Physics Exam Questions

• Figure 1 shows a jet engine.

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

  State what happens to the momentum of the air as it passes through the engine. (1)

  (1) Momentum increases

  
• Explain, using appropriate laws of motion, why the air exerts a force on the engine in the forward direction. (3)

  (1) (rate of change of momentum so) force acting on the air (Newton 2)

  (1) air exerts force (on engine) of the same/equal magnitude/size

  (1) but opposite in direction (Newton 3)
• In one second a mass of 210 kg of air enters at A. 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. (1)

  (1) (use of F = ^mv/t) F = 210 × 570 = 120 000 (N) (119 700)
• 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 Figure 2.

  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)

  (1) momentum is a vector OR momentum has direction

  (1) there is a change( in the air’s)direction

  
• 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 × 10^4 kg. (1)

  (1) (use of F = ma) a = (-) 190 000/7.0 × 104 = 2.7 (2.71) (m s-2)
• 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)(use of v^2 = u^2 +2as) 0 = 682 – 2 × 2.7 × s

  s = 682/(2 × 2.7)= 860 (m) (856)

  accept range 850 – 860
• Suggest why in practice the decelerating force provided by the deflector plates may not remain constant. (2)

  (1) rate of intake of air decreases (as plane slows) OR volume/mass/amount of air (passing through engine) per second decreases

  (1) smaller rate of change of momentum.
• Figure 3 is a diagram of a microwave oven.

  A student wants to use the stationary waves formed in the microwave oven to measure the frequency of the microwaves emitted by the transmitter.

  Suggest how stationary waves are formed in the microwave oven. (2)

  (1) waves are reflected (from the oven wall)

  (1) and superpose/interfere with wave travelling in opposite direction/incident waves/transmitted wave

  
• The student removes the turntable and places a bar of chocolate on the floor of the oven. He then switches the oven on for about one minute. When the chocolate is removed the student observes that there are three small patches of melted chocolate with unmelted chocolate between them. Figure 4 is a fullsized diagram of the chocolate bar.

  Suggest why the chocolate only melts in the positions shown. (2)

  (1) energy/amplitude is maximum

  (1) (chocolate melts at) antinode

  
• Calculate, by making suitable measurements on Figure 4, the frequency of the microwaves used by the oven. (5)

  clear evidence that used first and third antinode distance from first to third antinodes= 0.118±0.001 (m)

  OR distance between two adjacent antinodes = 0.059±0.001(m) wavelength = 0.118 (m)

  frequency = 3.0 × 108 /0.118

  frequency = 2.5 × 109 (Hz)
• Explain why most microwave ovens contain a rotating turntable on which the food is placed during cooking. (1)

  (1) position of antinode/maximum energy/maximum amplitude/nodes (in food) continually changes
• State what is meant by tensile stress and tensile strain. (2)

  (1) tensile stress is the force exerted per/over crosssectional area

  (1) tensile strain is the extension per/over original length
• Figure 5 shows the tensile stress–tensile strain graphs for four materials, A, B, C and D, up to their breaking stress.

  Identify a property of material A using evidence from the graph to support your choice. (2)/(4)

  (1) material is brittle

  (1) shown on graph by little or no of plastic behaviour OR by linear behaviour/straight line to breaking stress

  OR

  (1) material has high Young modulus OR material is stiff

  (1) shown on graph by large gradient/steep line (compared to other materials)

  
• A cylindrical specimen of material A under test has a diameter of 1.5 × 10^–4 m and a breaking stress of 1.3 GPa. Calculate the tensile force acting on the specimen at its breaking point. (3)

  (1) area = pi × (1.5 × 10-4)2/4 = 1.77 × 10^-8

  (1) tensile force = 1.77 × 10^-8

  (1) 23 (N)