1. The problem statement, all variables and given/known data Particle 1 of mass m moving with speed v in the +x direction has an elastic collision with particle 2 of mass 3m which was originally at rest. After the collision, particle 2 is moving in the +x direction. What is its speed? 2. Relevant equations deltap = 0 pf1 + pf2 = pi1 + pi2 3. The attempt at a solution Wouldn't it just be pf2 = pi1 3mvf2 = mv1 vf2 = v1/3 How come the answer is v1/2 instead? Help please..