You are hurt when you kick a stone. Why?

An apple falls from a tree because of gravitational attraction between the earth and the apple. If $F_1$ is the magnitude of force exerted by the earth on the apple and $F_2$ is the magnitude of force exerted by the apple on the earth,then:

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?

Two friends on roller-skates are standing $5\, m$ apart facing each other. One of them throws a ball of $2\, kg$ towards the other,who catches it. How will this activity affect the position of the two? Explain your answer.

$A$ girl of mass $50 \, kg$ jumps out of a moving boat of mass $300 \, kg$ onto the bank with a horizontal velocity of $3 \, m s^{-1}$. With what velocity will the boat begin to move backwards (in $, m s^{-1}$)?

$A$ water sprinkler used for grass lawns begins to rotate as soon as the water is supplied. Explain the principle on which it works.