A firefighter climbs a vertical ladder to reach the top floor of a burning building. A second firefighter, who has a greater body mass, climbs the same ladder to the same height in a shorter time. Explain why the second firefighter has a greater power output than the first, even though both firefighters do work against gravity.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Two firefighters climb an identical vertical ladder. Firefighter A has a mass of 70 kg and reaches the top in 12 s. Firefighter B has a mass of 90 kg and reaches the top in 8 s.
Model answer (4 marks)
Work done against gravity is W = Fd, where F = mg. Firefighter B has a larger mass, so his weight mg is greater and he does more work climbing the same height.
Power is the rate of doing work: P = W/t. B’s work is larger and his time t is shorter, so both a larger numerator and a smaller denominator give a higher power.
Therefore B’s power output is greater than A’s, even though both do work against gravity.
Power is the rate of doing work: P = W/t. B’s work is larger and his time t is shorter, so both a larger numerator and a smaller denominator give a higher power.
Therefore B’s power output is greater than A’s, even though both do work against gravity.
Examiner tips
- Show the formula W = mgd and note B’s larger m. State P = W/t and explain why a larger W and smaller t give higher P. Use the word ‘greater’ or ‘larger’ to compare B to A. Keep the answer to 4–5 short sentences.
Common mistakes
- Using the wrong formula for work (e.g. W = Fv instead of W = Fd). Forgetting that power is work divided by time. Not recognising that B’s mass is larger, so his weight and work are larger. Mixing up the direction of force or the sign of work.
Mark scheme (4 marks)
- Work done equals force multiplied by distance (W = Fd), and a greater mass means a greater gravitational force acting downwards, so Firefighter B does more work climbing the same height.
- Power is the rate at which work is done (power = work done ÷ time taken), so a greater amount of work done in a shorter time gives a larger power output.
- Firefighter B does more work than Firefighter A (because greater mass → greater weight → greater work done against gravity over the same height).
- Firefighter B also takes less time, so both a larger numerator (work done) and a smaller denominator (time) result in a larger value of power.
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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