Weight and mass

[Fran Poodry <FPoodry@AOL.COM>]

> Consider the following useful ideas (avoid/ignore
> buoyancy,holding on to ropes, etc, etc):
> a)   the gravitational force between you and the earth
> b)   the reading when you stand on a scale on the earth
> c)   the reading when you stand on a scale in any
> other reference frame.
>
> All of these clearly have something to do with
> weight, so which one should
> we call "the weight"?

I have a different answer.
a) force of gravity on you from the earth, OR force of gravity on the earth from you, depending on which vector you are looking at
b) normal force on you from the scale, or vice-versa (as above)
c) same as b)

I always ask students to define forces in terms of the type of force and the objects involved, so that I can tell if they are talking about the same thing as each other, or the same thing I am talking about.  We define the reading of a scale as the reading of the normal force between the person on the scale and the scale itself, which has the same magnitude as the normal force between the person and whatever they are standing on.  If there is no vertical acceleration, it is also the same as the force between the person and the earth.
Students get very confused if you try to bring "weight" and "apparent weight" into the discussion, I find.

Fran Poodry
Williamstown High School
Williamstown, NJ