State the law of conservation of momentum. How does conservation of momentum take place in each of the following?

(a) a rocket taking off from the ground

(b) flying of a jet aeroplane

Law of conservation of momentum consists of two laws:

1) Law of linear momentum

When the external force on a system of bodies is zero, then the total linear momentum of the system of bodies remains constant.

2) Law of angular momentum

When the external torque acting on a system of bodies is zero, then the angular momentum of the system of bodies is a constant.

(a) Hot gases and jet are the two separate components. Before the hot gases are ejected, the total linear momentum is zero (stationary). The total instantaneous momentum of both is conserved. The rate of change in the momentum of hot gases downward direction is equal to the rate of change in momentum of the rest of rocket in vertically upward direction. So it takes off at very high acceleration, due to the thrust.

(b) Fuel and jet are two components. The rates of burning and ejection of exhaust gases in the fuel burning chamber are such that the desired increase in the momentum is given to jet in the forward direction.