Elastic collision
An elastic collision is an encounter between two bodies in which the total kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

 of the two bodies after the encounter is equal to their total kinetic energy before the encounter. Elastic collisions occur only if there is no net conversion of kinetic energy into other forms.

During the collision of small objects, kinetic energy is first converted to potential energy
Potential energy
In physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...

 associated with a repulsive force
Repulsive force
Repulsive force may refer to:* A repulsive force of an Accelerating universe, which according to certain theories causes planets and matter to get farther and farther apart* Like charges repelling according to Coulomb's law...

 between the particles (when the particles move against this force, i.e.