A collision is an event in which two or more bodies exert forces on each other for a relatively short time. Although the concept of a collision can be extended to include any interaction between objects, it is usually only used when referring to high-speed interactions, such as those involving moving vehicles.
In physics, collisions occur when two objects interact with each other. The nature of the interaction depends on the type of forces that are involved. For example, if two objects collide and stick together (as happens when two pieces of gum are chewed together), then the force that held them together was likely an adhesive force. If two objects collide and bounce off each other (as happens when a tennis ball hits a racket), then the force between them was likely an elastic force. If one object collides with another and both objects stop moving (as happens when someone walks into a wall), then the force between them was likely a frictional force. Collisions can also be classified according to whether they are elastic or inelastic. An elastic collision is one in which there is no net loss of kinetic energy, while an inelastic collision is one in which some kinetic energy is converted into another form of energy (such as heat).
The vast majority of collisions that occur in everyday life are Elastic collisions In order for a collision to be perfectly elastic, there must be no net conversion of energy into any other form during the course of the collision; i.e., all forms of energy must be conserved. One simple way to achieve this would be if both masses were initially at rest and neither had any internal structure; i.e., they were both point masses. In this case, since KE = 1/2mv^2 and PE = 0, we would have Ei = EF . Another way to have a perfectly elastic collision would be if one mass was very much larger than the other and neither had any internal structure; again, since KE = 1/2mv^2 but now PE ≠ 0 , we would still have Ei = EF . A third possibility for achieving perfectly elastic collisions would involve highly reflective surfaces so that all incident kinetic energy could be reflected back out elastically; e