. Give me the theorems and I shall find the
proofs easily enough. Bernard Riemann
William L. Bahn
Jim Carr <j...@ibms48.scri.fsu.edu> wrote in article
<60c0nd$h6h$1...@news.fsu.edu>...
> Jim Carr wrote:
> |
> | >My point: So what if 100 years ago (or 30 for that matter) people
> | >did or did not use slug for the unit for mass. We do now.
> |
> | No we do not. We use the kilogram. Legally we use the pound.
> | We teach the slug in physics because (my hypothesis) the glb
> | and the poundal blew away too many future physicists and both
> | were fading from favor with the adoption of SI.
>
> Andy Bell <gt5...@prism.gatech.edu> writes:
> >
> >This is an engineering newsgroup, not a business/legal/whatever
> >newsgroup.
>
> This discussion is cross-posted between sci.physics, alt.usage.english,
> sci.engr, sci.math, sci.misc, and sci.lang because the discussion
> transcends specific newsgroups. It is also not specific to the US.
>
> It was interesting to hear the comment by a British engineer and
> your observations about current US textbooks; I am also interested
> to know if slugs are used in engineering practice in the US.
>
I don't think any absolute or blanket statement can really be made. There
is an awful lot of historical baggage associated with engineering data. I
have not personally seen the slug used in any actual engineering setting.
But I very regularly see the pound used as both a unit of mass and a unit
of force. As a result I also see calculations where the acceleration due to
gravity is either forgotten or used where it shouldn't be. What I have
always (so far) found to be true is that if you do a dimensional analysis
on the data or the equation you can determine if the pound is being used
for mass or for force (usually it's quite obvious) and then you can assume
that the data is valid for the definition that a 1 lbm object has a
gravitational weight of 1lbf. As this point I almost always jump into the
metric system (at least partially - so I often have intermediate units of
kg*ft/sec) and come back to the English system later if I need to.
> >In engineering, the slug and the kilogram are the units for mass.
>
> However, by law, the pound is a unit of mass in the United States
> when used in commerce. I know that the US DOT has shifted to the
> use of metric for construction, but it would be interesting to
> know how specs are written in other areas. Do slugs or poundals
> ever appear in specs?
>
Could you please supply a reference showing that the law makes the pound a
unit of mass? I have several sources that define pound both as a unit of
mass and as a unit of force (including the CRC) and find pound-mass,
pound-force and poundals in many of them as well. I realize that none of
these have the weight of law, but we should also keep in mind that what the
legal definition of something is is sometimes not consistent with the most
technically correct and precise definition. Ideally this would not be the
case, but in reality it sometimes is. BTW: My CRC does not have an entry in
its definitions section for "slug" though it does have conversion factors
for it.
<snip>
> >
> >And what does it matter physics or engineering anyway? Disciplines
> >do not change or manufacture units of measurement to suit their needs.
>
> To the contrary, the slug is an example of just that:
I agree. In fact most units were manufactured precisely for that reason. To
the best of my knowledge, as long as we have a unit for length, mass, time
and electric charge (or current) we have a complete set of units.
Everything else is a manufactured unit defined to suit some field's needs.
Even the remaining SI "base" units (with the possible exception of the
mole) can be written in terms of the other four.
> >However, I do have an issue with your statement that the slug IS NOT
> >a unit for mass. I am having a difficult time understanding this whole
> >discussion. Simple: Force is pound/newton....Mass is slug/kilogram.
> >Has been taught for years, and will be taught for time to come.
>
> It may have been taught that way, but the pound is a unit of
> mass by law in the United States. That is why your book found
> it necessary to add the "force" adjective to it, a distinction
> dropped from physics textbooks like Halliday and Resnick (1960).
I'm not sure you can extrapolate the deeper implications of the use of
pound-force in a text book that far. I have a couple of 1980's era physics
texts that use pound-mass and pound-force explicitly in the initial part of
the book where they anticipate students really struggling with the
distinctions. Likewise, I think the use of slugs in introductory physics
texts is widespread because it creates a one-to-one relationship between
the units of force, mass and the acceleration due to gravity between the
two systems.
If you use the pound as a unit of mass (equal to 0.45359237kg) then you
have a problem with the unit of force unless you abandon the pound-force
and go to the poundal. If you don't, then you can't use F=ma in the English
system since 1 pound (pound-force) is the force due to a standard gravity
on a 1 pound (pound-mass) object. So a=1 pound/pound (or 1 lbf/lbm). This
is correct, but it is not consistent (on the surface) with a=32.2ft/s^2
since in the MKS system you have a=9.81m/s^2=9.81N/kg. It would be nice if
the same were true in the English system, namely that
a=32.2(force_unit)/(mass_unit). This is possible if you use the pound as a
unit of mass and the poundal as a unit of force or if you use the pound as
a unit of force and the slug as a unit of mass.
While using pounds and slugs probably does make it quite a bit easier for
new students to grasp the concepts of force and mass and SI and English
units, it probably also does a big disservice because they walk out of that
class believing the word of God has been handed down to them. In reality,
they will have to deal with the fact that, if they are going to use the
English system much then they will have to give up a stringent and blind
definition of the pound one way or the other - they will have to learn to
determine the intended meaning by the context in which it is use. Even the
common usage of the kilogram will sometimes force them to recognize that
the intended use is a kg-force, like when they are given a pressure reading
in kg/m^2.
William L. Bahn
NOTE: sci.lang removed per request.
Gene Nygaard <gnyg...@crosby.ndak.net> wrote in article
<8751623...@dejanews.com>...
> In article <342984...@cfer.ualberta.ca>,
> Paul Skoczylas <P.Sko...@cfer.ualberta.ca> wrote:
> >
> >
> > However, we use the two-pan balance to INDIRECTLY MEASURE mass, but
this
> > is done by COMPARING forces.
> >
> > Clear as mud?
> >
>
> Exactly. It's what we can MEASURE that matters, not the mechanism behind
> it. No balance (in the classic meaning, not including spring scales and
> the sort), whether a double pan balance or a steelyard or a beam balance)
> can MEASURE those forces at all, without an independent measurement of
> acceleration to go with the measurement of mass the balance gives you.
Likewise, no balance can MEASURE the mass of the objects on it at all. It
requires an independent measurement of the mass of your reference object.
The balance doesn't actually MEASURE anything, since a measurement implies
getting a value for the magnitude of something. This is what's referred to
as a null measurement - directly and by itself it tells you only that the
relevant inputs are equivalent. In the case of a pan balance, it tells you
that the moments about the pivot point due to the system of masses is zero.
