Work and powerIB MYP Physics: Subtopic test

1. 1
   A removal worker in Lagos pushes a crate across a warehouse floor with a constant horizontal force of 150 N. The crate moves 8.0 m in 20 s at a steady speed.

   (a)

   How much work does the worker do on the crate?

   [1 mark]

   • A19 J
   • B1200 J
   • C24 000 J
   • D158 J

   (b)

   What is the power developed by the worker?

   [1 mark]

   • A7.5 W
   • B24 000 W
   • C0.40 W
   • D60 W

   (c)

   The crate moves at a steady speed, so its kinetic energy does not change. Explain where the energy transferred by the worker goes.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A motor in a lift at an office tower in Singapore raises the lift car and its passengers, which have a total weight of 8000 N, through a height of 30 m in 40 s.

   (a)

   How much work does the lift motor do in raising the lift car?

   [1 mark]

   • A267 J
   • B8030 J
   • C240 000 J
   • D9 600 000 J

   (b)

   The motor is replaced by a different motor which does the same lift in 30 s. What is the power of the new motor?

   [1 mark]

   • A8.0 kW
   • B6.0 kW
   • C7200 kW
   • D8000 kW

   (c)

   Explain why the new motor has a greater power than the original motor even though it does not do more work.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates whether carrying a bag changes the power she develops when she runs up a staircase. The staircase rises 4.0 m vertically and the student has a weight of 500 N. Without a bag, she completes the climb in 5.2 s, 4.8 s and 5.0 s on three trials, timed with a stopwatch.

   (a)

   Identify the independent variable, the dependent variable and one control variable in an investigation into the effect of carrying a bag on her power.

   [3 marks]

   (b)

   The work she does is her weight multiplied by the height climbed. Calculate the mean time and the power she develops without the bag, and explain why she repeats the timings.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A school in Abu Dhabi replaces 200 halogen bulbs, each with a power of 50 W, with 200 LED bulbs, each with a power of 8 W. The lights are switched on for 10 hours every day.

   (a)

   Calculate the energy saved by the school each day, in joules, by using the LED bulbs instead of the halogen bulbs.

   [6 marks]

   (b)

   Discuss and evaluate whether the school should replace its halogen bulbs with LED bulbs.

   [6 marks]

   Total for question 4: 12 marks

End of questions