Re: What is weight? (was Re: Internal or external?)

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Sun, 10 Oct 1999, Cliff Parker wrote:

> Wouldn't the weight of the 100 kg body be 1000 N in all cases described.
> Other forces act on the object such as the force of the elevator or the
> water on the body but the weight does not change in the conditions
> described.  At least not if one uses w = mg.

You could make a pretty consistent model out of this viewpoint if you

a) were willing to suppose that there is something special (i.e.,
privileged) about the reference frame in which the elevator's various
accelerations are being specified, and

b) defined weight as m*g with g measured in that privileged frame.

Furthermore, you'd be in good company--e.g., Sir Isaac Newton.  However,
for nearly a century now, we have understood that the Newtonian model is
flawed--flawed so subtly that it doesn't matter in many situations, not at
all subtly flawed in others.

One important aspect of our new, improved model is the understanding that
the only special frames are freely falling frames, that they are
necessarily restricted in extent (i.e., "local"), and that they are
special because, in them, g = 0.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm