State the law of conservation of momentum. How does conservation of momentum take place in each of the following?
$(i)$ $A$ rocket taking off from ground.
$(ii)$ Flying of a jet aeroplane.

(N/A) Law of conservation of momentum:
It states that in an isolated system,the total momentum of the system remains conserved (constant) if no external unbalanced force acts on it.
$(i)$ $A$ rocket taking off from ground: The chemicals inside the rocket burn and produce a high-velocity blast of hot gases. These gases are ejected through the tail nozzle in the downward direction. According to the law of conservation of momentum,the rocket gains an equal and opposite momentum in the upward direction,causing it to lift off.
$(ii)$ Flying of a jet aeroplane: In jet aeroplanes,a large volume of gases produced by the combustion of fuel is ejected through a jet in the backward direction at high velocity. The backward momentum of these gases results in an equal and opposite forward momentum for the aeroplane,which provides the necessary thrust for flight.