What is the mathematical formula and $SI$ unit of momentum?

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

When a net force acts on an object,the object will be accelerated in the direction of the force with an acceleration proportional to:

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.

An object of mass $2\, kg$ is sliding with a constant velocity of $4\, m\, s^{-1}$ on a frictionless horizontal table. The force required to keep the object moving with the same velocity is (in $, N$)

Explain the following:
$(i)$ Why does a boatman push the river bank backward with a long bamboo pole when launching his boat into the water?
$(ii)$ Why does a boat tend to leave the shore when passengers are alighting from it? How does the boatman overcome this difficulty?
$(iii)$ Why does a gun recoil when fired?
$(iv)$ How do rockets operate in space?