From that it is just as accurate to say that the weights (the force on each
object due to gravity) of the two objects are equal as it is to say that
the masses of the two objects are equal. In fact, fundamentally the balance
is directly indicating that the weights are equal and from that we are
concluding that the masses are equal.
The same is true of a beam balance. It is only indicating that the moments
due to the system of objects are equal and opposite. After taking the
moments into account, we then come up with a ratio between the force acting
on the unknown object to the net force acting on the counterweights. From
this we are concluding that the ratio of the mass of the unknown object to
the mass of the counterweights is the same. We are relying on apriori
knowledge of the ratio between the mass of the counterweights and the mass
of some reference object (an independent measurement) to determine the
magnitude of the mass.
In both cases, the fact that we can determine the magnitude of the mass and
not, in general, the magnitude of the weight does not mean that we are
measuring the mass and not the weight. We are using a system that responds
to the weights of the objects and are relying on apriori knowledge about
the internal workings and behavior of our system to extract the
"measurement" of the mass.
goldbach
My 1960 edition of H&R, "Physics for Students of Science and
Engineering" uses slug. No foot note about it.
quote by A.M. Worthington, Dynamics of Rotation (4th ed 1902),
p. vii. He tried to introduce the use of the term "slug" but it
did not seem to take until at least into the 30's.
--
Larry R. Shultis <gold...@idcnet.com>
"He who reifies mathematics, lives in a
fantasy world and he who reifies
metaphors will see spookiness in the universe" (lrs)
"Envy will hasten to his dark corner whence he will
summon his even more hideous cousin, malicious glee"
Kierkegaard
"Advocates attempt to prove their models right, but
scientists prove those models wrong".
John Nurick
On 24 Sep 1997 21:29:17 GMT, j...@ibms48.scri.fsu.edu (Jim Carr) wrote:
...
> My earlier comments drawn from the OED:
> ----- start insert -----
...
> 1973 {\it Nature} 30 July 184/3 The statement that the unit
> of mass in the British system is the slug is several years
> out of date.
...
> ----- end insert ----
> I should add that later searches have not located a US text
> using slug before 1960. My guess is that it migrated to the
> US from Britain during WW II and worked its way into textbooks
> when they were rewritten in the late 50s or 60s.
> We have also seen an article from a British engineer saying
> they used pounds/poundals rather than slug/pound, consistent
> with some of the remarks quoted above.
I specialised in maths & physics at school in England in the 1960s,
then did a little engineering at university. We worked mainly in MKS
and pound/poundal, but were also taught about CGS and slug/pound as
alternatives to be used when convenient (and of course so we didn't
get confused if we found them).
There was never any suggestion that the slug was a legally-defined
unit like the kg or pound (pound mass), just that one could sometimes
simplify the calculations by using it.
John
I dislocated my e-mail address, and the doctor says it will be
six months before I can see a specialist.
John Nurick
On 24 Sep 97 11:57:53 +0000, "Mark Barton"
<mba...@icrr.no.u-tokyo.spam.ac.jp> wrote:
>By
>Einstein's equivalence principle, the only possible way of measuring mass
>is to look at the force required to cause an object to move non-inertially.
Surely you can (in principle) just count the atoms or particles.
William L. Bahn
Per the request of someone in this group, I have removed sci.lang from my
posts on this topic and will try to remember to do so in future replies.
If someone from sci.lang IS following this thread, please either start
following it in one of the other groups or post a reply indicating that it
is alive in sci.lang.
jmfb...@ma.ultranet.com
More data on Mr. Carr's question on the use of slugs in the literature:
In my Schaum's College Physics, 7th edition, 1961, Frederick J. Bueche
says in the preface: "...In this substantially revised seventh edition,
major changes have been made to keep pace with the most recent concepts,
methods, and terminology. Although the English gravitational and cgs
systems are still included, so as to provide familiarity with them, the
text now uses the SI as its fundamental units system."
Re: _Modern University Physics_ by Richards, Sears, Wehr, and Zemansky,
1960. In their preface, they say, "This text is both new and old....This
text is old in that it is almost entirely taken from two other texts:
_University Physics_ by Sears and Zemansky and _Physcis of the Atom_ by
Wehr and Richards. Some parts have been rewritten, particularly the
discussion of work, energy, and momentum. Considerable material in the
source books has been omitted. This shortening has been accomplished for
the following reasons. First, there was some material common to both
books. Second, most students who use this book will take additional
courses which will include topics we have deleted. Third, some topics are
applications which will be covered in the laboratory. Fourth, some
traditional topics, such as color and musical scales, are of secondary
importance to physics, inasmuch as they involve psychology as much as
physics.....Both the English and metric systems of units have been
introduced and used. The metric mks system is used throughout, and the
English engineering system is used for topics where engineers use them..."
Then on page 85, under the topic "Systems of units" the text reads,
"...It is often convenient to introduce a single term for the combination
of units in which a physical quantity is expressed, and the unit above,
1 lb/(ft/sec^2), is called one slug. (This term arose from the concept of
mass as inertia or sluggishness.) Thus
Sigma F (lb) = m (slugs) x a (ft/sec^2)
[where F, m, and a are italisized and x is the multiplication indicator]
..."
/BAH
Gene Nygaard
In article <01bcc979$cdcf4530$0400a8c0@BAHN>,<good discussion of units used in engineering snipped--thanks>
>
> > >In engineering, the slug and the kilogram are the units for mass.
> >
> > However, by law, the pound is a unit of mass in the United States
> > when used in commerce. I know that the US DOT has shifted to the
> > use of metric for construction, but it would be interesting to
> > know how specs are written in other areas. Do slugs or poundals
> > ever appear in specs?
> >
>
> Could you please supply a reference showing that the law makes the pound a
> unit of mass? I have several sources that define pound both as a unit of
> mass and as a unit of force (including the CRC) and find pound-mass,
> pound-force and poundals in many of them as well. I realize that none of
> these have the weight of law, but we should also keep in mind that what the
> legal definition of something is is sometimes not consistent with the most
> technically correct and precise definition. Ideally this would not be the
> case, but in reality it sometimes is. BTW: My CRC does not have an entry in
> its definitions section for "slug" though it does have conversion factors
> for it.
>
I have dealt with this in part on my web page, in the "What law?" section
where I discuss a 1941 textbook which claims the opposite of what Jim
Carr is claiming.
I don't think there is any statute in the United States which defines
pound in either way. But "law" is much broader than statutes; it
includes treaties and regulations.
Much of our aceptance of metric units, and the definitions of the
International System of Units, etc. is based in part on the Meter
Convention, or Treaty of the Meter, a treaty originally signed by the
United States and 16 other countries in 1875, and by several others
since.
