For a perfectly elastic collision,the coefficient of restitution $e$ is

Six identical balls are lined in a straight groove made on a horizontal frictionless surface as shown. Two similar balls each moving with a velocity $v$ collide elastically with the row of $6$ balls from the left. What will happen?

$A$ particle of mass $m$ moving with a velocity $u$ makes an elastic one-dimensional collision with a stationary particle of mass $m$. They are in contact for a total time $T$. The contact force increases linearly from $0$ to $F_0$ in time $\frac{T}{4}$, remains constant for a further time $\frac{T}{2}$, and decreases linearly from $F_0$ to $0$ in the final time $\frac{T}{4}$, as shown in the graph. The magnitude of $F_0$ is:

$A$ ball of mass $m$ moving with velocity $V$ makes a head-on elastic collision with a ball of the same mass moving with velocity $2V$ towards it. Taking the direction of $V$ as positive,the velocities of the two balls after the collision are:

$A$ ball is dropped from a height of $10 \ m$ and hits the roof of an elevator moving downwards with a velocity of $1 \ m/s$. What is the rebound velocity of the ball in $m/s$?

$A$ neutron with velocity $v$ strikes a stationary deuterium atom. By what factor does its kinetic energy change?