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 ..........

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

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?

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

$A$ particle of mass $m$ moving with velocity $v$ makes a perfectly elastic collision with a simple pendulum bob of mass $m$. After the collision,the maximum height attained by the bob is:

$Assertion$ : If collision occurs between two elastic bodies,their kinetic energy decreases during the time of collision.
$Reason$ : During collision,intermolecular space decreases and hence elastic potential energy increases.