(a) State the law of conservation of momentum.
(b) Discuss the conservation of momentum in each of the following cases :
(i) a rocket taking off from ground.
(ii) flying of a jet aeroplane.

(a) The law of conservation of momentum states the total momentum of a system remains unchanged in the absence of any external force.

(b) (i) A rocket taking off from ground
Answer: In rocket, a large volume of gases produced by the combustion of fuel is allowed to escape through it in the backward direction. Due to the very high velocity, the backward rushing gases have a large momentum. They impart an equal and opposite momentum to the rocket due to which the rocket moves forward with a great speed.

(ii) Jet aeroplanes utilize the principle of action and reaction. In modern jet aircrafts, the hot gases obtained by the rapid burning of fuel rush out of a jet at the rear end of the aircraft at a great speed. The equal and opposite momentum of the gases heading backwards pushes the aircraft forward at a great speed.