Work, energy and powerAQA A-Level Physics: Subtopic test

1. 1
   A student pulls a suitcase across a level floor using a handle. The handle makes an angle of 30° above the horizontal and the force in the handle is a constant 40 N. The suitcase moves 25 m along the floor in 20 s.

   (a)

   What is the work done by the student on the suitcase?

   [1 mark]

   • A1000 J
   • B870 J
   • C500 J
   • D1150 J

   (b)

   What is the average power developed by the student?

   [1 mark]

   • A50 W
   • B25 W
   • C870 W
   • D43 W

   (c)

   The student raises the handle so that it makes an angle of 60° above the horizontal, with the same force and distance. State and explain the effect on the work done.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A car travels along a straight, level road at a constant speed of 25 m s⁻¹. The total resistive force on the car is 600 N. The engine's fuel supplies energy at the rate of 60 kW.

   (a)

   What is the driving force on the car?

   [1 mark]

   • A24 N
   • B15 000 N
   • C600 N
   • D625 N

   (b)

   What is the useful power delivered to the wheels?

   [1 mark]

   • A15 kW
   • B24 W
   • C0.60 kW
   • D625 W

   (c)

   Calculate the efficiency of the car's engine and drive system.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A crate of mass 20 kg is at rest on a smooth horizontal floor. A horizontal force pushes the crate in a straight line. The force increases uniformly from 0 to 80 N over the first 5.0 m of the crate's motion, then stays constant at 80 N for a further 3.0 m. Friction is negligible.

   (a)

   State the significance of the area under a force-displacement graph and calculate the work done on the crate in the first 5.0 m.

   [3 marks]

   (b)

   Calculate the speed of the crate after it has moved the full 8.0 m.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A cyclist and bicycle have a total mass of 85 kg. The cyclist climbs a straight hill, inclined at 6.0° to the horizontal, at a constant speed of 5.0 m s⁻¹. The total resistive force is 20 N, assumed constant. Take g = 9.81 N kg⁻¹. The cyclist's body is 24% efficient at converting chemical energy to useful work.

   (a)

   Calculate the useful power the cyclist must deliver and the rate at which the cyclist's body must transfer chemical energy.

   [6 marks]

   (b)

   A manufacturer fits the bicycle with an electric motor that provides a useful power of 250 W. Evaluate whether the motor alone could keep the cyclist climbing at 5.0 m s⁻¹, and determine the effect on the power needed of doubling the speed. Assume the resistive force stays at 20 N.

   [6 marks]

   Total for question 4: 12 marks

End of questions