Collision of objects:
In a closed system, when two or more particles collide, the sum of momentum of two particles before and after collision will always be the same. In elastic collision, kinetic energy and momentum of a particle remains conserved. Whereas in inelastic collision, momentum remains same before and after collision but some of the particle's kinetic energy may be converted to other forms of energy.
A moving particle or object may possess kinetic energy and it depends mainly on its motion and mass. The kinetic energy is converted into potential energy and converted back to kinetic energy during collision of small particles.