The momentum of a body of given mass is proportional to its

$A$ javelin throw is marked foul if an athlete crosses over the line marked for throw. Explain why the athletes often fail to stop themselves before the line.

$A$ large truck and a mini bus both moving with a velocity of magnitude $V$ have a head-on collision and both of them come to a halt after that. If the collision lasts for $1 \, s$:
$(a)$ Which vehicle experiences the greater force of impact?
$(b)$ Which vehicle experiences the greater change in momentum?
$(c)$ Which vehicle experiences the greater acceleration?

The $SI$ unit of force is

Give reasons and state the laws related to these statements:
$(a)$ It is easier to push an empty box than to push a box full of books.
$(b)$ It is difficult for a fireman to hold a hose which ejects a large amount of water with high velocity.

Give an example of a force that merely changes the direction of motion of a body.