A tennis ball is dropped on a horizontal smooth surface. It bounces back to its original position after hitting the surface. The force on the ball during the collision is proportional to the length of compression of the ball. Which one of the following sketches describes the variation of its kinetic energy $K$ with time $t$ most appropriately? The figures are only illustrative and not to the scale.

$A$ section of fixed smooth circular track of radius $R$ in vertical plane is shown in the figure. $A$ block is released from position $A$ and leaves the track at $B$. The radius of curvature of its trajectory when it just leaves the track at $B$ is:

A small ball falling vertically downward with constant velocity $4m/s$ strikes elastically $a$ massive inclined cart moving with velocity $4m/s$ horizontally as shown. The velocity of the rebound of the ball is

A bomb is projected upwards. At topmost point it explodes in three identical fragments. First fragment comes to ground in $10\  sec$. and others in $20\  sec$ each. Then the height reached by the original bomb is.........$m$

In an elastic collision of two billiard balls, which of the following quantities remain conserved during the short time of collision of the balls ? (i.e. when they are in contact)

$(a)$ Kinetic energy.

$(b)$ Total linear momentum.

Give reason for your answer in each case.

Write the principle of conservation of mechanical energy for non-conservative force.