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

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 ..........

Two masses $m_{a}$ and $m_{b}$ moving with velocities $v_{a}$ and $v_{b}$ in opposite directions collide elastically. After the collision,$m_{a}$ and $m_{b}$ move with velocities $v_{b}$ and $v_{a}$ respectively. The ratio $m_{a} : m_{b}$ is:

$A$ smooth sphere $A$ is moving on a frictionless horizontal plane with an angular speed $\omega$ and its center of mass has a linear velocity $v$. It undergoes an elastic collision with an identical sphere $B$,which is initially at rest. After the collision,their angular speeds are $\omega_A$ and $\omega_B$ respectively. Neglecting friction,which of the following is true?

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

The coefficient of restitution $e$ for a perfectly elastic collision is