IGCSE §1: Forces and Motion · IGCSE
Moments and Balance
Revision notes on Moments and Balance for the OCR A-level Physics specification (H556). Free to read, with 5 practice questions in the app.
Moment — the turning effect of a force about a pivot:
moment = force × perpendicular distance from the pivot
measured in newton metres (N m). Force is in newtons and distance in metres.
Perpendicular matters. The distance used is the one at right angles to the line of action of the force, not the length of the spanner or the distance along a slope. A force applied along the line through the pivot has no turning effect at all, however large it is.
The principle of moments — for an object in equilibrium:
sum of clockwise moments = sum of anticlockwise moments
about any point. If the moments balance and the forces balance, the object neither turns nor accelerates.
Working a beam problem:
- Mark the pivot.
- For each force, find its distance from the pivot and decide whether it turns the beam clockwise or anticlockwise.
- Set the total clockwise moment equal to the total anticlockwise moment.
- Solve for the unknown.
Example: a 600 N child sits 1.5 m from the pivot of a seesaw. For a 450 N child to balance it, the anticlockwise moment must equal the clockwise one: 600 × 1.5 = 450 × d, so d = 900 ÷ 450 = 2.0 m from the pivot. The lighter child sits further out — which is exactly what children work out for themselves.
Centre of gravity — the single point at which the whole weight of an object can be taken to act. For a uniform beam it is at the middle, and the beam's own weight is drawn as one arrow acting from there.
Stability — an object topples when a vertical line down from its centre of gravity falls outside its base. A wide base and a low centre of gravity therefore make an object stable, which is why a racing car is low and wide and a double-decker bus is tested for tipping.
Everyday levers — a spanner, a crowbar, a wheelbarrow and a pair of scissors all increase the moment by increasing the distance from the pivot, letting a modest force produce a large turning effect.
5 questions on Moments and Balance
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.
Start Moments and Balance