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.