I hate to think of all the hours I've spent in my life reading and arguing
and teaching about "weight" and "mass."
In teaching introductory calculus-level physics, I don't use the term
"weight" at all. Well, since the textbook probably uses the term, I will
have to point out what I'm using in place of what the textbook calls "weight."
On free-body force diagrams, I use mg and call it "the gravitational force
on the object." I NEVER use W (except for "work"). I call mg a "field
force" as opposed to a "contact force" and say the gravitational field g
(always a vector, but I can't do it on my email program) depends on ALL the
objects in the universe, but, near the earth, is mainly due to the earth.
In the problem with an object on top of another object resting on the
earth, there is an mg on each of the two objects. Yes, it's mainly due to
the earth, but it conceptually includes the negligible effect of the other
object. I will likely say it's really m1g1 and m2g2 but that g1 and g2 are
close enough in magnitude to treat them as a single g.
In problems involving scales in elevators and the like, I will ask for the
scale reading and use a term like the normal force of the scale on the
person or vice versa.
I will point out that the "weightless" astronaut would not need a normal
force from a scale, that mg is the net force in that case.
For a person standing on a scale on the rotating earth, the gravitational
force is still mg, there is a normal force of the scale on the person which
is a little less than mg to allow for a small centripetal acceleration. I
would never say that a correction has to be made to g.
It has been many years since I used the term "weight" in a course. I have
never regretted giving it up, and I feel that I and the students understand
everything better without it.
Laurent Hodges, Professor of Physics
12 Physics Hall, Iowa State University, Ames, IA 50011-3160
lhodges@iastate.edu http://www.public.iastate.edu/~lhodges