## No conservation of momentum with bouncing ball?

So I was thinking about the conservation of momentum. If you throw a handball at a wall, the wall will provide an equal normal force, thus sending the handball back at the same velocity (in a perfect scenario). The ball has a momentum vector, the wall never moves, and thus only has a zero-amplitude vector. But in this closed system, the net momentum vector changes! How?

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 Mentor The ball is not perfectly elastic: some of the spring energy in the bounce is converted to heat.

 Quote by russ_watters The ball is not perfectly elastic: some of the spring energy in the bounce is converted to heat.
So what would happen if the ball was perfectly elastic? It would still bounce off, right?

Mentor

## No conservation of momentum with bouncing ball?

It would bounce back with an equal momentum to what it started with.

 Sound is also a form of energy in which the initial energy of the ball gets converted to, so it loses energy there. In a perfectly elastic system the momentum is conserved completely and none is wasted. meaning p = p' (momentum before = momentum after)

 Quote by russ_watters It would bounce back with an equal momentum to what it started with.
But the vector is in a completely different direction!

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Gold Member
 Quote by nhmllr So I was thinking about the conservation of momentum. If you throw a handball at a wall, the wall will provide an equal normal force, thus sending the handball back at the same velocity (in a perfect scenario). The ball has a momentum vector, the wall never moves, and thus only has a zero-amplitude vector. But in this closed system, the net momentum vector changes! How?
the wall does move, except that it's connected to the ground (aka Earth) which means it only appears to not move. If you really could isolate the ball and wall/earth system, the momentum would be conserved. Of course, look at the masses you're talking about and you can understand why the wall seems to not move.

 Quote by Pengwuino the wall does move, except that it's connected to the ground (aka Earth) which means it only appears to not move. If you really could isolate the ball and wall/earth system, the momentum would be conserved. Of course, look at the masses you're talking about and you can understand why the wall seems to not move.
Ah- I see. little mass x big velocity = huge mass x tiny velocity. So I suppose in deep space the wall would actually start moving back, and the conservation would be more obvious.

Thanks

 Recognitions: Gold Member Science Advisor Don't forget that when you threw the ball, the conservation law also applied and the Earth rotated backwards a tiny bit. The amount it moves forwards would be twice that value for a perfectly elastic collision and an equal value for a totally inelastic collision. All the Mv's add up to zero in every case.