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Re: [Phys-l] Weightless



On Nov 20, 2006, at 2:33 PM, Anthony Lapinski wrote:

Saying that scales read your "weight" has ambiguity (in certain situations). Again, take one up in the orbiting shuttle. All scales read zero. Then if the astronauts have no weight, what force keeps them orbiting the Earth?

The gravitational force!

I understand that lots of people (including apparently the folks at NIST) want weight to be the same thing as gravitational force, but I have yet to hear one compelling reason for that. Moreover, I submit that weight is demonstrably NOT the same thing as gravitational force in common parlance. Again, if astronauts are not "weightless," then I simply don't see any good use for the word.

If I were king ...

"gravitational force" would be a frame-dependent vector quantity equal to the mass of an object times the local gravitational field vector (which is easily determined by measuring the local acceleration of a freely falling object).

"weight" would be a frame-independent scalar quantity equal to the magnitude of the gravitational force in the instantaneous rest frame of the object.

Thus ...

the weight of a person on an accelerating elevator would be different from the weight of the same person on the ground even though the gravitational force on both would be the same in any given reference frame.

and

astronauts would be weightless even though a nonzero gravitational force acts on them in the Earth frame.

It's all SO simple, neat, and sensible that way, don'cha think?

John Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://outlawsofphysics.com>