Re: A weighty subject

[Leigh Palmer <palmer@SFU.CA>]

> ...The conceptual problem (I think) is WHAT IS THE SOURCE
> OF THIS FORCE CALLED WEIGHT.

That is a perfectly good question, but it should not be a
conceptual problem. I maintain that it is not a reach for
a student to accept the proposition that everything in his
laboratory has weight without having to inquire about its
cause. If the student asks then one can simply tell him
that the force is mostly due to gravity, and if he persists
then give him the universal law of gravitation and caution
him that there are other small effects. If he wants to
know what the other small effects are tell him that the
Earth is not spherically symmetric, and that centrifugal
force makes the next largest contribution to the weight
after the gravitational force.

I see no problems that can be encountered in satisfying the
student's curiosity, but I am quite sure that the *concept*
of weight is as close to intuitive as any concept can get
without a detailed understanding of it mechanisms.

I think I should clarify what I mean by "Weight is what the scale
reads." I assume an ideal scale, in static equilibrium in the frame,
which registers the force with which it is squeezed from both sides
equally, and reports it truthfully. There is no bouyancy to account
for; ideally this is done in a vacuum. If bouyancy is significant in
a particular calculation then I require it to be included explicitly.
The same goes for for gravitational effects of other bodies included
within the laboratory.

When teaching physics I think there is great value in affirming what
the students already believe whenever it can be done, and one of
those things is that scales report weight. I see far too many
students who don't believe physics describes reality.

Leigh