When two objects come in direct contact with each other, it is called a collision. During a collision, two or more objects exert forces on each other in a relatively short amount of time. A collision can be categorized as either an elastic or inelastic collision. If two or more objects approach each other, collide and then bounce off, moving away from each other with the same relative speed at which they approached each other, the total kinetic energy of the system is said to be conserved. This is an elastic collision. In an elastic collision, there is no net change in the kinetic energy of the system, and the system's total momentum and kinetic energy are conserved. Before and after the collision, the kinetic energy remains the same and does not convert to any other form of energy such as heat or light.

A closed system always has its momentum and kinetic energy conserved during an elastic collision. Energy momentum problems confined to a plane usually have two unknowns and can be solved using the following approach:

1. Define a closed system
2. Write down the expression for the conservation of momentum
3. Write down the expression for the conservation of kinetic energy
4. Solve for the unknowns, using the two equations and standard methods

This text is adapted from Openstax, University Physics Volume 1, Section 9.4: Types of Collisions.