# Momentum conservation?

1. May 10, 2012

### jsmith613

Momentum conservation???

Consider a ball falling towards the earth. I understand here how momentum is conserved here (ball momentum increases in one direction and earth momentum increases in the other).
BUT when the ball collides with the floor and it changes direction its momentum change is 2mv. How is momentum consereved in the whole system?

2. May 10, 2012

### Infinitum

Re: Momentum conservation???

The earth collided with the ball too

3. May 10, 2012

### jsmith613

Re: Momentum conservation???

so do you mean the earth's change in momentum is 2mv?

4. May 10, 2012

### Infinitum

Re: Momentum conservation???

The change in Earth's momentum is

$$2M_{earth}V_{earth}$$

Where Vearth is the original velocity with which the earth was moving(neglecting rotation/revolution), and that is equal to 2mv.

Last edited: May 10, 2012
5. May 10, 2012

### jsmith613

Re: Momentum conservation???

ok so to check that means the earth is moving DOWNWARD (however negligible the motion)

6. May 10, 2012

### Infinitum

Re: Momentum conservation???

Pretty much, yes. Though downward isn't exactly defined in this sense. More appropriate to say earth is moving opposite to the original direction of motion, assuming the collision was head-on.