1. The problem statement, all variables and given/known data a ball falls in a truck loaded with sand with velocity v having horizontal and vertical component. The angle under which it enters the truck is α, the mass of the ball is m of the track loaded with sand is M. What happens with the momentum after the collision? 3. The attempt at a solution This should be considered as completely inelastic collision. In vertical direction the momentum before the collision is the momentum of the ball mcosv. After the collision the track is not moving in horizontal direction, but the momentum should be conserved. The ball sinks in the sand, so is this the explanation? In horizontal direction the momentum before the collision is msinv. What is the momentum after? There is friction during the collision. It should be (m+M)v', but what about the friction?