Question

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

Answer 1

(i) When a rocket takes off, hot burning gases eject at very high pressure and speed in a downward direction. As the linear momentum of the gases and the rocket is conserved before ejaculation of the gases. Hence, to keep the momentum conserved after the ejection of high-speed gases, an upward thrust works on the rocket which takes it upward with high acceleration. That's how a rocket takes off of the ground.

(ii) In a Jet aeroplane, the rate of burning of the fuel and the rate of ejection of exhaust gases is kept in such a way, that it provides the right amount of forwarding push which helps the jet to gain the desired change in momentum and accelerate accordingly.

So, the conservation of momentum is also applicable here. Ejection of exhaust gases at high speed in the backward direction disturbs the linear momentum of the jet. Hence, a forward thrust acts on the jet which accelerates it in such a way that it suffers an equal change in momentum in forwarding direction. That's how a jet aeroplane flies at high speeds.