Q180S – Jets and rockets
These short questions increase in difficulty and ask you to relate impulse to change of momentum.
1.	Thrust SSC is a supersonic car powered by 2 jet engines giving a total thrust of 180 kN. 
	Calculate the impulse applied to the car when the engines run for 4 seconds.

Assume the thrust is the only force acting on the car, which has a mass of 10 000 kg.  Calculate the increase in speed of the car after the 4 s.

2.	One suggestion for powering spacecraft engines of the future is an ion engine. A beam of ions (charged atoms) is fired backwards, propelling the spacecraft forwards. In one test, xenon ions were used.   Consider how using xenon ions would compare with using krypton ions, which are lighter.  If equal numbers of each ion were propelled back per second, at the same speed, which type of ion would you expect to give more thrust? Explain your answer

The mass of a xenon ion is 2.2 ´ 10–25 kg, and it can be ejected at a speed of 3.1 ´ 104 m s–1.   Calculate the number of ions that would have to be emitted per second to generate a thrust of 0.1 N (a typical value of the thrust from such an engine).

3.	When holding a hose firefighters need to ensure that they are not pushed backwards, especially if the water is ejected at a high speed.  Explain why firefighters experience a backwards force. 

20 kg of water is ejected horizontally in 10 s, the speed of the water leaving the nozzle is 30 m s–1.
Calculate the force experienced by a firefighter holding the hose.

4.	A spacecraft is approaching the planet Zog and needs to slow down. To do so it fires a jet of gas forwards. 
Explain how firing gas forwards slows the rocket down. 

The rocket has a mass of 50 000 kg. The gas can be fired forward at a speed of 5 000 m s-1 relative to the rocket. 
Calculate the mass of gas must the rocket eject to reduce its speed by 5 m s-1. Ignore the change in the rocket’s mass due to the ejection of gas.

5.	Air of density 1.3 kg m–3 strikes a sail of area 15 m2. The air is initially moving at 5 m s–1 and assume it is brought to rest when it hits the sail. 
Calculate the mass of air brought to rest in each second. 

Hence calculate the force the air exerts on the sail.

6.	A kestrel is a bird of prey which searches for prey by hovering above grassy areas. 
	Using your ideas about momentum, suggest how a kestrel is able to hover.

The kestrel has a mass of 200 g and it pushes down a column of air of area 600 cm2. 
Estimate the downward speed given to the air by the kestrel
g = 9.8 N kg–1, density of air = 1.3 kg m–3.

Estimate the minimum power the kestrel needs to hover. 

Suggest why there are no large birds which can hover. (Some large birds, such as buzzards and condors, may appear to hover, but are not really doing so. They use upwards currents of air – ‘thermals’ – to stay up.)