What is a head-on collision?

An object of mass $m_{1}$ collides with another object of mass $m_{2}$,which is at rest. After the collision,the objects move with equal speeds in opposite directions. The ratio of the masses $m_{2} : m_{1}$ is

In a carrom-board game,the striker and the coins are identical and of mass $m$. In a particular hit,the coin is hit when it is placed close to the edge of the board as shown in the figure,such that the coin travels parallel to the edge after the collision. If the striker is moving with speed $v$ before the strike,then the net impulse on the striker during the collision,if it moves perpendicular to the edge after the collision,is (assume all collisions to be perfectly elastic):

$A$ uniform rod of mass $M$ and length $L$ is placed on a smooth horizontal surface. $A$ particle of mass $m$ moving with velocity $v$ strikes the rod at one end perpendicularly and comes to rest. The velocity of the centre of mass of the rod after the collision is:

If $n$ balls hit elastically and normally on a surface per unit time and all balls of mass $m$ are moving with the same velocity $u$, then the force on the surface is ..........

Which of the following potential energy curves in the figure cannot possibly describe the elastic collision of two billiard balls? Here $r$ is the distance between the centres of the balls.