Turning effect of forces

Key Definition The moment of a force measures its turning effect about a pivot. It equals the product of the force and the perpendicular distance from the pivot to the line of action of the force, with the SI unit of newton-metre (N m).

moment = force × perpendicular distance from the pivot to the line of action of the force

A larger force or a greater perpendicular distance produces a larger moment, because moment is directly proportional to both quantities.

Moments act in either a clockwise or anticlockwise sense, and correctly identifying the direction of each moment is essential when solving problems.

Key Definition The principle of moments states that for an object in equilibrium, the sum of the clockwise moments about any pivot equals the sum of the anticlockwise moments.

For an object to be in equilibrium, two conditions must be satisfied simultaneously:

• There is no resultant force acting on the object
• There is no resultant moment acting on the object

A balanced beam, a seesaw, and a lever in use are all governed by these conditions.

Extended Extended students must also apply the principle of moments to situations with more than one force on each side of the pivot, summing the individual moments on each side before equating.

Moment balance experiment

The moment balance experiment confirms the principle directly — when a metre rule balances horizontally on a pivot, the measured clockwise and anticlockwise moments are found to be equal, demonstrating that no resultant moment acts on the system.