Types of Collision

Before we begin to apply the conservation of linear momentum, we first clarify what is meant by a collision and distinguish between two types of collision. By "collision", we mean an interaction between two objects which is over in a very short time, compared to the time of observation. With this as a working definition, we may divide time up into three regions, before collision, during collision and after collision.

Collisions may be either elastic or inelastic. Linear momentum is conserved in both cases. A perfectly elastic collision is defined as one in which the total kinetic energy of the particles is also conserved:

(Elastic) (5.9)

The word "perfectly" is often omitted. Notice that this is a scalar equation. During an elastic collision the kinetic energy of the particles is wholly or partly stored as potential energy and then completely recovered as kinetic energy. Colli­sions between hard steel balls come close to being elastic. In atomic and nuclear systems, elastic collisions are quite common.

In an inelastic collision, the total kinetic energy of the particles changes. Some of the kinetic energy is stored as potential energy associated with a change in internal structure or state, and is not immediately recovered. Some of the kinetic energy may be used to raise the system (e.g., an atom) to a state with higher energy. Or, it may be converted into thermal energy of vibrating atoms and molecules or into light, sound, or some other form of energy. (The total energy, which includes all forms, is always conserved.) In a completely inelastic collision, the two bodies couple or stick together. You may come across the term superelastic collision. This refers to the possibility that the total kinetic energy actually increases as a result of the collision. This may occur because a compressed spring, or an explosive charge, is triggered and releases stored energy.

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