Define momentum and state its $SI$ unit. State the principle of conservation of momentum.

$(a)$ Why are the wheels of vehicles provided with mudguards?
$(b)$ $A$ cricket player lowers his hands while catching a ball. Why?
$(c)$ Why is a glass pane of a window shattered when a flying pebble hits it?

$(a)$ Define momentum. Write its $SI$ unit.
$(b)$ $A$ car and a truck have the same momentum. Whose velocity is more and why?
$(c)$ $A$ bullet of mass $20 \ g$ moving with a speed of $500 \ m s^{-1}$ strikes a wooden block of mass $1 \ kg$ and gets embedded in it. Find the speed with which the block moves along with the bullet.

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.

State whether the following statement is true or false:
The force of friction is smaller in solids than in liquids.

Suppose a ball of mass $m$ is thrown vertically upward with an initial speed $v$. Its speed decreases continuously until it becomes zero. Thereafter,the ball begins to fall downward and attains the speed $v$ again before striking the ground. It implies that the magnitude of the initial and final momentum of the ball are the same. Yet,it is not an example of the conservation of momentum. Explain why?