[Phys-L] Re: TPT article on momentum & reference frames

[clement at HAL-PC.ORG (John Clement)]

You are correct for all practical purposes, but the principle of equivalence
applies to a uniform gravitational field and the field at the satellite is
not uniform.  The very slight gradient means you can theoretically detect
that you are not in an inertial frame, but since the extent of the satellite
is small compared to the distance from the Earth, the gradient is normally
undetectable.  I would put this in the same class as making the time short
so that external forces have a negligible effect, so indeed momentum would
be conserved.

John M. Clement
Houston, TX

> Hi all,
>
> I got my copy of the October issue of The Physics Teacher a few
> days ago; there was a quite interesting article
> "When is momentum conserved and what about car crashes
> in Virginia?". However, it seems to me that he made a small (?)
> mistake when writing about an orbiting shuttle:
>
> "Astronauts inside the shuttle would find that momentum is
> conserved for collisions of objects in *their* accelerated
> noninertial reference frame". (the emphasis added by me)
>
> I have  thought that the frame *inside* the shuttle would be
> a free float frame (as Spacetime Physics by Taylor & Wheeler
> put it). Hence it would be a (local) inertial frame instead
> of "accelerated noninertial reference frame". Of course,
> the shuttle is accelerating as judged from the reference
> frame fixed to the Earth.
>
>
> Regards,
>
> Antti
>
> Antti Savinainen, Ph.D., B.Ed.
> Senior Lecturer in Physics and Mathematics
> Kuopio Lyseo High School
> Finland
> E-mail: <antti.savinainen@kuopio.fi>
> Website: <http://kotisivu.mtv3.fi/physics/>
>
>
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