1. The problem statement, all variables and given/known data A train car with mass m1 = 550 kg is moving to the right with a speed of v1 = 7.2 m/s and collides with a second train car. The two cars latch together during the collision and then move off to the right at vf = 4.7 m/s.2)What is the mass of the second train car? 2. Relevant equations momentum before = momentum after. kinetic energy before does not equal kinetic after. 3. The attempt at a solution okay, I assumed the second car is not moving and solved for v final using this: m1v1+m2v2=(m1+m2)v final. My answer was wrong. Any ideas. My guess is that I shouldn't assume the second train car isn't moving but I don't know another way to solve it in that case.