Define the following.
Coefficient of restitution

Explanation

Co-efficient of restitution is defined as the ration of velocity of separation (relative velocity) after collision to the velocity of approach (relative velocity) before collision, i.e.,
`e=("velocity of separation (after collision)")/("velocity of approach (before collision)")`
`=((v_(2)-v_(1)))/((u_(1)-u_(2)))`