A building lift (elevator) carries passengers upwards at a steady speed. Explain why a lift motor with a greater power output is needed to move the same passengers through the same height in a shorter time.

Pearson Edexcel International GCSE Physics (4PH1) — 4.2 Work and power · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Two identical lifts in a hotel carry passengers of the same total weight through the same vertical height. Lift A completes the journey in 20 s, whilst Lift B completes the identical journey in 10 s.

Model answer (4 marks)

Both lifts do the same amount of work because the force required to lift the passengers is the same and the vertical distance is identical. Power is the rate at which work is done, i.e. power = work ÷ time. Therefore, if the same work is completed in a shorter time, the power must be greater. Lift B, which takes 10 s, requires twice the power of Lift A, which takes 20 s, because the same amount of work is done in half the time.

Examiner tips

  • Define work and power clearly; link power to work ÷ time; state that the same work is done in half the time → power doubles.
  • Use the word ‘greater’ or ‘twice’ to show the quantitative relationship.

Common mistakes

  • Confusing power with force; writing ‘force’ instead of ‘power’.
  • Failing to mention that the work is the same for both lifts.

Mark scheme (4 marks)

  1. Both lifts do the same amount of work / transfer the same amount of energy (because force and distance are identical)
  2. Power is the rate at which work is done / energy is transferred
  3. A shorter time (with the same work done) means a greater power is required
  4. Correct qualitative link: Lift B (10 s) requires twice the power of Lift A (20 s) because the same work is done in half the time

Key terms in this question

power

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