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

[Jack Uretsky <jlu@hep.anl.gov>]

Hi Marc-
	I find your answer unduly complicated.  A counterpart of this 
question is in Hake's origianl SDI labs.  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 earch.
	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.
				Regards,
					Jack


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

> Howdy-
>
> A student asked me this question recently, and I am not
> sure about my answer.
>
> He asked, "Imagine a rocket hovering off the ground,
> applying a thrust equal to the rocket's weight. Gas is
> moving downwards getting downwards momentum. What is
> getting upwards momentum?"
>
> Okay formulate your answer....
> (Scroll down to see mine.)
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> I said that we should decide that the direction that the
> gas is moving is negative, and opposite of that direction
> is positive. The gas gets negative momentum and the rocket
> gets positive momentum due to conservation of momentum, but
> the rocket is pulling on the planet -- the planet doesn't
> just pull on the rocket but the rocket also pulls on the
> planet! Though the distance between the rocket and the
> planet doesn't increase, the rocket and planet, together,
> get some velocity in the positive direction.
>
> This is before the gas hits the ground. After the gas
> starts hitting the ground the situation becomes more
> complicated. In the simplest case, the gas imparts its
> momentum to the planet which stops the rocket's and the
> planet's motion in the positive direction.
>
> What say you?
>
> Marc "Zeke" Kossover
>
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