Re: Rocket action

[brian whatcott <inet@INTELLISYS.NET>]

I too, think this is a splendid puzzle for a mind experiment.
I force myself to recall, that even when the rocket and its exhaust
are BOTH exiting stage left, pronto....there is still a long long
trail of exhaust that continues to dribble past the zero position
and when this no longer happens there is a plume that also shows a
distribution of speeds further to the right.

The left directed momentum component is thereby always neatly balanced
 by the rightwards exhaust heading out towards Alpha Centauri.

At 13:54 12/10/99 -0500, Rick wrote:
> 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

brian whatcott <inet@intellisys.net>
Altus OK