Moment of a Force

The moment of a force is its turning effect about a pivot (fulcrum).

Equation

Moment = force x perpendicular distance from the pivot

Unit

Newton-metres (Nm).

Principle of moments

For an object in rotational equilibrium (not rotating), the sum of the clockwise moments about any point equals the sum of the anticlockwise moments.

Example

A 40 N force acts at 0.3 m from a pivot. Moment = 40 x 0.3 = 12 Nm.

For balance: if a 20 N force acts on the opposite side, it must be at 12 / 20 = 0.6 m from the pivot.

Key points

• The distance must be the perpendicular distance from the line of action of the force to the pivot.
• If the force is not perpendicular to the line from the pivot, you must resolve it or use the perpendicular component.

In exam questions

Draw a diagram showing the pivot, forces and distances. Write the moment equation clearly. If the question involves balance, set clockwise moments equal to anticlockwise moments.