There may be something in the Code of Federal Regulations defining
pounds, but I don't have a specific citation. The original legal
definition of the pound avoirdupois as a fraction of a kilogram
(0.4535924277 kg at that time) was accomplished in April 1893 simply by
an order of T.C. Mendenhall, Superintendent of Weights and Measures.
Since a pound avoirdupois had also been defined as 7000 grains troy for a
long time, this may have conflicted somewhat with the only time a pound
had ever been defined in the statutes of the U.S.--in or about 1828 the
Troy Pound of the Mint had been defined as a specific artifact. In
either 1911 or 1913, Congress acted on this by changing this specific
definition of the troy pound to be the pound as maintained by whatever
our national standards laboratory was called at the time (later it was
the National Bureau of Standards, now replaced by NIST, the National
Institute of Standards and Technology). I believe this and other
statutes delegate the power to set the legal definition to an executive
branch agency, and form the legal basis for whatever regulation or other
agency action constitutes the primary definition of the pound today.
In any case, this is not just a United States phenomenon. It is a
worldwide legal definition. In 1959, the directors of the national
standards laboratories of all the major countries then using English
units to any great extent (the U.S., Canada, the U.K., Australia, New
Zealand, South Africa) got together to agree on common definition of the
pound (as well as units of length). They were the ones who defined it as
a unit of mass equal to 0.45359237 kg exactly, slightly less than the
1893-1959 U.S. avoirdupois pound. I think I've seen it mentioned that
there is a specific statute on this in the United Kingdom, but I don't
have a citation for it. I also don't know the specific legal basis for
accepting this definition in any other country, but it was accepted by
all of them. It is the legal definition for science, as well as for
commerce. I am pretty sure that this international agreement never
reached the level of a treaty.
> <snip>
<more good stuff snipped>
>
> If you use the pound as a unit of mass (equal to 0.45359237kg) then you
> have a problem with the unit of force unless you abandon the pound-force
> and go to the poundal. If you don't, then you can't use F=ma in the English
> system since 1 pound (pound-force) is the force due to a standard gravity
> on a 1 pound (pound-mass) object. So a=1 pound/pound (or 1 lbf/lbm). This
> is correct, but it is not consistent (on the surface) with a=32.2ft/s^2
> since in the MKS system you have a=9.81m/s^2=9.81N/kg. It would be nice if
> the same were true in the English system, namely that
> a=32.2(force_unit)/(mass_unit). This is possible if you use the pound as a
> unit of mass and the poundal as a unit of force or if you use the pound as
> a unit of force and the slug as a unit of mass.
The same is true in any English system, even if the English system you
are using is the one which uses the pound force as a "base" unit and the
slug as a "derived" unit. Even though the pound mass is not a part of
this system per se, as the system is actually used it is part of the
metasystem. The pound mass (0.45359237 kg since 1959) is used to define
the pound force, by assuming an exact value for the acceleration of
gravity. The value established by a 1901 resolution of the General
Conference on Weights and Measures is the one used for this--the standard
acceleration of gravity is 9.80665 meters per second squared, the same
value used with metric units. Thus a pound force is exactly equal to
4.4482216152605 newtons.
>
> While using pounds and slugs probably does make it quite a bit easier for
> new students to grasp the concepts of force and mass and SI and English
> units, it probably also does a big disservice because they walk out of that
> class believing the word of God has been handed down to them. In reality,
> they will have to deal with the fact that, if they are going to use the
> English system much then they will have to give up a stringent and blind
> definition of the pound one way or the other - they will have to learn to
> determine the intended meaning by the context in which it is use. Even the
> common usage of the kilogram will sometimes force them to recognize that
> the intended use is a kg-force, like when they are given a pressure reading
> in kg/m^2.
>
Amen.
Gene Nygaard
Other newsgroups reinstated
In article <60cqol$kke$2...@news.sas.ab.ca>,
jat...@freenet.edmonton.ab.ca () wrote:
>
> G.M.Sigut (si...@awu.id.ethz.ch) wrote:
> : gnyg...@crosby.ndak.net (Gene Nygaard) writes:
>
> : >
> : > What is weight? Is it different from mass?
> : >
>
> : I am truly amazed at the amount of discussion this theme brought forth.
> : I believed it was thrashed through and understood ages ago. Just to confuse
> : everybody concerned: kgf (kilogram force) is (was?) called kp (kilopond)
> : in most(?) non-english speaking countries.(kiloPOND, NOT kilopound!)
>
> Not to mention that things "weigh" so-and-so many kilos. For years, I
> taught a course in dynamics and you wouldn't believe how many times I had
> to correct people on that point.
>
There's nothing to "correct." That's an ancient and perfectly valid use
of "weigh." Check out my web page. Accept the fact that "weigh" and
"weight" are ambiguous, and that attempts at limitation of their use in
physics and engineering to a specific definition are jargon.
OTOH, people who use "kilos" for kilograms should be corrected.
Gene Nygaard
In article <01bcc97d$80971050$0400a8c0@BAHN>,
That's true, of course, unless one side of the balance happens to be the
International Prototype Kilogram in today's world, or any of the official
standards for a shekel or a beqa or a libra or a pound or a rotl or any
of the various other units that have been established as equal to the
mass of a particular stone or piece of metal by various kings and queens
and other government officials throughout the ages, Hammurabi and Solomon
and all the Caesars and Ptolemys and Cleopatras and Henrys and Johns and
so on.
But the thing is, such standards have been set ever since prehistoric
times, and comparisons of masses that untimately relate back to that
standard are and always have been the primary purpose of balances.
> In both cases, the fact that we can determine the magnitude of the mass and
> not, in general, the magnitude of the weight does not mean that we are
> measuring the mass and not the weight. We are using a system that responds
> to the weights of the objects and are relying on apriori knowledge about
> the internal workings and behavior of our system to extract the
> "measurement" of the mass.
Isn't determining magnitudes what "measuring" is all about? You've lost
me here. You yourself said in your first paragraph above "a measurement
implies getting a value for the magnitude of something." Like I said,
you can measure weight with a balance, as long as that "weight" means the
same thing as mass. That's what you can determine the magnitude of with a
balance, not force.
Geoff Waters
Any chance of sparing sci.lang the rest of this debate?
John Chalmers
Slugs and poundals were certainly in the textbooks I saw in college in the
1960's. The strangest unit I've come across was one used in the
fermentation industry -- kilograms product per 10,000 gallons of "broth."
But if fermenters are built according to the traditional US system
(15,000 ---> 100,0000 gallons) and pharmaceuticals are sold on the world
market by the kg, it makes sense.
--John
Paul Skoczylas
Dick Brewster wrote:
> My observation has been that the SI purists for the most part are
> uncreative people who don't have much to contribute so turn to
> criticism of other systems. Why else would a person pride
> themself on not understanding another person?
I liked this post, because it emphasizes the fact that units are
abritrary.
Anyway, I consider myself to be somewhat of an SI purist, in that I get
really upset when people use SI incorrectly. Examples: kilogram-force,
spelling newton with a capital letter (or using n of the abbreviation),
using a capital K for kilo-, etc.)
However, I have no problem with using the imperial system, if it is used
correctly. I find it nice for some types of calculations (particulary
stress/strain), but cumbersome for others (eg. fluid flow). The worst
types of calculations for imperial are those that use mass, because I
find it very confusing to deal with the units and conversion factors.
This is probably why so many engineering relations out there consist of
"magic equations" where the inputs/outputs are in whatever convenient
units, and the constants and conversions are taken care of in the
"magic" constant at the front of the equation. These equations are
particularly common in the oil business, where I work. They make the
calculations nice and easy, but I personally don't like them, because I
like to keep track of the units as I go through a calculation. For this
reason, I like using SI for many things: I can see a base equation which
I can carry units through and have them cancel out appropriately.
I use both SI and imperial daily, and I like both of them for certain
things. If I had to choose absolutely between one or the other, SI
would win hands down, though.
My $0.02
-Paul
Stan Bischof
: : And what does it matter physics or engineering anyway? Disciplines
: : do not change or manufacture units of measurement to suit their needs.
: Here I'd have to disagree. Disciplines do indeed develop their own units of measurement to suit their needs. Ever heard of a "barne" outside of particle physics, or a "point" outside of graphic arts, or a "board-foot" outside of the lumber industry?? There's many many examples of units which are discipline specific.
Forgot my favorite example: The "alternate energy" crowd uses the WATT
interchangeably as an energy or power unit. They seem to understand
the usage from context.
Stan Bischof
st...@sr.hp.com
Chad English
Mark Barton wrote:
[snip]
> This is a matter of semantics, but I don't think equating "compare" and
> "measure" is a good idea, or fair to Mr Nygaard. First, note that using a
> pan balance by placing it in the gravitational field of a large mass gives
> a value for the "passive gravitational mass" of the object, whether you
> choose to call it a measurement or not. Similarly using it on a platform
> accelerated by rockets gives a value for the "inertial mass". Although
> these are conceptually different in Newtonian physics, according to our
> best theory, General Relativity, they are fundamentally the same thing. By
> Einstein's equivalence principle, the only possible way of measuring mass
> is to look at the force required to cause an object to move non-inertially.
> period does not escape this. If you define "measure" in such a way as to
> disqualify all procedures which involve forces as an intermediate you
> produce the nonsensical result that a measurement of mass is impossible in
> principle - nothing but nothing is a measurement of mass. The important
> feature of the pan balance (as opposed to a simple spring balance) is that
> it is automatically self-calibrating, so that the numbers that come out are
> automatically values of mass.
I actually thought about the semantics too, but after further thought I
think "compare" and "measure" are in essence the same thing, with a
different purpose. Generally I think the term "measure" is used when
refering to getting a number (using a specific unit/scale) out of the
process, whereas "compare" is not necessarily using a standard (e.g.,
two apples on a balance).
Even when "measuring" the length of a line, you are comparing it's
length to that of some standard length. While the two terms are not
identical, I don't see that the differences are relevant to the current
discussion.
Now, I want to clear up the suggestion (that I inferred from the above)
that I said you can't uses forces to measure mass. I never said (or
meant to say) that. In fact I can't think of a single way to measure
mass without using deduction (or equations) rather than direct
comparisons.
What I did say was that the statement "Balances measure mass, not force"
(which has been said or implied many times, including the web page in
question) is not correct. Balances compare moments. Using the moment
arms you can deduce a comparison of forces. Assuming (or knowing) that
all external forces (or differences in external forces) are negligble,
and the objects are in the same gravitional field and under the same
acceleration, one can deduce a comparison between masses.
If it just said you can use balances to measure (or compare) mass, then
there is no problem (though technically one could argue against it as is
appearing here lately). But implying that they don't measure forces, or
that the comparison of masses is more fundamental than the comparison of
forces on a balance, is where the problem arises.
I want to keep out of the technical arguments about relativity and so
forth since those don't affect the practical use.
--
Chad English
ceng...@mae.carleton.ca
http://www.mae.carleton.ca/~cenglish
Mark Barton
On Thu, 25 Sep 1997 11:13, John Nurick
<mailto:j.nu...@ialday.ipexpay.omcay> wrote:
>On 24 Sep 97 11:57:53 +0000, "Mark Barton"
><mba...@icrr.no.u-tokyo.spam.ac.jp> wrote:
>
>>Einstein's equivalence principle, the only possible way of measuring mass
>>is to look at the force required to cause an object to move
non-inertially.
>
That's a good criticism, but I think it is probably fair to rebut it by
pointing out that eventually you have to fall back upon a prior measurement
of the mass of the atoms or you don't actually have a "measurement" of the
mass of the object in the sense I was recommending - you only have an atom
count. Calibration is a major and essential component of measurement, and,
for me, the whole issue in the balance debate.
Cheers,
Mark B.
----------------
Please remove the spam filter from my address before replying.
Chad English
Gene Nygaard wrote:
> You must have a strange definition of "to measure."
At least to me, "measure" means a comparison with a standard. Measuring
the length of a line with a ruler means you are (ultimately) comparing
the length of the line with whatever the standard is for those units.
In the case of the meter, I believe the standard now deals with the
wavelength of a cesium atom or something like that. In fact, using a
balance with a known standard is *exactly* how they used to measure
things way back when, and still do in some places (we did in high school
physics and chemistry).
I don't see how this is "strange".
Tak To
Mark Baker wrote:
>
> TT> Note that there are other systems of terms. In UK, for example,
> TT> the unit of mass is "pound" and the unit of force is "poundal"
> TT> or "pound weight". (I am referring to what is taught in physics
> TT> classes, not the general everyday usage.)
>
> I think it _very_ unlikely that you'll find a physics class in the
> UK that teaches about any of these.
>
> To ordinary people here, mass and weight are the same thing and
> are measured in pounds or kilograms; the only people who care about
> the distinction never use anything other than SI.
Thanks for the information.
I was constrasting "pound/poundal" with "slug/pound". Of course
if pound is not used in the school then the point it moot. A UK
physics text book from the pre-SI days (say, the 60s) should
make my point.
