During a collision the objects involved generally apply equal-and-opposite forces on one another. There are usually no external forces, so the momentum of the system of objects is conserved.

Generally, momentum is conserved in all types of collisions.

There are four classes of collisions based on what happens during the collision and, in particular, what happens to the total kinetic energy of the system.

1. Super-elastic: Kinetic energy is larger after the collision (e.g., an explosion).
   
   
2. Elastic: Kinetic energy is conserved.
   
   
3. Inelastic: Kinetic energy is smaller after the collision.
   
   
4. Completely inelastic: Kinetic energy is smaller, and the objects stick together, after the collision.