If they return to their original shapes after collision, such a collision is called perfectly elastic. If none of the energy associated with their motion is transformed into other forms, such as heat or sound, then kinetic energy is also conserved. However, if the collision between the bodies is not perfectly elastic, some of the kinetic energy will be used up in causing permanent deformation or transfered into other forms such as heat and sound. Conservation of kinetic energy does not apply to these situations. Such a collision is not perfectly elastic and is called semielastic .
All real collisions are semielastic.
In every collision momentum is conserved but kinetic energy is only conserved in perfectly elastic collisions. A collision where bodies become joined together and are permanently deformed is non-elastic collision, usually called an inelastic collision. An example of an inelastic collision is a bullet fired into a block of wood. Suppose that the bullet has a mass m and is fired horizontally with a velocity v. The bullet becomes embedded in the block of wood of mass M which is initially at rest and is suspended by strings from a common point.