# Conservation of momentum question

1. Dec 12, 2015

### David112234

1. The problem statement, all variables and given/known data

2. Relevant equations
m1v1+m2v2=m1v1+m1v1 after

3. The attempt at a solution
because the wall is not moving so its velocity is 0
therefore
m1v1 = m1-v1 after
I put -v because it is the same just bounces in the opposite X direction
I found somewhere online the answer should be 2 * mv * sin(θ)
I dont understand why

2. Dec 12, 2015

### haruspex

Conservation of momentum only applies to a closed system, i.e. no outside forces. The wall exerts a force. If you include the wall, the Earth exerts a force on the wall. If you include the Earth then in fact it will acquire a velocity from the impact, but so small you could not possibly measure it.
Just consider the change in momentum of the ball (as a vector).

3. Dec 12, 2015

### David112234

It is the same as before only its x component changes, first it goes right, then it goes left

4. Dec 13, 2015

### haruspex

The magnitude is the same as before, but momentum and velocity are vectors. What is the net change in velocity?

5. Dec 13, 2015

### David112234

Oh so I do vector addition
I did the math out and got the same answer, thank you