A collision occurs when two bodies exchange momentum. The collisions below are all elastic.

Now you know what an elastic collision looks like. To be a perfectly elastic collision two laws must be true.

1. Law of conservation of energy
2. Law of conservation of momentum

In other works if the sum of the kinetic energies before a collision equals the sum of the kinetic energies after the collision AND the sum of the momentums before a collision equals the sum of the momentums after the collision, then it is a PERFECTLY elastic collision.

In this course the rotational kinetic energy of any colliding balls is assumed to be so small that it is ignored.

The collisions below are completely elastic.

Elastic and "perfectly elastic" collisoipns look the same. They wil lhave the same number of bodies before and after the collision. The law of conservation of momentum can applied to both types of collisions. However, a plain elastic collision does not conserve all of the kinetic energy.

Inelastic collision are easy to identify. The number of bodies before the collision does not equal the number of bodies after the collisiopn. In the examples above there are 2 bodies before the collision. The bodies stick together forming smaller bodies.

Completely inelastic collisions are where the momentum is still conserved but the maximum amount of kinetic energy is lost during the collision. This lost kinetic energy is transfer into other forms of energy such as thermal energy, light, and/or sound energy.

For our problem solving purposes, any time a problem says it is an "inelastic collision." What is meant is that the collision is actually "completely inelastic."