Why is it advised to tie luggage with a rope on the roof of buses?

You are hurt when you kick a stone. Why?

While playing football,the goalkeeper did not get sufficient time to stop a fast ball shot towards him. Why did he hurt his hand while doing so?

If a man jumps out from a boat to the bank of the river,the boat moves backwards. Identify the action and reaction in this situation.

$(a)$ 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?

$(a)$ Define momentum. Write its $SI$ unit.
$(b)$ $A$ bullet of mass $0.02 \ kg$ is fired from a gun weighing $7.5 \ kg$. If the initial velocity of the bullet is $200 \ m \ s^{-1}$,calculate the speed with which the gun recoils.