1. The problem statement, all variables and given/known data Two railway trucks of masses m and 3m move towards each other in opposite directions with speeds 2v and v respectively. These trucks collide and stick together after the collision. What is the speed of the trucks after the collision? 2. Relevant equations Conservation of momentum: m1v1 + m2v2 = (m1+m2)Vtotal 3. The attempt at a solution m(2v) + 3m(-v) = (m+3m)V V= -4/v ∴ speed = 4/v Answer given: 5v/4 Apparently they added the 2mv and 3mv together for the initial momentum. Is the book wrong or am I wrong?