# Homework Help: 2-D collison question involving one moving objecy and one stationary object

1. Oct 6, 2008

### meeklobraca

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

A 20.0kg curling rock is moving due east when it collides with a stationary rock of equal mass. After the collison the first rock is travelling at 1.50 m/s 47 degrees north of east, and the other rock is travelling 1.71 m/s 40 degrees south of east. Determine the velocity of the first rock before the collison.

2. Relevant equations

p=mv

3. The attempt at a solution

I can sort of figure out how to do this if your given the velocity of the moving rock, but im kind of at a loss as to how to figure it out going the other way. THe attempt I made was that the momentum before and after have to equal the same. So I figured the momentum of the two rocks after the collison equalled 64.2N. but taking that and diving it by the mass of the before rock doesnt equal the answer.

Can someone point me in the right direction for trying to figure out this question?

Thanks!!