There is "rhyme and reason" in these things once you catch on. Since
we seem more familiar with mks units, let's start there.
* * * mks and/or SI * * *
We define a unit of mass called the kilogram. Its standard is a hunk
of metal in a vault outside Paris, France.
We define a force called a newton. It's the force required to give
that one kilogram mass an acceleration of 1 m/s^2.
Since g is roughly 9.8 m/s^2, a one kilogram mass has a weight of about
9.8 N.
Some people say that 9.8 N is a "kilogram-force" abbreviated kgf and
also called a "kilopond" abbreviated kp.
Some people sat that a mass of 1/9.8 kg can be called a newton-mass
because it is the amount of mass that would weigh one newton when
subjected to a one-g environment.
* * * US Customary Units * * *
The unit of mass is the slug.
The unit of force required to accelerate the slug at 1 ft/s^2 is the
pound.
Since g is roughly 32 ft/s^2, a one slug mass has a weight of about 32
pounds.
Some people say that 32 pounds is a "slug-force."
Some people say that 1/32 slug can be called a pound-mass because it is
the amount of mass that would weight one pound when subjected to a
one-g environment.
* * *
Now notice that up to here the sentences describing the US Customary
Units have been exactly parallel to the mks sentences.
We have objects throughout our lecture rooms and labs that we can hold
up and say, "If I want to give this object an acceleration of 1 m/s^2,
I need to exert a 1 newton force on it." Of course the object we hold
up is a 1 kg mass.
If we want to do the same thing in USCU we would say "If I want to give
this object an acceleration of 1 ft/s^2, I need to exert a 1 pound
force on it." However, good luck... at least good luck holding it up
with one hand. The object I would need to hold up would be a slug, and
it would weigh 32 pounds. Imagine about six-and-a-half 5-lb packs of
sugar.
One the other hand, we might have a hunk of metal that constitutes a
one-pound weight; or just imagine a standard 4-stick package of butter.
If I hold that up and say, "If I want to give this object an
acceleration of 1 ft/s^2, I need to exert a force of ..." Hmmm, what
shall I call this. How about a "poundal?" So we say that the
one-pound weight has a mass of one pound-mass, and we give it an
acceleration of 1 ft/s^2 by exerting a force of one poundal.
This is just a way of getting a more manageably-sized object (other
than the slug) into the USCU.
Some people think that feet and inches are more manageable (more
appropriately sized) than meters and centimeters. I might put myself
in that camp. Thus, I might think that USCU is more convenient for
measuring length. But I think the kilogram is much more manageable
than the slug. So USCU might win on length and lose on mass and force.
The USCU people who would rather not lose have invented the pound-mass
and poundal to compete with kilograms and newtons.
In some respects I appreciate the attempt to make the USCU more
parallel to mks/SI. But in doing so we essentially have created a
third units system at least for some parts of USCU. Is it any wonder
we lost the Mars satellite?
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817