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.