Collision:

When two bodies are approaching each other, a force comes into play between them for a finite time and brings about a measurable change in their velocities, momentum and energy according to the respective laws of conservation, a collision is said to have taken place.

However, in physics, it is clearly understood that collision does not necessarily means physical contact between two bodies. When two bodies or particles collide, a force of interaction between that bodies or particles comes into play during the collision for a finite small time and as a result, a change in their relative motion occurs.

Let us consider the duration of the collision between two colliding bodies or particles be from \( t \) to \( (t+\Delta{t}) \), then a time \( t_1 \) which is less than \( t \) is known as time before collision, and a time \( t_2 \) which is greater than \( (t+\Delta{t}) \) is known as time after collision.

If the nature of the colliding particles do not change after collision then this type of collision is known as scattering.

Examples: the deflection of an \( \alpha \) particle by an atomic nucleus, the deflection of the passing comet near the solar system, etc.