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Re: Rocket action



Let me elaborate on the meaning of my message so that Brian can respond.

I was just commenting on what to me seems less than an obvious situation.
We know that in this situation momentum is conserved (at zero) since there
are no external forces. This is equivalent to saying that the center of
mass remains fixed.

But now consider the rocket--moving to the left from an initial position P0.
At first it is not hard to see that a small mass of fuel/exhaust is expelled
and is moving to the right of P0 with a large velocity while the rocket and
remaining fuel moves off to the left with a much lower velocity--conserving
momentum and leaving the CM fixed. But now if the engine IS NOT throttled
such that the velocity of the exhaust relative to the engine remains fixed,
the velocity of the exhaust relative to P0 is constantly decreasing in the
rightwards direction and eventually (with enough fuel/thrust) the exhaust
gasses expelled at some later time will actually have a velocity towards the
left of P0. It is at this point that I don't have a good intuitive feel for
how the CM remains fixed. I"m sure I can do the math, I just have trouble
'seeing' it. Thus my comment was aimed only at the fact (at least for me)
that the rocket problem is non-trivial, and non-intuitive. I have no
problem when I consider the motion from a force/acceleration point of
view--but from the momentum, CM point of view, it gets more hazy. ;-)

Rick

----- Original Message -----
From: "Robert A Cohen" <bbq@ESU.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, December 10, 1999 1:22 PM
Subject: Re: Rocket action


On Thu, 9 Dec 1999, brian whatcott wrote:

At 15:53 12/9/99 -0500, Robert Cohen wrote:
On Fri, 3 Dec 1999, brian whatcott wrote:

At 08:52 12/2/99 -0500, Rick Tarara wrote:

It is easy to 'see' that the rocket can continue to accelerate under
a
sustained thrust, but less obvious how the center of mass (rocket
starting
in space, originally at rest) remains fixed and at rest, especially
once the
speed of the rocket exceeds the speed of the exhaust. ;-)
///
Richard W. Tarara

I believe Rick would want to reconsider this mind experiment.

brian whatcott <inet@intellisys.net>
Altus OK


Why reconsider it? Is something wrong with it?


I have written up my objection to the list two or three
times by now. Is the objection not intelligible?

The idea of a time progression until a rocket outraces its
exhaust is a physics textbook fairy tale in essence, if you
consider that its exhaust can be throttled to low velocity
at some indefinitely early stage.

I'm sorry but you've lost me. What is it, specifically, about Rick's mind
experiment that you object to? Is it his statement that the center of
mass remains fixed? It seems reasonable to me and I'd like a clear
explanation for why it is wrong. By the way, I've only been a member of
this list for a short while and so I may have missed your earlier posts
(although I checked the archive and couldn't come up with anything).

----------------------------------------------------------
| Robert Cohen Department of Physics |
| East Stroudsburg University |
| bbq@esu.edu East Stroudsburg, PA 18301 |
| http://www.esu.edu/~bbq/ (570) 422-3428 |
----------------------------------------------------------