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



That's one trouble with analogies, they often mis-state the problem. When dealing with gravity, it's not the same force, it's the same acceleration - quite a different matter. You are, in effect -if I understand you- saying that a heavier body will fall more slowly than a light body. Even Aristotle would blush!
Regards,
Jack

On Tue, 25 Jul 2006, Folkerts, Timothy J wrote:

Hi Timothy-
Are you saying that heavy bodies fall faster than light bodies?
Aristotle, you've finally been vindicated!
Regards,
Jack

Um... No, Jack, I'm not saying that at all. You must have misunderstood my analogy (or perhaps the earlier analogy that I was modifying).

I'm saying that when the same magnitude of force (ie from the spring) is applied to two objects (ie low friction carts of different mass on a horizontal track), the smaller mass will have a larger acceleration, and hence move faster and farther than the heavy cart. I think that is all in accordance with Newtonian Mechanics.

Tim F


On Tue, 25 Jul 2006, Folkerts, Timothy J wrote:

I think a better example would be one very light cart connected by the spring to a very heavy cart. If you let go, they will both move toward each other, but the light cart will move more.

If a rocket is connected to the light cart to maintain a constant distance from the heavy cart, then the whole system will slowly accelerate (the same as if a solid bar connected the two while the rocket fired).

To make a better analogy with the earth, you might want to consider a large plastic bag surrounding the carts to contain the air. That complicates the system, but makes a fairly tight analogy.


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.



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