A worker uses a machine to lift heavy crates from the ground onto a shelf. The machine takes longer than expected to complete the task. Explain what is meant by the power of the machine and describe what effect the longer time taken has on the power of the machine, assuming the same crates are lifted to the same shelf.

Edexcel GCSE Physics (1PH0) — 8.1 Energy — forces doing work · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Power is the rate at which energy is transferred, i.e. the rate at which work is done.
Power = work ÷ time.
The work done lifting the crates to the shelf is the same as before – the energy transferred is unchanged.
Because the time taken is longer, the denominator in the power equation increases, so the power of the machine decreases.

Examiner tips

  • Define power as rate of energy transfer or work per unit time; include the formula. State that work remains unchanged. Explain that increased time reduces power. Use correct units (W = J s⁻¹).

Common mistakes

  • Confusing power with work or energy. Forgetting to mention that work stays the same. Using the wrong formula (e.g. power = force × distance).

Mark scheme (4 marks)

  1. Power is the rate at which energy is transferred (or the rate at which work is done)
  2. Power is calculated by dividing the work done (or energy transferred) by the time taken
  3. The work done (energy transferred) lifting the crates to the shelf remains the same / is unchanged
  4. Because time taken increases (and work done stays the same), the power of the machine decreases

Key terms in this question

power · time taken

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