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What is moment of a force?

The moment of a force is its turning effect about a pivot, equal to the force multiplied by the perpendicular distance to the pivot.

Moment of a force explained

For a balanced object, total clockwise moments equal total anticlockwise moments — the principle of moments used in levers, beams and see-saws.
The distance must be perpendicular to the line of action of the force, so angled forces need resolving before substituting.

Key formula

M=F×d⊥M = F \times d_{\perp}

Worked example

A 20 N20\,\text{N} force at 0.5 m0.5\,\text{m} from the pivot gives a moment of 10 N m10\,\text{N m}.

Examiner tip

Take moments about the point where an unknown force acts — that removes it from the equation and leaves one unknown.

Moment of a force: common questions

What is the centre of mass?
The point where the whole weight of an object can be taken to act, which is where you place its weight arrow when taking moments.
What are the units of a moment?
Newton-metres (N m). It is not a joule, even though the units look similar, because the distance is perpendicular rather than along the force.

Related terms

Resultant force

The resultant force is the single force that has the same effect as all the forces acting on an object combined.

Newton's second law

Newton's second law states that the resultant force on an object equals its mass multiplied by its acceleration.

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