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

["Folkerts, Timothy J" <FolkertsT@bartonccc.edu>]

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.