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Re: [Phys-l] Rocket Hovering and Conservation of Momentum


I think Oren Quist might mean "system" when he said frame of reference. If the entire universe is your system then, momentum is conserved, but it is not necessarily conserved for a single object or collection of objects which you define as your system. Oren has chosen to define his system such that it excludes the Earth except for the gravitational force associated with the Earth.

Although one can do this, and often get the right answers, it leaves a bad taste in my mouth, the same way that talking about centripetal force rather than radial acceleration leave a bad taste. If only a little more work you can do all of the physics. I know engineers design things by calculating the centripetal force, but it is not really a good way to approach problems while learning physics.

I would suggest that when solving a problem, if one's first attempt at understanding and solving it indicates that momentum is not conserved, then one needs to consider adding something to your chosen system.

Roger Haar************************************************************
John Denker wrote:
Quist, Oren wrote:

It depends upon your reference frame !

No, it doesn't. All accurate observers agree that momentum is conserved.
Always. Really and truly. This is true for special relativity as well
as Galilean relativity.

An external force will change the momentum -- N2!

OK, so what? Change is not the same as non-conservation.
I'll see your N2 and raise you an N3.

Momentum is conserved -- N3!

I don't think it is a matter of "constancy" or "conservation". I think
it is a matter of being careful to choose your reference frame.

Please show us a reference frame in which N3 is violated, i.e.
in which momentum is not conserved.
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