# Homework Help: Consevation of momentum

1. Apr 3, 2005

### alcatras

HELP is NEEDED!

A 1kg ball hits the wall with V= 5m/s and moves in opposite direction with V =4m/s. P1=5kgm/sec and P2=-4kgm/sec. But P1 must be equal to P2 from conservation of momentum , isn't that true?

THANKS

2. Apr 3, 2005

### xanthym

NO. Conservation of Momentum does not apply to the ball (as a single entity) because the wall applied a force to the ball, thus changing the ball's momentum. Anytime a net EXTERNAL force is applied to an entity, that entity's momentum will change (and not be conserved).

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Last edited: Apr 3, 2005
3. Apr 3, 2005

### SpaceTiger

Staff Emeritus
When you're considering conservation of momentum, you have to consider the momentum of the whole system, not just the ball. When it collides with the wall, some momentum is imparted to the particles in the wall. The wall, however, is stuck to the earth, so momentum is then imparted to the earth! Can you use the equation for momentum to answer why wouldn't notice such a change in the earth's velocity?