Moments, Levers and Gears — AQA GCSE Physics (Separate)
A moment is the turning effect of a force. Levers and gears use moments to make tasks easier.
The moment of a force
The moment is the turning effect of a force about a pivot:
$$M = F \times d$$
- M = moment (newton metres, Nm)
- F = force (N)
- d = perpendicular distance from the pivot to the line of action of the force (m)
The principle of moments
If an object is balanced (in equilibrium), the total clockwise moment about a pivot equals the total anticlockwise moment:
clockwise moment = anticlockwise moment
This lets you calculate an unknown force or distance on a balanced beam (e.g. a seesaw).
Levers
A lever uses a pivot to increase the force (or distance) applied. By applying a force at a greater distance from the pivot, a smaller effort can produce a larger turning effect — a force multiplier (e.g. a spanner, crowbar, wheelbarrow).
Gears
Gears transmit the turning effect of a force. A larger gear turns more slowly but with a greater moment (more turning force); a smaller gear turns faster but with a smaller moment. Gears are used to change the speed and turning force of a rotating system.
Exam tips
- Learn moment = force × perpendicular distance (Nm).
- Use the principle of moments (clockwise = anticlockwise) for balanced objects.
- Levers act as force multipliers by applying force further from the pivot.
- A larger gear gives a bigger moment but turns slower.