$A$ water tanker filled up to $2/3$ of its height is moving with a uniform speed. On sudden application of the brake,the water in the tank would

$(a)$ State Newton's second law of motion. Use this law to find a method to measure force acting on an object.
$(b)$ From a rifle of mass $4 \ kg$,a bullet of mass $50 \ g$ is fired horizontally with an initial velocity of $40 \ m \ s^{-1}$. Calculate the initial recoil velocity of the rifle.

$(a)$ Define momentum of a body.
$(b)$ $A$ ball is thrown vertically upward. What is its momentum at the highest point?
$(c)$ State the law of conservation of momentum.

Explain the following:
$(i)$ Why do we jerk wet clothes before spreading them on a line?
$(ii)$ Why does dust fly off when a carpet is hit with a stick?
$(iii)$ Why do fruits fall off the branches in a strong wind?
$(iv)$ Why does a pillion rider fall forward when the driver of a two-wheeler suddenly applies brakes?

The rate of change of momentum is

Two cars each of mass $1000 \ kg$ are moving in a straight line but in opposite directions. The velocity of each car is $5 \ m s^{-1}$ before the collision during which they stick together. What will be the velocity of the combined cars after collision (in $m s^{-1}$)?