GCSE Physics

Moments and pressure

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Subtopics and learning goals

Calculate moments and apply the principle of moments to balanced systems.
Calculate moments and apply the principle of moments.
Calculate pressure using force and area.
Explain how changing lever arm or contact area changes an effect.

Revision notes

A force can produce a turning effect about a pivot. The moment of a force is moment = force × perpendicular distance from the pivot to the line of action of the force, M = Fd. The perpendicular distance matters: measuring to the wrong point or along a sloping force gives the wrong moment. For an object in rotational equilibrium, the total clockwise moment about a chosen pivot equals the total anticlockwise moment. Levers use this idea to trade force for distance.

Moments and turning effects

A force can produce a turning effect about a pivot. The moment is M = Fd, where d is the perpendicular distance from the pivot to the force's line of action. The perpendicular distance matters: increasing lever length can increase turning effect without changing force.

For an object in rotational equilibrium, total clockwise moment equals total anticlockwise moment. This principle is used in balances, seesaws and lever systems. In calculations, choose one pivot, identify each perpendicular distance and keep units consistent.

Levers and mechanical advantage

Levers allow a smaller force to produce a larger turning effect by increasing distance from the pivot. This does not create energy. In an ideal system, a smaller input force acts through a larger distance while the load moves through a smaller distance. Real systems also lose energy through friction and deformation.

Pressure on surfaces

Pressure is force per unit area, p = \(\dfrac{F}{A}\). For the same force, a smaller contact area produces greater pressure. This explains why sharp blades cut effectively and why wide tyres, snowshoes or tracks can reduce pressure on soft ground.

Area must be in square metres for pressure in pascals. Be especially careful converting cm² to m²: the conversion factor is squared.

Worked example

Worked example 1: A 30 N force acts 0.40 m from a pivot at right angles to a spanner. M = Fd = 30 × 0.40 = 12 N m.

Worked example 2: A 180 N force acts 0.40 m from a pivot. To balance it with a force 0.60 m on the other side, equate moments: 180 × 0.40 = F × 0.60, so F = 120 N.

A 30 N force acting perpendicular to a lever 0.40 m from a pivot produces a moment of 30 × 0.40 = 12 N m.

Worked example 3: A 150 N force 0.40 m from a pivot balances a force F at 0.60 m: 150 × 0.40 = F × 0.60, so F = 100 N.

Misconception check

A larger contact area does not create a larger pressure for the same force. The same force spread over a larger area gives a smaller pressure.

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