Page 137 - Science Coursebook

9.8 The principle of moments

Using moments
When children play on a seesaw, they balance it by changing their positions. They
move along the beam until it is balanced. They are solving the balancing problem by
trial-and-error.

However, if we know the children’s weights, we can calculate their moments and work
out how to balance the beam.

Activity 9.8

Balancing a beam

Balance a ruler on a pivot made from a wooden rod or a pencil. You are going to
investigate how you can apply forces to this beam and keep it balanced.

Try placing different weights on opposite sides of the pivot. Move them until the
beam is balanced. Calculate the moment of each force.
Can you find the balancing rule?

The principle of moments

From the activity, you should have discovered the principle of moments.
For a beam to be balanced, the clockwise moment on it must equal the anticlockwise
moment acting on it.

Summary
• moment of a force = force × distance from pivot
• The principle of moments states that, for a beam to be balanced, the
clockwise moment acting on it must equal the anticlockwise moment
acting on it.

9 Forces in action 135