MomentsEdexcel GCSE Physics: Revision notes

A force can cause an object to rotate about a fixed point called a pivot (or fulcrum). This turning effect of a force is called a moment.

Examples include opening a door, using a spanner to turn a nut, or a see-saw balancing about its pivot. The further from the pivot a force is applied, and the larger the force, the greater the turning effect.

moment (N m) = force (N) × distance normal to the direction of the force (m)

• The distance must be measured perpendicular (at right angles) to the line of action of the force, from the pivot
• Moments are measured in newton-metres (N m)
• A larger force or a longer distance from the pivot produces a larger moment

When an object is in rotational equilibrium (not rotating, or turning at a constant rate), the moments about the pivot are balanced.

Principle of moments: sum of clockwise moments = sum of anticlockwise moments

This is used to solve problems involving see-saws, beams and balances, where forces on either side of a pivot must produce equal and opposite turning effects for the system to stay balanced.

Levers use the principle of moments to make a task easier: applying a force over a large distance from the pivot creates a large moment, which can move a load close to the pivot using a much smaller effort force. This is how a crowbar or a pair of scissors works.

Gears transmit rotational forces between connected toothed wheels. A small gear turning a larger gear can increase the turning force (moment) transmitted, though it turns the larger gear more slowly — gears allow rotational effort and speed to be traded off against each other.