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

["Quist, Oren" <Oren.Quist@SDSTATE.EDU>]

Some interesting comments on this thread, but, in my humble opinion, we
are making things too hard for the high school student.

In terms of the rocket ship.

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.

The force on the rocket ship (equaling its weight) equals the change in
momentum per time of the expelled gas.

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 Jack
Uretsky
Sent: Monday, July 24, 2006 7:37 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Rocket Hovering and Conservation of Momentum

The change in momentum of the gas after it leaves the rocket cannot, at 
leasst in first order, have any effect on the rocket.  I don't
understand 
your remark after the "but".
 			Regards,
 				Jack



On Mon, 24 Jul 2006, Marc "Zeke" Kossover wrote:

> --- Jack Uretsky <jlu@hep.anl.gov> wrote:
>
> > The answer to
> > what is getting
> > upward momentum is, simply, the earth.  The added
> > downward momentum from
> > the gas is from the gravitational attraction of the
> > each.
>
> This is what I originally thought (and the student, too)
> but it leaves out the fact that the rocket does get some
> positive momentum as well. The rocket end up upwards from
> it's original position, even though it remains the same
> distance from the planet.
>
> >  	I advocate keeping it simple and accompanying the
> > answer with a
> > diagram showing the gas, the earth, the mutual forces
> > acting on each, and
> > nothing else.
>
> Sure, but he was wondering about momentum conservation. And
> I think that he was leading me towards asking the more
> vexing question about momentum conservation in a plane with
> level flight.
>
> Marc "Zeke" Kossover
>
>
>
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