CBSE Class 11-science Answered

In an elastic collision the kinetic energy and momentum is conserved.

I think you wish to know what happens during the actual collision...

During the short time when collision is taking place, the billiard balls squash a little near the surface of contact, but then they sring back. Some of the kinetic energy of the system is stored temporarily as elastic potential energy, but at the end it is reconverted to kinetic energy.

Since the billiard balls deform very little when they collide and quickly spring back from any deformation that they might undergo, so we say that kinetic energy is conserved. The force of interaction between the balls is almost perfectly conservative. This means that no energy is lost. It was temporarily converted to another form and then reverts back to its original form.