A crane lifts a steel beam vertically upwards at a steady speed. The crane's motor transfers 18 000 J of energy to the beam in 12 seconds. Explain what is meant by the power of the crane's motor and describe how the values given allow you to determine it, stating whether this power output is relatively large or small compared to a typical household light bulb rated at 60 W.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A crane on a construction site lifts a steel beam vertically upwards at a steady speed. The crane's motor transfers 18 000 J of energy to the beam in 12 seconds.
Model answer (4 marks)
Power is the rate at which energy is transferred, i.e. the work done per unit time.
The power of the crane’s motor is calculated by dividing the energy transferred (18 000 J) by the time taken (12 s):
P = 18 000 J ÷ 12 s = 1 500 W.
A power output of 1 500 W is far greater than the 60 W rating of a typical household light bulb, so the crane’s motor has a much larger power output.
The power of the crane’s motor is calculated by dividing the energy transferred (18 000 J) by the time taken (12 s):
P = 18 000 J ÷ 12 s = 1 500 W.
A power output of 1 500 W is far greater than the 60 W rating of a typical household light bulb, so the crane’s motor has a much larger power output.
Examiner tips
- Define power as energy per unit time; show the calculation; compare to 60 W; keep answer concise and use correct units.
- Use the word ‘rate’ and include the division step; state the numerical result explicitly; mention ‘far greater’ to show comparison.
Common mistakes
- Using the wrong unit (e.g. J/s instead of W); forgetting to divide by time; mis‑calculating 18 000 ÷ 12 as 1 500; not comparing to 60 W or using the wrong comparison word.
Mark scheme (4 marks)
- Power is the rate at which energy is transferred (or the rate at which work is done)
- Power is calculated by dividing the work done (energy transferred) by the time taken
- Correct power value stated as 1 500 W
- 1 500 W is much greater than 60 W, so the crane's motor has a much larger power output than the light bulb
Key terms in this question
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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