Collision is a situation in which interacting bodies experience large force for a short interval of time. The line along which the internal forces act during collision is known as the line of collision. Franck Hertz experiment explains about the elastic and inelastic collision.

Depending upon the velocity of the body with respect to line of collision the collisions are of two types:

It happens when velocities of both the particles are along the line of collision (as shown in the figure). This can be regarded as a collision in one dimension.

### Oblique Collision:

It happens when any of the two bodies have velocity at an angle with the line of collision. In case of an oblique collision the component of velocity perpendicular to the line of collision remains unchanged. This can be regarded as collision in two dimensions.

Apart from the above two classification collisions can also be classified on the basis of whether kinetic energy remains constant or not. If the kinetic energy of the system remains constant then it is known as elastic collision. While there are situations when some of the kinetic energy gets converted into heat, deformation of shape etc. These types of collision are known as inelastic collision.

We often define another quantity known coefficient of restitution (e). It is defined as ratio of velocity of separation by velocity of approach along the line of collision. So here,

$e$ = $\frac{v_2 – v_1}{u_1 – u_2}$

Depending on the values of e we can infer the type of collision.

For e = 1, Elastic collision

0 < e < 1, inelastic collision

e = 0, perfectly inelastic collision