For every action, there is an equal and oposite reaction, or something like that. F = MA The Force on the rocket is equal to the Mass x Acceleration of the particles (atoms, whatever) in the exhaust. A rocket is said to be 100% efficient when the rocket's speed is equal to the speed of the exhaust (I think that is correct, not a rocket scientist[8D]) so a rocket which is travekling through space faster than its exhaust is exiting is more than 100% efficient.