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 ..........

$A$ heavy steel ball of mass greater than $1\, kg$ moving with a speed of $2\, m/s$ collides head-on with a stationary ping-pong ball of mass less than $0.1\, g$. The collision is elastic. After the collision,the ping-pong ball moves approximately with a speed of ......... $m/s$.

$A$ body of mass $30 \ kg$ moving with a velocity $20 \ ms^{-1}$ undergoes a one-dimensional elastic collision with another ball of the same mass moving in the opposite direction with a velocity of $30 \ ms^{-1}$. After the collision,the velocities of the first and second bodies respectively are:

It is found that if a neutron suffers an elastic collinear collision with deuterium at rest,the fractional loss of its energy is $P_d$; while for its similar collision with a carbon nucleus at rest,the fractional loss of energy is $P_c$. The values of $P_d$ and $P_c$ are respectively:

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.

$A$ smooth sphere $A$ moves on a frictionless horizontal surface with an angular velocity $\omega$ and a linear velocity $v$ of its center of mass. It undergoes an elastic collision with another identical sphere $B$ at rest. Neglecting friction everywhere,if the angular speeds after the collision are $\omega_A$ and $\omega_B$ respectively,then: