# Collisions thought i was doing it right?

1. Nov 1, 2007

### quickclick330

In outer space a rock with mass 4 kg, and velocity < 3900, -3000, 3200 > m/s, struck a rock with mass 17 kg and velocity < 300, -260, 260 > m/s. After the collision, the 4 kg rock's velocity is < 3500, -2300, 3700 > m/s.
What is the final velocity of the 17 kg rock?

m1 = 4 kg
v1 = < 3900, -3000, 3200 > m/s

m2 = 17 kg
v2 = < 3500, -2300, 3700 > m/s

Attempt at a solution:

pf = pi1 + pi2

(m1 + m2)*vf = m1v1 + m2v2
...solve for v2

as i was typing this i realized one of my mistakes is that the left side isn't (m1 +m2) but i'm still not sure what this should be?

Thanks for the help!:-)

2. Nov 1, 2007

### Feldoh

It's an elastic collision so the two rocks will not necessarily be moving with the same velocity. You cannot assume that vf will be the velocity of both masses.

3. Nov 1, 2007

### quickclick330

what method would i use to solve for v2f then?

4. Nov 1, 2007

### Feldoh

Well if this is a closed system then the rocks masses will remain constant, so will the total momentum in the system, so we can see that m1vi1 + m2vi2 = m1vf1 + m2vf2

You have know m1, m2, vi1, vi2, vf1 you have one unknown...