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

["Marc \"Zeke\" Kossover" <zeke_kossover@yahoo.com>]

--- 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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