$A$ ball is thrown vertically upwards. What is its momentum at the highest point?

Explain the following:
$(i)$ Why is it difficult to walk on marshy land?
$(ii)$ Why does an air-filled balloon rise up slightly when punctured from below?
$(iii)$ Why does a swimmer push water backward with his hands in order to swim in the forward direction?

$(a)$ Define momentum. Write its $SI$ unit.
$(b)$ $A$ bullet of mass $0.02 \ kg$ is fired from a gun weighing $7.5 \ kg$. If the initial velocity of the bullet is $200 \ m \ s^{-1}$,calculate the speed with which the gun recoils.

Write the relation for the collision of two balls $A$ and $B$ and state the related law.

Show that the rate of change of momentum $=$ mass $\times$ acceleration.

$A$ bullet of mass $10\, g$ is fired from a rifle. The bullet takes $0.003\, s$ to move through its barrel and leaves with a velocity of $300\, m s^{-1}$. What is the force exerted on the bullet by the rifle (in $, N$)?