MODEL RELEASED. Collision experiments. Two students using an air track to investigate the law of conservation of linear momentum. The air track reduces friction. When two objects on the track hit each other, their collective momentum (mass x velocity) must be the same before and after the collision. Collisions can be elastic, where total kinetic energy is preserved, or inelastic. As an example, if two objects of equal mass and equal, but opposing, velocities collide then if the collision is perfectly elastic they will rebound at the same velocity. If it is perfectly inelastic then they will meet and stop.