IGCSE §4: Energy Resources and Energy Transfer · IGCSE
Work Done and Power
Revision notes on Work Done and Power for the OCR A-level Physics specification (H556). Free to read, with 5 practice questions in the app.
Work done — the energy transferred when a force moves an object through a distance:
W = F d
with work in joules (J), force in newtons and distance in metres. The distance must be measured in the direction of the force.
Work done and energy transferred are the same quantity, measured in the same unit. Saying a force "does 50 J of work" and saying it "transfers 50 J of energy" are two ways of saying one thing.
When no work is done. Carrying a heavy bag horizontally does no work against gravity, because the weight acts downwards and the movement is sideways — the distance in the direction of the force is zero. Your arms get tired for physiological reasons, not because the physics says work is being done on the bag. Likewise, pushing against a wall that does not move does no work at all, however hard you push.
Power — the rate of doing work, or the rate of transferring energy:
P = W / t
with power in watts (W), work in joules and time in seconds. One watt is one joule per second.
Power says how quickly, not how much. Two cranes lifting identical loads to the same height do the same work; the more powerful one simply finishes sooner.
Example: a 600 N force pushes a crate 4.0 m in 8.0 s. The work done is W = Fd = 600 × 4.0 = 2400 J, and the power is P = W ÷ t = 2400 ÷ 8.0 = 300 W.
Work done against friction is transferred to the thermal store of the surroundings, which is why brakes get hot and why rubbing your hands warms them. A car braking from speed transfers its entire kinetic store to the brake discs and the air.
Deriving the useful shortcut. Lifting a mass at steady speed means the upward force equals the weight, mg, and the distance is the height h. So the work done is W = mgh — which is exactly the gain in gravitational potential energy. The equation is not a separate fact to memorise; it is W = Fd applied to lifting.
5 questions on Work Done and Power
Reading is not revising. This topic has 5 questions in the app — multiple choice, calculations, and written answers marked against the mark scheme, point by point. Miss one and it comes back later with its options reshuffled, so you have to know it rather than remember where the answer sat.
- Every topic of Edexcel International GCSE Physics (4PH1)
- Flashcards and every equation, linked to the topic it comes from
- Full mock papers in the real paper structure
20 minutes a day is free. No card, nothing to cancel — and the paid version never renews itself either.
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