What's (lbm) and (lbf)?
Donald G. Shead
~ I'd Like to Know ~
What, exactly, is the difference between a pound mass (lbm), and a
pound force (lbf)? Does anybody know?
Donald Shead <u10...@snet.net>
Mitchell Patenaude
In article <01bc73f8$d305fd40$132b3ccc@default>,
Donald G. Shead <u10...@snet.net> wrote:
> What, exactly, is the difference between a pound mass (lbm), and a
>pound force (lbf)? Does anybody know?
32.
Well.. not exactly... since they're completely different units.
The difference is the same as the difference between mass and force in
the metric (SI) system, but is confused by the fact they go by the
same name in the english system, and are often used interchangeably.
A pound mass (lbm) is about .4545 kg. This is a measurement of the
object's inertia, ala Newton's second law F=ma (where F is a force,
and a is the acceleration of an object of mass m being acted upon
by that force).
A pound force (lbf) is the force exerted by *Earth's gravity* on
a 1 pound mass. This will produce a 1 g (~ 32 f s^-2) acceleration.
To keep from having to put in "fudge factors" like 32, and still
be able to use nice units like f s^-1, there is a unit of mass
in the english system called a "slug" which is equal to 32 lbm,
so that 1 lbf will produce an acceleration of 1 f s^-2.
The difference in the quantities can be illustrated by the following
thought problem. An object with a mass of 1 lbm will have a
weight (i.e. force exerted by gravity) of 1 lbf here in
"Sunny California"(tm). On the moon however, it will weigh only
about 1/6 lbf, since the moon's gravity is about 1/6 as strong as
Earth's, but will have the same mass, and therefore inertia, which
means that you won't be able to throw it any faster with your arm
than you would on Earth. (It will however, travel much further
due to the much weaker gravitational acceleration. Derivation
of how much further as a function of initial angle and velocity is
left as an exercise to the reader with too much free time.)
>Donald Shead <u10...@snet.net>
-- Mitch Patenaude (m...@sonic.net)
--
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Alexander Sperduti
Donald G. Shead <u10...@snet.net> wrote in article
<01bc73f8$d305fd40$132b3ccc@default>...
> ~ I'd Like to Know ~
>
> What, exactly, is the difference between a pound mass (lbm), and a
> pound force (lbf)? Does anybody know?
>
Simply put, F=ma, so (lbf)=(lbm)*g, with g defined as 32.16 ft/s/s. This
holds true even in space, since the relationship between lbf and lbm is a
definition.
Bernd Nebendahl
How long will it take to get rid of those completely useless units like
pounds, feet, inch, miles, gallons, there is an uncountable number of
them. There is a system of units called SI, which avoids to spend 50% of
ur time to look up tables to come from one unit to the other.
I wonder how they found the moon in 69 using those units :-)
Bernd
--
Bernd Nebendahl | Voice : ++49-711-685-5175
2. Phys. Inst., Uni Stuttgart | Fax : ++49-711-685-5285
Pfaffenwaldring 57 | e-mail:
b.neb...@physik.uni-stuttgart.de
D-70550 Stuttgart, Germany | URL: http://home.pages.de/~b.nebendahl/
Dennis Kehoe
Alexander Sperduti wrote:
>
> Donald G. Shead <u10...@snet.net> wrote in article
> <01bc73f8$d305fd40$132b3ccc@default>...
> > ~ I'd Like to Know ~
> >
> > What, exactly, is the difference between a pound mass (lbm), and a
> > pound force (lbf)? Does anybody know?
> >
>
> Simply put, F=ma, so (lbf)=(lbm)*g, with g defined as 32.16 ft/s/s. This
> holds true even in space, since the relationship between lbf and lbm is a
> definition.
That sounds good enough for me, but I have to add that in teaching high
school physics, this has always bothered me. Most books that present
the non-metric pound as a unit of force, use the slug as a unit of
mass. But engineers in particular seem to want to use lbm. I tell my
class it would be much like deciding to invent a new unit for
temperature and calling it the Joule - even though it is used elsewhere
for energy. Then every time "Joule" comes up you have to specify
whether it is the temperature joule or the energy joule. How idiotic it
seems.
Dennis Kehoe
A J Sperduti
Dennis Kehoe <dke...@tir.com> wrote in article
<5omg2a$p...@agate.berkeley.edu>...
>
> That sounds good enough for me, but I have to add that in teaching high
> school physics, this has always bothered me. Most books that present
> the non-metric pound as a unit of force, use the slug as a unit of
> mass. But engineers in particular seem to want to use lbm. I tell my
> class it would be much like deciding to invent a new unit for
> temperature and calling it the Joule - even though it is used elsewhere
> for energy. Then every time "Joule" comes up you have to specify
> whether it is the temperature joule or the energy joule. How idiotic it
> seems.
>
> Dennis Kehoe
Thanks for the info. I was never even taught that the slug was the English
unit of mass! Where did this idiotic use of lbf and lbm first appear, and
how did it end up in college thermodynamics courses? Out of curiosity if 1
lbm = X slug, what is X?
[Moderator's note: Let me see if I can get this right. The slug is
the "standard" unit that goes along with the foot, second, and pound
(for force) So one lbf is one slug-foot per second squared. And we
know that one lbf is also one lbm x 32.16 feet per second squared. So
I'd say that a slug is 32.16 pounds of mass. Did I get that right?
-TB]
Darrell Loyd
According to my CRC "Handbook of Chemestry and Physics" 54th Edition
(1973-1974)
1 slug = 32.1740 Pounds = 14.5939 Kilograms
I studied physics at LSU 1975, was taught slug was mass, pound was force
"only".
[Moderator's note: Quoted text deleted. -TB]
Bill Rowe
In article <5ougdg$m...@agate.berkeley.edu>, dl...@gte.net says...
>
>According to my CRC "Handbook of Chemestry and Physics" 54th Edition
>(1973-1974)
>1 slug = 32.1740 Pounds = 14.5939 Kilograms
However, the 65th edition defines pound on page F-96 as " A unit of mass...".
Right below this is a definiton for poundal defined as the force need to give
a 1 lb mass an acceleration of 1 ft/sec^2
So using these definitions one would say a 1 lb mass weighs 32 poundals on
earth just as one would say a 1 kg mass weighs 9.8 newtons.
Oz
In article <5p9h6o$1...@agate.berkeley.edu>, Bill Rowe
<ghos...@sprynet.com> writes
>However, the 65th edition defines pound on page F-96 as " A unit of mass...".
>Right below this is a definiton for poundal defined as the force need to give
>a 1 lb mass an acceleration of 1 ft/sec^2
I confirm pounds and poundals as the system taught in the UK in the
1960's, before we became metricated.
Notice the logical abbreviations used in the old Imperial system. I mean
it's obvious that 'lb' stands for 'pound', that 'cwt' is a hundredweight
of 112 lbs and as for stones, well.
It is gratifying to note that the oldest and most rebellious of the old
colonies sticks so stubbornly to the archaic iron age units. Still I
suppose colonies are always a little behind the times.
Indeed the other quasi-metric units beloved of astronomers and nuclear
physicists like angstroms and parsecs and so on seem to be producing yet
another layer of units that aren't SI.
It's odd that plumbers and carpenters in the UK (who are not renowned
for their high intelligence) managed to change from Imperial to metric
relatively easily, but those high-powered astro-nuclear physicists find
a change to SI units quite beyond their capabilities.
--
'Oz "Is it better to seem ignorant and learn,
- or seem wise and stay ignorant?"