$A$ sledge of mass $80 \, kg$ moves over the frozen surface of a lake with a velocity of $30 \, m s^{-1}$ and comes to rest in $5 \, s$. Find its negative acceleration and the frictional force between the lake and the sledge.

Amar, Javed, and Rita were riding in a bus that was moving with a high velocity on a highway when a bug hit the windshield and its dead body got stuck on the windscreen. Amar and Javed began discussing the situation. Amar suggested that the change in momentum of the bug is much greater than that of the bus. After all, argued Amar, there was no noticeable change in the velocity of the bus compared to the obvious change in the velocity of the bug. Javed opined that since the bus was moving with a larger velocity, so it exerted a larger force on the bug, as a result of which it died. Rita disagreed entirely, arguing that both the bug and bus experienced the same force and a change in their momentum. Who do you agree with? Support your answer.

Two persons manage to push a motorcar of mass $1200 \, kg$ at a uniform velocity along a level road. The same motorcar can be pushed by three persons to produce an acceleration of $0.2 \, m \, s^{-2}$. With what force (in $N$) does each person push the motorcar? (Assume that all persons push the motorcar with the same muscular effort.)

Give the magnitude and direction of the net force acting on:
$(a)$ a drop of rain falling down with a constant speed.
$(b)$ a cork of mass $10 \,g$ floating on water.
$(c)$ a kite skilfully held stationary in the sky.

An object of mass $1 \,kg$ travelling in a straight line with a velocity of $10\, m\, s^{-1}$ collides with,and sticks to,a stationary wooden block of mass $5\, kg$. Then they both move off together in the same straight line. Calculate the total momentum just before the impact and just after the impact. Also,calculate the velocity of the combined object.

$A$ machine gun can fire $50 \, g$ bullets with a velocity of $150 \, m s^{-1}$. $A$ $60 \, kg$ stone is moving towards the machine gun with a velocity of $10 \, m s^{-1}$. How many bullets must be fired from the gun to just stop the stone in its tracks?