As I used to explain it to students: Properly speaking, a _force_ is a
push or a pull on an object. Properly speaking, _mass_ is a measure of
the amount of matter in an object. _Weight_ is a particular force, i.e.
the force of gravity on an object.
So in a U.S. grocery if you buy 2.2 pounds of cheese, you're buying the
amount of cheese upon which the earth's gravity exerts a force of two
pounds. It's a roundabout way of specifying the mass you want, but it
works as long as you're just talking cheese, etc.
In a European country, you'd specify you wanted a kilogram of cheese,
which is about 2.2 pounds worth. There, you're directly specifying the
amount of cheese you want.
That makes it sound like the Europeans are much smarter. But they turn
things around and sometimes measure forces in kilograms, or pressure in
kg/cm^2 etc.
Where it makes a difference is in calculations involving force, mass and
acceleration. Or other engineering calculations. If you don't clearly
understand whether you're dealing with force or with mass, you get
answers that are very, very wrong.
>
> When I was in school, years ago, we were quite strictly made to write
> either lb_f (pound force) or lb_m (pound mass), and to include unit
> conversions from one to the other using constants g (the nominal force
> of graivty at the surface of the Earth) and g_c (a unit conversion factor).
>
> The conversion is:
>
> lb_f = lb_m * g / g_c
>
> In English units g = 32.2 ft/s^2
> g_c = 32.2 lb_m ft/s^2 lb_f
>
> but if you didn't include the conversion, you failed.
Exactly! And students who ignored all that got answers that were wrong
by a factor of 32.2.
As I explained it, g_c ("Gee sub C") is just a conversion factor, in the
same way that (12 in / 1 ft) is a conversion factor. If a person
diligently showed units in their computations, it was obvious when it
was needed.
Most conversion factors have no names, and it always seemed weird to me
that they gave that conversion factor a name. Thousands of students got
endlessly confused between the acceleration of gravity
g, which is 32.2 ft/sec^2
and that conversion factor
g_c, which is 32.2 (lbm*ft)/(lbf*sec^2)
Diligent attention to units on ALL quantities straightens out that
confusion. At least, for most students.
And BTW, I found that engineers typically pay attention to units like
that. To my astonishment, some professors teaching basic physics did not.
--
- Frank Krygowski