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



Hi,

I have two very different suggestions for understanding this class of problems.

1. Use the basic ideas of general relativity in which the "natural", force-free path in the presence of gravity is falling. Then, if the rocket is hovering, it is actually accelerating upward. ( This is a very simplified explanation. )

2. Consider two aircarts connected by a spring. The track runs from left to right and the spring is initially stretched so the carts would pull toward each other. If one holds the left cart so that it is stationary in the lab frame, the right cart accelerates toward it. Now replace the person holding the left cart with a rocket that keeps the left cart stationary in the lab frame. (This would require a controller on the rocket the adjusts the thrust constantly.) The right cart accelerates to the left and thus gains momentum, balancing the conservation of momentum equation.

One catch with this type of problem is correctly apply Newton's Third law to gravity between two masses, but replace gravity with a spring and the action and reaction become at least somewhat clearer.

Thanks
Roger Haar
U of AZ Physics

Jack Uretsky wrote:
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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