Turning forces are found in many everyday situations and are essential for machines to function. Levers and gears make use of these turning forces to provide an advantage.

Part of

If an object is balanced, the total clockwise moment about a point is equal to the total anticlockwise moment about the same point. This is called the Principle of Moments. Total clockwise moment = Total anticlockwise moment.

- Question
The diagram below shows two masses balanced on a level beam.

How far is the 10 N weight from the pivot?

The beam is balanced and so from the

**Principal of Moments**we know that:Total clockwise moment about the pivot = Total anticlockwise moment about the pivot.

Calculate each individual moment first:

**Anticlockwise moment**Perpendicular distance from the pivot = d m.

Force F = 10 N.

Anticlockwise moment = F x d = 10 N x d m = 10d Nm.

**Clockwise moment**Perpendicular distance from the pivot = 1 m.

Force F = 20 N.

Clockwise moment = F x d = 20 N x 1 m = 20 Nm.

**Total clockwise moment = Total anticlockwise moment**10d = 20.

d = 2 m

The 20 N weight is 2 m from the pivot.

- Question
A parent and child are at opposite sides of a playground see-saw. The parent sits 0.8 m from the pivot. The child sits 2.4 m from the pivot and weighs 250 N.

Calculate the weight of the parent if the see-saw is balanced.

The see-saw is balanced and so from the Principal of Moments we know that:

Total clockwise moment about the pivot = Total anticlockwise moment about the pivot

**Anticlockwise moment**The anticlockwise moment is the child's moment = Fd.

Perpendicular distance of child from the pivot = 2.4 m.

Force F = 250 N.

Anticlockwise moment = 250 N × 2.4 m = 600 Nm.

**Clockwise moment**The clockwise moment is the parent's moment = Fd.

Perpendicular distance of adult from the pivot = 0.8 m.

Force F = F N.

Clockwise moment = F N x 0.8 m = 0.8F Nm.

**Total clockwise moment = Total anticlockwise moment.**0.8 F = 600.

F = 600 Nm ÷ 0.8 m.

F = 750 N.

The parent’s weight equals 750 N.