Collisions thought i was doing it right?

[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!:-)

 

[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.

 

[quickclick330]

what method would i use to solve for v2f then?

 

[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...