Page 136 - Science Coursebook

9.8 The principle of moments

If you have ever played on a seesaw, you will
have learnt about balancing the turning effects

of forces.
A seesaw is a long beam balanced on a pivot.
The pivot is half way along the beam.

In the photograph, you can tell that the girl (on
the right) is heavier than the boy because her end
of the beam is lower. Her weight has a greater

turning effect than the boy’s weight.
The diagram represents the seesaw. It shows the
pivot and the two forces acting downwards on These children have a seesaw in their school
the beam. playground in The Gambia.

Question
1 Look at the photograph of the pivot
children on the seesaw. Does the
girl’s weight turn the beam clockwise
or anticlockwise?
weight weight
of boy of girl

This diagram shows the forces acting on the seesaw.
Balancing up

How can the children balance the seesaw? The girl can do this by
moving towards the pivot. Then her weight will have less turning

effect, because it will be closer to the pivot.

Question

A+I 2 Suggest two ways in which the boy could balance the beam.

Moment of a force

The turning effect of a force depends on two things:

• The greater the force, the greater its turning effect.
• The further the force is from the pivot, the greater its turning effect.

We can calculate the moment of a force like this:
moment = force × distance from pivot

The bigger the moment of a force, the greater its turning effect.

134 9 Forces in action