Page 138 - Science Coursebook

9.9 Calculating moments

The principle of moments tells us that, when a beam is
balanced, the clockwise moment acting on it is equal to
the anticlockwise moment.

clockwise moment = anticlockwise moment
0.8 m 0.4 m
The diagram shows the forces acting on a beam,
and their distances from the pivot. The beam is
balanced because the moments of the two forces
are equal.
50 N 100 N

A balanced beam
Question

1 a In the diagram, which force has a clockwise moment (turning e ect)?

b Calculate the moment of this force.
c Calculate the moment of the other force.
d Is the beam balanced? Explain how you can tell.

Calculating a distance
If we know that a beam is balanced, we can 12 cm x
calculate the distance of a force from the pivot.

25 N 15 N
Example: In the diagram, the beam is balanced. We
do not know the distance x from the pivot to the 15 N
force, but we can work it out like this:
clockwise moment = anticlockwise moment
25 N × 12 cm = 15 N × x

300 = 15 x
300
x = = 20 cm
15
So the force must act at 20 cm from the pivot.

Question
2 A seesaw is 4.0 m long with a pivot at its midpoint. A boy who weighs 400 N
sits at a distance of 1.5 m from the pivot. His sister weighs 300 N.
a Draw a diagram to show the beam, the pivot and the forces and their
distances from the pivot.
b Calculate the distance at which the girl must sit if the beam is to
be balanced.

136 9 Forces in action