Page 139 - Science Coursebook

9.9 Calculating moments

Calculating a force 0.35 m 0.2 m
Similarly, we can calculate the force needed to
balance a beam.

10 N F
Example: In the diagram above, the beam is balanced. We do not know the force F,
which is needed to keep it balanced, but we can work it out like this:

clockwise moment = anticlockwise moment
10 N × 0.35 m = F × 0.20 m
3.5 = 0.20 F
3.5
F = = 17.5 N
0.20
So a force of 17.5 N is needed to balance the beam.

Question
3 Calculate the force F shown in the diagram.
40 cm 25 cm

75 N F

Activity 9.9

Moment challenge

Devise two ‘balanced beam’ problems for your partner to solve:
• one to calculate an unknown distance
• one to calculate an unknown force.
You can give each problem in words or as a diagram. Check that you agree with
your partner about the answers.

Summary
• If we know that a beam is balanced, we can use the principle of
moments to calculate an unknown distance or an unknown force.

9 Forces in action 137