Re: summary of weight

[Leigh Palmer <palmer@SFU.CA>]

> Camp 2: (As exemplified, I think, by John Denker)
>
>   Weight is what you get when you multiply the mass of an object
>   by the acceleration of a freely falling object at the same
>   location *as measured within some chosen reference frame*
>   (or, loosely, "weight depends on the reference frame.")
>
> Camp 3: (As exemplified by me and, I think, Leigh, but maybe Leigh really
> agrees with John Denker. I might even come around myself.)
>
>   Weight is what you get when you multiply the mass of an object
>   by the acceleration of a freely falling object at the same
>   location *relative to the object*
>   (or, loosely, "weight is what a scale reads.")

My "weight" is, indeed, relative to frame of reference. An object
could, therefore, have more than one weight. The weight I advocate
for teaching physics will almost always be relative to a frame in
which the objects being analyzed *can be* accelerating. A freely
falling object is *not* weightless relative to the laboratory in
which it is falling. It may *feel* itself to be weightless, but I
would like to be able to say that the accelerating force has
magnitude equal to the object's weight in the laboratory frame.

> Then there is the question of whether or not weight is a vector, which I
> think is not central and will simply leave alone.

While this question is not central, its answer is trivial. If
weight is to be a useful concept then all things are acted upon by
forces with magnitude proportional to their masses and in a common
direction. We define this direction as "down" and the magnitudes
of these forces as the weights of the objects. Incidentally, it
has never been my practice to bring Coriolis force into this model.
Once the Coriolis force becomes significant one must resort to a
more sophisticated model.

The single important point here is whether one wishes to include
the centrifugal force contribution in reckoning what one means by
"weight". I certainly want to include it because my scale sees it,
and I see absolutely no reason why I should not include it. There
are some who hold the superstition that centrifugal force is,
somehow, not respectable. I maintain that the issue does not arise
when one first defines weight; why worry about it at all? When the
time comes to introduce the concept of centrifugal force (which I
consider entirely respectable), its contribution to the weight is
an excellent example to discuss.

Leigh