# Conservation of Momentum when an Object Rebounds

1. Apr 14, 2013

### Mikry

This is my first post(of hopefully many!), so I hope I've made this thread in the right subforum...

Anyway! Let me set the scene:
Say you throw a ball against a wall, a perfectly elastic collision takes place and the ball bounces back off the wall with the same speed.

Now I understand the mechanics of this in terms of kenetic and potencial energy: the kenetic energy is turned into some form of potencial energy and then converted into kenetic energy again.

But I can't seem to wrap my mind around what's going on with the momentum in this situation. The momentum travels in one direction before it hits the wall, and then afterwards it travels in the complete opposite direction. I don't understand how the momentum is being conserved or what the external force is that's causing the momentum to change...

Could someone please explain what's going on there?

2. Apr 14, 2013

### Staff: Mentor

Momentum is conserved when there is no external force acting on the system. In this case, the momentum of the ball is not conserved because it is acted on by a force from the wall.

Now, if you consider the ball and the whole earth all as one system then for the collision there is no external force. So to conserve momentum the ball must have produced some miniscule change in the velocity of the earth.