A force pair is the forces two objects apply to each other.

In a force pair, one force is called the action force. The other force is called the reaction force.

For every action force, there is a reaction force that is equal in strength but opposite in direction.

In any collision, one object transfers momentum to another object.

The total momentum of a group of objects stays the same unless outside forces act on the objects.

Elastic collision happens when objects bounce off each other.

Inelastic collision happens when objects collide and stick together.