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

I don't agree !

It depends upon your reference frame !

An external force will change the momentum -- N2!

The original question was something like -- How could the gas be gaining
momentum, when the rocket ship isn't.

It is the change in momentum (per time) of the gas that produces the
force on the rocket ship to counter-act gravity.

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.

Oren Quist
South Dakota State University

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John
Denker
Sent: Thursday, July 27, 2006 1:13 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Rocket Hovering and Conservation of Momentum

Quist, Oren wrote:

Momentum is conserved only if there are no external forces. There is
an
external force on the rocket ship (gravity) -- don't expect momentum
to
be conserved for the rocket ship.

Pleeeease don't say that. That is at best an abuse of the terminology,
and is very likely to spread misconceptions.

There is a crucial difference between /constancy/ and /conservation/

a) Constancy means simply that the momentum in a given region doesn't
change.

b) Conservation means that the momentum in a given region doesn't
change
_except by flowing across the boundary_.


Momentum is *always* conserved. Always. This is one of the grandest
and most
fundamental principles of physics. You should never pass up an
opportunity
to teach this point.

There is no law of constancy of momentum ... although you can find
special
situations where the momentum is constant, for example if there is no
momentum
flowing across the boundary.

The idea of conservative flow can be made very precise; see e.g.
http://www.av8n.com/physics/conservative-flow.htm
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