$A$ man weighing $60 \ kg$ runs along the rails with a velocity of $18 \ km \ h^{-1}$ and jumps into a car of mass $1$ quintal $(100 \ kg)$ standing on the rails. Calculate the velocity with which the car will start travelling along the rails. (in $m \ s^{-1}$)

Newton's second law of motion gives us a measure of

The $SI$ unit of force is

Explain the following:
$(i)$ Why do the doors of a corridor train open or shut when the train starts or stops?
$(ii)$ Why is it necessary to run along with a moving bus in the same direction while alighting from it?
$(iii)$ Why does a player run for some distance before taking a long jump?
$(iv)$ Why does a person fall off the back of a stationary horse if the horse starts off suddenly?
$(v)$ Why is a neat hole formed in a glass pane if a bullet is fired at it from a close range?

$(i)$ Name the property of bodies to resist a change in their velocity.
$(ii)$ What is the relationship between force and acceleration?
$(iii)$ What name is given to the product of mass and velocity of a body?
$(iv)$ Which physical quantity corresponds to the rate of change of momentum?
$(v)$ Name the principle on which a rocket works.

What is momentum? Write its $SI$ unit. Interpret force in terms of momentum. Represent the following graphically:
$(a)$ Momentum versus velocity when mass is fixed.
$(b)$ Momentum versus mass when velocity is constant.