Tak
----------------------------------------------------------------------
Tak To (617) 949-1377
Aspen Technology, Inc Fax: (617) 949-1030
10 Canal Park, Cambridge, Ma 02141. tak...@aspentech.com.-
----------------------------------------------------------------------
Disclaimer: I do no speak for Aspen Technology. [taode takto ~{LU5B~}]
Chad English
Chris Dearlove wrote:
> Incidentally whilst I might agree in theory with people who
> say balances compare forces, the point is that this is equivalent
> to comparing masses (assuming virtually constant local
> gravitation) and so the important question is what you put on
> them. This is usually unknown object on one side, standard
> calibrated known masses on the other. When you've finished
> you know the unknown object's mass, but not the force it
> exerts.
> --
> Christopher Dearlove Personal comments only.
Agreed. I think the original purpose of saying it has been lost
though. I never meant to get technical or nitpicky. The web page
implied that balances measure mass, not force (as a spring scale does).
It's the "not force" that I had a problem with, since they do, in fact,
measure force (actually moments).
For all practical purposes, balances *can* be used measure mass. But
they *do* measure force.
As for all this talk about whether the term "weight" means force of
gravity or mass, I don't know. I always thought weight meant force and
a weight given in kg was just a misuse of the term. However, I'm
willing to accept that it is supposed to mean mass, if the history
lesson is indeed true, and that a weight in pounds means it's mass.
However, what word do we use to describe the force of gravity on an
object then?
(I really think a lot of this confusion could have been avoided if they
never used the same term for two things -- pound-mass and pound-force.
And what's this I hear about people using the term "kg-f"? Gee, that
won't confuse anybody now.)
Gene Nygaard
Paul Skoczylas <P.Sko...@cfer.ualberta.ca> wrote:
>Jim Carr wrote:
>>
>> It may have been taught that way, but the pound is a unit of
>> mass by law in the United States. That is why your book found
>> it necessary to add the "force" adjective to it, a distinction
>> dropped from physics textbooks like Halliday and Resnick (1960).
>It is interesting to note that my 1988 copy of Halliday and Resnick
>barely even acknowledges the existence of unit systems other than SI,
>and certainly never _uses_ them.
>Clearly, there are several variations on the system known as "English".
>One can use pounds for force and mass, one can use poundals for force
>and pounds for mass, or one can use slugs for mass and pounds for
>force. These have all been used in the past, and there are probably
>more. That doesn't make any of them incorrect or invalid. They are all
>valid when used properly. The "English" system is confused further by
>the fact that many of the units have different definitions depending on
>how or where they are used.
>When and where these systems were popular, and who teaches them, is
>absolutely irrelevant.
I find that very relevant and interesting. The way these things have
been taught plays a major role in the confusion evident in the
examples on my web page.
> The fact that US law says that the pound is a
>unit of mass for commerce is irrelevant to most of the world, and
>irrelevant to scientists and enginners within the US, who will either
>use SI, or use pounds for force and/or mass, as appropriate.
Actually, this is a worldwide definition, for science as well as for
commerce. I've dealt with this somewhat on my web page under the
"What law" section and in more detail in a reply I've just posted to
William L. Bahn.
>The
>"pound" in the omnipresent unit "psi" is virtually always a force, after
>all. It is especially interesting that the legal US unit of mass pound
>is actually _defined_ by US law using the kilogram!
This isn't anything new. The pounds have been fractions of a kilogram
since 1893 in the United States.
This also isn't anything limited to the United States; it's worldwide.
In 1959, Canada and all the other major countries using English units
to any extent (U.S., U.K., Australia, New Zealand, South Africa) got
together to agree on a common definition of the pound (as well as
length units, but not gallons and bushels). They defined the
avoirdupois pound as 0.45359237 kilogram.
BTW, I think Canada had also redefined their customary units in terms
of the metric system before 1959. Canada didn't have to change its
definition of feet and yards, etc., in 1959 because they already used
that definition. The common 1959 definition of length units was
exactly 2 parts per million shorter than the old U.S. definition, and
approximately 2 parts per million longer than the old U.K.. definition
where it was still based on independent standards until 1959.
The legal basis for this is not by statute in the United States, but
it is nonetheless "by law." The authority to define these units has
been delegated to an executive agency, much as the power to define the
metre or the kilogram has been given to the international
organizations established under the Treaty of the Meter, or Meter
Convention, of 1875.
What is the basis for the legal definition of the pound avoirdupois as
0.45359237 kg in Canada?
Gene Nygaard
http://ourworld.compuserve.com/homepages/Gene_Nygaard/weight.htm
Chad English
Gene Nygaard wrote:
>
> In article <60b4be$1...@bertrand.ccs.carleton.ca>,
> ceng...@mae.carleton.ca (Chad English) wrote:
> >
> > Bill Baldwin (rev...@gte.net) wrote:
> >
> > : Huh? We're clearly not using the same definition of the word "balance"
> > : here. A balance, by definition I would have thought, compares two weights
> > : by BALANCING them against each other. So a one kilogram object will ALWAYS
> > : exactly balance another one kilogram object here, on the moon, on Mars...
> > : anywhere. However, on a non-balance type scale, i.e. a spring scale such as
> > : many bathrooms sport, a one kilogram object will register one kilogram on
> > : earth but considerably less (1/6?) on the moon. That is because it is
> > : exerting less force against the springs while the springs maintain a
> > : constant force in the reverse direction regardless of gravity. Clear as
> > : mud.
> >
> > A balance actually compares the forces on the two ends, regardless of the
> > masses (if there are any). It can usually be assumed that the masses are
> > also equal if other external forces (including buoyancy) can be neglected.
> >
> > --> But the balance cannot MEASURE the magnitude of those forces. If the
> balance is moved to a place with a different acceleration of gravity, the
> force on the side with the unknown masses is greater, but so is the force
> on the side with the known masses. The same mass that will balance a
> known mass on Earth (weighed in a vacuum, or with both the known and
> unknown masses of the same density), will also balance on the Moon. Even
> though the magnitude of the force on Earth is about 6 times greater than
> that on the Moon, a balance cannot measure that difference. To determine
> forces with a balance, you also need an independent measure of the
> acceleration of gravity.
>
> A balance does MEASURE the magnitude of the mass.
>
> Gene Nygaard
Well, I was hoping to avoid getting too technical, but with this
perspective in mind I'll add a couple of things. While I agree somewhat
with the above point about force, a balance also does not "measure" the
magnitude of a mass. How about this (avoiding relativity):
1. A balance compares two moments. Knowing the moment arms, one can
deduce a comparison of forces. If one of the forces is "known" based on
a standard, you can *measure* the other force. (This avoids your point
of moving to the Moon, because I said the *force is known*.) (I've
defined "measure" here as a comparison with a standard unit.)
