When two particles collide elastically,then impulsive force can't change

The rebound coefficient between a tennis ball and a racket is defined as $e = v_2/v_1$,where $v_1$ is the incoming speed of the ball and $v_2$ is the speed of the ball after rebound while the racket is at rest. $A$ tennis ball falls from height $H$ to a racket at rest and bounces back to $0.8 H$. $A$ tennis player is using the racket to hit an incoming tennis ball traveling at $150 \ km/hr$ and the racket is moving at $100 \ km/hr$ towards the ball. What is the speed of the ball after being hit? (Assume the mass of the racket $\gg$ that of the ball).

Which one of the following statements does not hold good when two balls of masses $m_1$ and $m_2$ undergo elastic collision?

$A$ sphere of mass $0.1 \ kg$ is attached to a string of length $1 \ m$. It is released from a horizontal position and collides with another sphere of equal mass $m$ at rest. Find the kinetic energy gained by the second sphere. The collision is perfectly elastic. (in $J$)

$A$ ball is thrown vertically downwards with velocity $\sqrt{2gh}$ from a height $h$. After colliding with the ground,it just reaches the starting point. The coefficient of restitution is:

$A$ ball of mass $1 \ kg$ moving along the $X$-direction collides elastically with a stationary ball of mass $m$. The first ball (mass $= 1 \ kg$) recoils at a right angle to its original direction of motion. If the second ball starts moving at an angle $30^{\circ}$ with the $X$-axis,the value of $m$ must be (in $kg$)