Re: A weighty subject

[Leigh Palmer <palmer@SFU.CA>]

> Leigh,
> Thanks for the succinct summary.  I would like some clarification on a
> couple of points you make in your carefully worded definition and how it
> relates to some past comments on this thread.  I've interpreted you as being
> in the weight is what the scale reads camp, as long as we are careful to not
> do the weighing in such situations where non-inertial forces may affect the
> scale reading, e.g. weighing the object under-water; I assume you mean that
> if we do do such a silly thing, we must take that into account in
> interpreting our readings.

Yes, of course. I hope I also implied that the scale is ideal
and calibrated correctly, and that it is a force measuring
device. I wish an object's weight to be defined as having
magnitude equivalent to the force it exerts on the scale.

> I'm not sure how to relat this to the accelerating upwards elevator. What
> would a geophysicist say "the acceleration of gravity" to be in the elevator
> reference frame?

What would the point of a geophysicist doing a gravimeter
measurement in an accelerating elevator be? My definition of
weight is chosen to be useful only in one circumstance, in a
region where it is practically sufficient to specify inertial
forces as being proportional to mass over the entire region
occupied by the system in question. Weight is always defined
relative to a frame of reference, but it may not be useful to
ask questions such as "What is the weight of an orbiting
astronaut with respect to the kitchen of his wife on Earth?"
Utility (and pedagogical propriety) is the goal here, not the
construction of multiple choice questions for standardized
physics examinations.

> I interpret the above statements to confine weight to
> effects of gravity, centrifugal forces, and perhaps coriolis and azimuthal
> forces? But what of inertial forces caused by accelerating frames that are
> not associated with the spinning motion of the earth?  I assume that you
> wish to include those as well; so I'd advocate rewording your definition
> above in a way to make that clear. (Have I understood you correctly?)

I think the limitation discussed above will answer your
question. All of this started because of a Hecht question
which asked for the "effective weight of a car rounding
the top of a hill". I have no trouble with the question if
what is wanted is the weight of the driver and every other
mass *within* the car; there might conceivably be some use
to which that information could be put if one wished to
explain the evolution of a system within the car. I fail to
see what use could be made of the information sought in the
question, however. It is like asking "What is the combined
weight of all the people on earth?" If one considers weight
to be a vector then this problem takes on a new dimension
(or two). It would be silly to expect a definition such as
mine to have application to such a question.

Leigh