$A$ cricket ball of mass $70 \, g$,moving with a velocity of $0.5 \, m s^{-1}$ is stopped by a player in $0.5 \, s$. What is the force applied by the player to stop the ball (in $, N$)?

The rate of change of momentum is

$(a)$ Two objects of masses $100\, g$ and $200\, g$ are moving along the same line and direction with velocities $2\, m s^{-1}$ and $1\, m s^{-1}$ respectively. They collide and after the collision,the first object moves at a velocity of $1.67\, m s^{-1}$. Determine the velocity of the second object.
$(b)$ If a man jumps out from a boat,the boat moves backwards. Why?

Define $1\, kg$ weight and express it in newton.

The $SI$ unit of force is

State the law of conservation of momentum. How does conservation of momentum take place in each of the following?
$(i)$ $A$ rocket taking off from ground.
$(ii)$ Flying of a jet aeroplane.