If someone jumps to the shore from a boat,the boat moves in the opposite direction. Explain.

$A$ man throws a ball of mass $0.5\, kg$ vertically upward with a velocity of $25\, m s^{-1}$. Find:
$(a)$ The initial momentum of the ball.
$(b)$ The momentum of the ball at the halfway mark of the maximum height (given $g = 10\, m s^{-2}$).

Using the second law of motion,derive the relation between force and acceleration. $A$ bullet of $10 \, g$ strikes a sandbag at a speed of $10^3 \, m \, s^{-1}$ and gets embedded after traveling $5 \, cm$. Calculate:
$(i)$ The resistive force exerted by the sand on the bullet.
$(ii)$ The time taken by the bullet to come to rest.

Suppose a ball of mass $m$ is thrown vertically upward with an initial speed $v$. Its speed decreases continuously until it becomes zero. Thereafter,the ball begins to fall downward and attains the speed $v$ again before striking the ground. It implies that the magnitude of the initial and final momentum of the ball are the same. Yet,it is not an example of the conservation of momentum. Explain why?

If a body is in motion,is it necessary that it is being acted upon by a force?

The action-reaction forces: