$A$ body of mass $m$ moving with velocity $v$ collides head-on with another body of mass $2m$ which is initially at rest. The ratio of the $K.E.$ of the colliding body before and after the collision will be: (in $: 1$)

$A$ 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 ......... $m/s$.

On a horizontal smooth surface,a disc of radius $r$ is placed at rest. Another disc of the same mass is moving towards it with an impact parameter equal to its own radius. What should be the radius of the incoming disc so that after the collision,the first disc moves at an angle of $45^{\circ}$ to the direction of motion of the incoming disc?

$A$ body of mass $m$ moving with velocity $v$ makes a head-on collision with another body of mass $2m$ which is initially at rest. What is the ratio of the kinetic energy of the colliding body before and after the collision (in $: 1$)?

When two bodies collide elastically,then

$Assertion$: $n$ small balls each of mass $m$ collide elastically each second on a surface with velocity $u$. The force experienced by the surface is $2mnu$.
$Reason$: On elastic collision,the ball rebounds with the same velocity.