2. If the objects are under the same graviational field and
acceleration, and external forces (or differences between external
forces) are neglible, a comparison of masses can be deduced. If one of
the masses is "known" based on a standard, you can *measure* the other
mass.
How's that. I hope all of the bases are covered now.
Stan Bischof
Andy Bell (gt5...@prism.gatech.edu) wrote:
: And what does it matter physics or engineering anyway? Disciplines
: do not change or manufacture units of measurement to suit their needs.
Here I'd have to disagree. Disciplines do indeed develop their own units of measurement to suit their needs. Ever heard of a "barne" outside of particle physics, or a "point" outside of graphic arts, or a "board-foot" outside of the lumber industry?? There's many many examples of units which are discipline specific.
Stan
Pat Marengo
Stan Bischof wrote:
[...]
> Here I'd have to disagree. Disciplines do indeed develop their own units of measurement to suit their needs. Ever heard of a "barne" outside of particle physics, or a "point" outside of graphic arts, or a "board-foot" outside of the lumber industry?? There's many many examples of units which are discipline specific.
Ever heard of a carriage return? They are standard in newsgroup
articles. Please use one every 60 to 70 characters. Some of us
don't bother reading lines longer than about 80 characters.
Andy Bell
I humbly retract all comments given by myself on this
topic. This discussion has gone way past my knowledge
(at least I admit it). My experience (I am young) does
not go into the 40's, 50's, 60's, or 70's for that matter.
And I believe my comments showed that.
However, I have learned a lot through this
discussion. Stuff that they don't teach in school (at least
not anymore). My belief that the SI system is less
complicated is made more concrete, that the English
system is defined by whichever book or reference
happens to be in front of you, and if a spec gives you
units including pounds perform dimensional analysis to
verify if this implies mass or weight.
I am in awe that this topic brought such heated and
diverse opinions. This is a testament to how different
one area can be. Before this thread, I would have said
that something as simple as a system of units would be
a subject with a widely known definition. I would not
say that today.
Regards,
Andy Bell
Pat Marengo
Andy Bell wrote:
[...]
> I am in awe that this topic brought such heated and
> diverse opinions. This is a testament to how different
> one area can be. Before this thread, I would have said
> that something as simple as a system of units would be
> a subject with a widely known definition. I would not
> say that today.
Very astute! And keep that in mind when approaching other
subjects you encounter. In one of my specialties, navigation
systems, I was amazed at the confusing complexity of the
massive amounts of text devoted to what seems a simple
concept. After several years, I encountered a guy who had
devoted several years of his life to studying the subject
from first principles. He approached and mastered the
subject in an unconventional manner so that in his mind
nearly all of the conventional discussion was just so much
smoke and mirrors which he never bothered to fully understand.
The guy could work up an analysis in a few days that would
take other guys in the company many weeks. It was a shame
that he didn't write a book on it, but the U.S. government
screwed him over big time on a patent, and he decided not
to share his ideas any more than necessary to do his job.
Pat Marengo
Dick Brewster wrote:
[...]
>
> I used the term for several weeks before anyone actually asked me
> what it was. Some SI purist twit that I worked with finally asked
> me, when he found out he went whining to our boss. Our boss told
> him get lost because his SI results were frequently wrong and my
> made up unit of mass results were always correct.
>
> My observation has been that the SI purists for the most part are
> uncreative people who don't have much to contribute so turn to
> criticism of other systems. Why else would a person pride
> themself on not understanding another person?
I've found that this attitude is very common among those at the
higher end of the technical bell curve. These people are smart
enough to easily handle the confusion of mangled units, quadruple
sign reversals, combinations of left and right handed reference
frames, and etc. so they balk at any attempt to introduce common
sense into things. It makes them relatively more valuable and
strokes their egos, but it is very foolish from the standpoint
of the productivity of the whole enterprise which includes many
more people with less capability. Unfortunately in this case, it
is the more capable people who get to choose how things will be
done. Progress is slowed by the rareness of very smart people
who can introduce and promote systems that work well for the grunt
workers who do the bulk of the work in any enterprise.
This has become an even greater problem in the software industry
where it is so very easy for the smartest people to introduce
complexity that can take months for others to comprehend.
Mark Barton
On Thu, 25 Sep 1997 20:41, Chad English
<mailto:ceng...@mae.carleton.ca.NOSPAM> wrote:
>Mark Barton wrote:
We apparently agree on a lot of things, but we part company here:
>What I did say was that the statement "Balances measure mass, not force"
>(which has been said or implied many times, including the web page in
>question) is not correct. Balances compare moments. Using the moment
>arms you can deduce a comparison of forces. Assuming (or knowing) that
>all external forces (or differences in external forces) are negligble,
>and the objects are in the same gravitional field and under the same
>acceleration, one can deduce a comparison between masses.
It's clear from the way you change randomly between them in the above
passage that for you "compare" and "measure" are exact synomyns. This has
been my criticism from the start, and I'm glad to see that I was not
misinterpreting you. There are three problems with this (i) it makes
"measure" into a redundant word, (ii) it leaves us with an concept ("output
correct values of physical quantity X") with no word, and (iii) it's not
how physicists normally use the word. Therefore I submit that this is a bad
usage, and Gene Nygaard has nothing to be ashamed of for not using it.
>If it just said you can use balances to measure (or compare) mass, then
>there is no problem (though technically one could argue against it as is
>appearing here lately). But implying that they don't measure forces, or
>that the comparison of masses is more fundamental than the comparison of
>forces on a balance, is where the problem arises.
In the sense I'm recommending, it's obvious that (pan) balances don't
measure forces - there are no force values output at any time, nor is the
output proportional to a force (as in a spring balance). They do "compare"
forces, but they don't "measure" them - these words are not and should not
be synonymous. The values that _are_ output, which are mass values,
continue to be correct mass values even under the precise circumstances
which ought to show up the difference between mass and force: a variation
in local gravity.
Christopher R Volpe
Mark Barton wrote:
>
> On Thu, 25 Sep 1997 11:13, John Nurick
> <mailto:j.nu...@ialday.ipexpay.omcay> wrote:
> >On 24 Sep 97 11:57:53 +0000, "Mark Barton"
> ><mba...@icrr.no.u-tokyo.spam.ac.jp> wrote:
> >
> >>By
> >>Einstein's equivalence principle, the only possible way of measuring mass
> >>is to look at the force required to cause an object to move
> non-inertially.
> >
> >Surely you can (in principle) just count the atoms or particles.
>
> That's a good criticism, but I think it is probably fair to rebut it by
> pointing out that eventually you have to fall back upon a prior measurement
> of the mass of the atoms or you don't actually have a "measurement" of the
> mass of the object in the sense I was recommending - you only have an atom
> count. Calibration is a major and essential component of measurement, and,
> for me, the whole issue in the balance debate.
>
Furthermore, the mass of an object is more than just the sum of the
masses of its component particles. Internal energy contributes to the
mass of a composite object as well. A hot brick has more mass than a
cold brick, even if they have the exact same number and type of atoms.
"Mass" is the total energy in the center-of-momentum inertial frame.
--
Chris Volpe Phone: (518) 387-7766
GE Corporate R&D Fax: (518) 387-6981
PO Box 8 Email: "volpecr" AT "crd.ge.com"
Schenectady, NY 12301 Web: http://www.crd.ge.com/~volpecr
Ray Butterworth
On Tue, 23 Sep 1997 11:56:23 GMT,
gnyg...@crosby.ndak.net (Gene Nygaard) wrote:
>What is weight? Is it different from mass?
>Balances measure mass, not force.
Wrong.
Balances compare the weight of an object
with the weight of a standard mass.
The object can then be said to have the same weight as the 1 kg mass,
but that doesn't mean that it has a 1 kg mass itself.
>Weight is equivalent to mass, in this definition, and this
>definition is a perfectly valid one.
Wrong.
Consider a two-arm balance.
On one arm tie an uninflated balloon.
On the other arm tie a balloon inflated with helium.
The inflated balloon has more mass than the other
since it also includes the mass of the helium.
But the inflated ballon has less weight than the other.
The balance tips in a direction indicating that the
inflated balloon has less of what is being measured
than the other balloon.
i.e. it is affected by the difference in weights,
not by the difference in masses.
Consider the Goodyear blimp.
Its weight is aproximately zero
(which is why it doesn't fall down),
but its mass is sustantially greater
(which is why it needs huge engines to move it).
Or consider a brick tied to a number of helium balloons.
On a balance, it might measure "10 grams" (1/3 ounce).
But that means that it has the same weight as a 10 gram mass,
not that it has a mass of 10 grams.
If you don't see the difference between these two objects
that have identical weights,
try kicking the 10 gram weight and then try kicking the brick.
JBarnes262
Now here are some simple questions :-)
Does *weight* vary if you measure a constant mass in a vacume ?
( density of air approx 1.25 Kg / m^3 )
How big a diferance does the phase of the moon make ?
How big a diferance does the latitude make ? ( acounting both for
centripetal acceleration, and equatorial "spread" )
Jonathan.
goldbach
--
Larry R. Shultis <gold...@idcnet.com>
"He who reifies mathematics, lives in a
fantasy world and he who reifies
metaphors will see spookiness in the
universe. (lrs)
"Envy will hasten to his dark corner whence he will
summon his even more hideous cousin, malicious glee"
Kierkegaard
"Advocates attempt to prove their models right, but
scientists prove those models wrong".
Jim Carr <j...@ibms48.scri.fsu.edu> wrote in article
<60cnbt$50$1...@news.fsu.edu>...
> "goldbach" <gold...@idcnet.com> writes:
> >
> >My 1960 edition of H&R, "Physics for Students of Science and
> >Engineering" uses slug. No foot note about it.
>
> I have the 1966 revision, and the commentary I alluded to
> is on page 91 (second page of section 5-6, right after the
> second displayed equation F[lb] = m[slugs] x a [ft/sec^2])
> in slightly smaller print than the rest. I have always
> assumed this is a slightly longer version of what was there
> in the first edition, set that way so the rest of the book
> did not have to be reset in type. "In this book forces will
> only be measured in pounds ...." starts the next paragraph.
The first (1960) edition which I used at
University of Wisconsin Madison in 1960 is the same, though
that stuff is on page 77 with the small print. There must have
been new material added for the 1966 edition. Page 76 has a
picture of bell jars with standard kilogram masses.
>
> Can you verify that this is a change?
>
> --
> James A. Carr <j...@scri.fsu.edu> | Commercial e-mail is _NOT_
> http://www.scri.fsu.edu/~jac/ | desired to this or any address
> Supercomputer Computations Res. Inst. | that resolves to my account
> Florida State, Tallahassee FL 32306 | for any reason at any time.
>
Gene Nygaard
Chad English <ceng...@mae.carleton.ca.NOSPAM> wrote:
>Mark Barton wrote:
>[snip]
>> This is a matter of semantics, but I don't think equating "compare" and
>> "measure" is a good idea, or fair to Mr Nygaard. First, note that using a
>> pan balance by placing it in the gravitational field of a large mass gives
>> a value for the "passive gravitational mass" of the object, whether you
>> choose to call it a measurement or not. Similarly using it on a platform
>> accelerated by rockets gives a value for the "inertial mass". Although
>> these are conceptually different in Newtonian physics, according to our
>> best theory, General Relativity, they are fundamentally the same thing. By
>> Einstein's equivalence principle, the only possible way of measuring mass
>> is to look at the force required to cause an object to move non-inertially.
>> The suggestion of mounting the mass on the end of spring and looking at the
>> period does not escape this. If you define "measure" in such a way as to
>> disqualify all procedures which involve forces as an intermediate you
>> produce the nonsensical result that a measurement of mass is impossible in
>> principle - nothing but nothing is a measurement of mass. The important
>> feature of the pan balance (as opposed to a simple spring balance) is that
>> it is automatically self-calibrating, so that the numbers that come out are
>> automatically values of mass.
>I actually thought about the semantics too, but after further thought I
>think "compare" and "measure" are in essence the same thing, with a
>different purpose. Generally I think the term "measure" is used when
>refering to getting a number (using a specific unit/scale) out of the
>process, whereas "compare" is not necessarily using a standard (e.g.,
>two apples on a balance).
That's the way I look at "measure" also--getting a number is what's
important. See below.
>Even when "measuring" the length of a line, you are comparing it's
>length to that of some standard length. While the two terms are not
>identical, I don't see that the differences are relevant to the current
>discussion.
>Now, I want to clear up the suggestion (that I inferred from the above)
>that I said you can't uses forces to measure mass. I never said (or
>meant to say) that. In fact I can't think of a single way to measure
>mass without using deduction (or equations) rather than direct
>comparisons.
>What I did say was that the statement "Balances measure mass, not force"
>(which has been said or implied many times, including the web page in
>question) is not correct. Balances compare moments. Using the moment
>arms you can deduce a comparison of forces. Assuming (or knowing) that
>all external forces (or differences in external forces) are negligble,
>and the objects are in the same gravitional field and under the same
>acceleration, one can deduce a comparison between masses.
