1. The problem statement, all variables and given/known data A locomotive of mass 21x10^3kg travelling at 2.0m/s runs into an identical stationary locomotive. If the collision is perfectly elastic and lasts for 1.5s, find the average force exerted on the stationary locomotive during the collision. 2. Relevant equations Find momentum before and after the collision (m1v2 + m2v2 = m1v2 + m2v2) Use impluse to find the average force (F = p/t) 3. The attempt at a solution I found the momentum before and after the collision. For the second part, i'm not sure which momentum to use. The final one? Can someone tell me if I'm right?