1. The problem statement, all variables and given/known data Two people are standing on a 2.0 m long platform, one at each end. The platform floats parallel to the ground on a cushion of air, like a hovercraft. one person throws a 6.0 kg ball to the other, who catches it. The ball travels nearly horizontally. Excluding the ball, the total mass of the platform and people is 118 kg. Because of the throw, this 118 kg mass recoils. How far does it move before coming to rest again? 2. Relevant equations Momentum after = momentum before 3. The attempt at a solution I realized that conservation of momentum applied, but I couldn't set up any equations.