>If it just said you can use balances to measure (or compare) mass, then
>there is no problem (though technically one could argue against it as is
>appearing here lately). But implying that they don't measure forces, or
>that the comparison of masses is more fundamental than the comparison of
>forces on a balance, is where the problem arises.
What happens if you move the balance to a different place? If you
increase the acceleration (of gravity, as balances are normally
used), the force increases, doesn't it? But the force increases
equally on both sides of the balance, so the reading you get from the
balance doesn't change. The balance still tells you the "weights" are
the same, even though the force is more than what it was before.
You don't get a number for the magnitude of the force from a balance.
Therefore, you don't "measure" force with a balance.
Oh, you can measure force with a balance, as long as you have an
independent measure of the acceleration. And as long as a balance is
always used in the same place, you can even get a fairly precise
measurement of that force. Whatever problems some people have with
saying that a balance measures mass, based on the fact that you do
need to know the mass on one side of the balance in terms of the
standard units of measure, carry over into the measurement of force
with a balance, in addition to the problems of having to measure
acceleration independently.
>I want to keep out of the technical arguments about relativity and so
>forth since those don't affect the practical use.
>--
>Chad English
>ceng...@mae.carleton.ca
>http://www.mae.carleton.ca/~cenglish
I agree on that point.
Gene Nygaard
http://ourworld.compuserve.com/homepages/Gene_Nygaard/weight.htm
Gene Nygaard
"William L. Bahn" <ba...@pcisys.net> wrote:
>Note: sci.lang has been removed because no one voiced an objection. One
>person asked that alt.useage.english be removed, but I have received
>e-mails from two people in that group that are following the thread so I'm
>going to leave it in. Please remember that all I can control is the groups
>in MY posts to this thread.
It doesn't look like you got it removed, from the way I found the
message on AltaVista. I can go along with that. I do feel that
alt.usage.english (note spelling) is the most appropriate of the
groups remaining. It does also deal with semantics, but any of the
linguists in alt.lang who have an interest in it probably follow
alt.usage.english anyway. I think the comments I have made about
"weight" apply to some degree to similar words in other languages as
well.
>Gene Nygaard <gnyg...@crosby.ndak.net> wrote in article
>> "William L. Bahn" <ba...@pcisys.net> wrote:
>> > Jim Carr <j...@ibms48.scri.fsu.edu> wrote in article
><much good info snipped>
>Thanks for the information. Having worked for NIST, I am somewhat (though
>not especially) familiar with the way in which standards are created and
>maintained. These usually involve the agreement of several committees from
>several countries in a forum that has sufficient international credibility
>that many nations will accept the standards agreed upon, even if they
>didn't participate in the process directly. It is my understanding that in
>the U.S. that the Department of Commerce, via NIST, is authorized by the
>legislature in accordance with Article 1, paragraph 8 subparagraph 5 of the
>U.S. Constitution which explicitly grants the Legislative branch the power
>to fix the standards of weight and measure.
>Congress doesn't sit down and codify these things because they have learned
>that it is best to leave these areas to the discretion of people that
>(hopefully) understand, or are at least aware of, the subtle nuances and
>hidden interdependencies between units of measure and the need to prevent a
>system from being overdefined. Ideally, each base quantity should have a
>single definition and all other units for that same measure should be
>referred back to that one definition - even if it's in another system of
>units entirely.
What I've got below led me to this legal authority for defining units
of measure from the United States Code, 15 U.S.C. 272:
272. Establishment, functions, and activities
(a) Establishment of National Institute of Standards and
Technology
There is established within the Department of Commerce a science,
engineering, technology, and measurement laboratory to be known
as the National Institute of Standards and Technology (hereafter
in this chapter referred to as the "Institute").
b) Functions of Secretary and Institute
...
(2) to develop, maintain, and retain custody of the national
standards of measurement, and provide the means and
methods for making measurements consistent with those
standards, including comparing standards used in scientific
investigations, engineering, manufacturing, commerce,
industry, and educational institutions with the standards
adopted or recognized by the Federal Government;
>Unless it has been corrected, there are actually two definitions that
>relate the units of length between the English and the SI systems. An inch
>is defined as being exactly 2.54 cm. But in the field of land surveying,
>the inch is defined as being a length such that there are exactly 39.37
>inches in one meter. Personally, I believe that the first is the legal
>definition (in the US) because it is the one supported by NIST which has a
>clear constitutional authority. But, for reasons probably having to do with
>politics and probably long forgotten, all legal descriptions of real
>property, including state boundaries, highway mile markers, etc are
>required (at least as of a few years ago) to use the second definition. And
>don't ask me why they just don't get off their ass and start making
>measurements directly in meters and then convert to miles and feet when
>necessary.
This really shouldn't apply to highway mile markers, in my opinion.
I think one reason it hasn't been changed is that the U.S. Geological
Survey has long used meters for new measurements, perhaps even in
1959.
This isn't completely forgotten, though probably not easy to find.
This is from the Secretary of Commerce's official announcement of the
1959 redefinitions of the yard and the pound.
Federal register of notice of 1 July 1959
quoted in _Weights and Measures Standards of the United States: a
brief history_, National Bureau of Standards Special Publication 447,
originally issued Oct. 1963, updated Mar 1976.
" Any data expressed in feet derived from and published as a result
of geodetic surveys within the United States will continue to bear the
following relationship as defined in 1893:
1 foot = 1200/3937 meter
"The foot unit definied by this equation shall be referred to as the
U.S. Survey Foot and it shall continue to be used, for the purpose
given herein, until such a time as it becomes desirable and expedient
to readjust the basic geodetic survey networks in the United States,
after which the ratio of a yard, equal to 0.914 4 meter shall apply."
>Well, that's a discrepancy of about 2ppm which adds up to potential
>cumulative errors of 20 or 30 feet in a country this size. There have been
>legal suits between states over who has water and mineral rights to streams
>and rivers on or close to a shared border and a few feet uncertainty is
>where the legal border really is makes a big difference sometimes. And just
>think if you owned property whose legal description of one side was based
>on a geographic feature such as a river bank and the other side was based
>on surveyed measurements. Any correction to the legal definitions might
>very well move one of your borders but not the other. And of course the
>party losing land will contend that the legal description should be changed
>to reflect the change in definitions and the party losing land will
>maintain that they have a legally binding document that describes the
>boundaries of their property and that's all there is to it.
I think those issues are probably fairly well settled in U.S. law.
After all, we have places where some of the original grants were in
varas (Texas, California, etc.) or arpents (Louisiana).
Gene Nygaard
http://ourworld.compuserve.com/homepages/Gene_Nygaard/weight.htm