Questions on Newton's Laws

Q2.

The diagram shows a jet engine.

Air enters the engine at A and is heated before leaving B at a much higher speed.

(a) State what happens to the momentum of the air as it passes through the engine.

[1 mark]

(b) Explain, using appropriate laws of motion, why the air exerts a force on the engine in the forward direction.

[3 marks]

The speed of this mass of air increases by 570 m s^–1 as it passes through the engine.

Calculate the force that the air exerts on the engine.

[2 marks]

(d) When an aircraft lands, its jet engines exert a decelerating force on the aircraft by making use of deflector plates.

These cause the air leaving the engines to be deflected at an angle to the direction the aircraft is travelling as shown in the diagram below:

The speed of the air leaving B is the same as the speed of the deflected air.

Explain why the momentum of the air changes.

[2 marks]

(e) The total horizontal decelerating force exerted on the deflector plates of the jet engines is 190 kN.

Calculate the deceleration of the aircraft when it has a mass of 7.0 × 10⁴ kg.

[1 mark]

(f) The aircraft lands on the runway travelling at a speed of 68 m s^–1 with the deflector plates acting.

Calculate the distance the aircraft travels along the runway until it comes to rest.

You may assume that the decelerating force acting on the jet engines remains constant.

[2 marks]

(g) Suggest why in practice the decelerating force provided by the deflector plates may not remain constant.

[2 marks]

(13 marks Total)