13.0 TURNING EFFECT OF A FORCE (MOMENTS)

When you open a door, use a spanner, or see-saw with a friend, you are witnessing the turning effect of a force. This effect, called a moment, depends on both the size of the force and its distance from the pivot. This chapter explains how to calculate moments, the principle that governs balanced objects, and how to handle parallel forces on beams.

13.1 MOMENT OF A FORCE

13.1.1 Definition

The moment of a force (often just called moment or torque) is the turning effect of the force about a pivot (or fulcrum). A force can make an object turn clockwise or anticlockwise.

Everyday examples:

• Pushing a door handle – the door turns on its hinges.
• Using a spanner to loosen a nut – the longer the spanner, the easier it is to turn.
• Children on a see-saw – a heavier child must sit closer to the pivot to balance.

13.1.2 Formula for Moment

The moment of a force about a point is calculated using:

Moment = Force × Perpendicular distance from the pivot to the line of action of the force

In symbols:

M = F × d

where:

• M = moment (in newton-metres, N m)
• F = force (in newtons, N)
• d = perpendicular distance (in metres, m) from the pivot to the line of the force.

Important: The distance must be measured perpendicularly to th