If force on a rocket having exhaust velocity of 300 m/sec is 210 N, then rate of combustion of the fuel is 

(1) 0.7 kg/s

(2) 1.4 kg/s

(3) 0.07 kg/s

(4) 10.7 kg/s

A 5000 kg rocket is set for vertical firing. The exhaust speed is 800 ms^–1. To give an initial upward acceleration of 20 ms^–2, the amount of gas ejected per second to supply the needed thrust will be (g = 10 ms^–2) 

(1) 127.5 kg s^–1

(2) 187.5 kg s^–1

(3) 185.5 kg s^–1

(4) 137.5 kg s^–1

A rocket has a mass of 100 kg. 90% of this is fuel. It ejects fuel vapours at the rate of 1 kg/sec with a velocity of 500 m/sec relative to the rocket. It is supposed that the rocket is outside the gravitational field. The initial upthrust on the rocket when it just starts moving upwards is:

(1) Zero

(2) 500 N

(3) 1000 N

(4) 2000 N

Rocket engines lift a rocket from the earth surface because hot gases with high velocity:

(1) push against the earth.

(2) push against the air.

(3) react against the rocket and push it up.

(4) heat up the air which lifts the rocket.