Moments and balance
Hang masses on a pivoted beam and find the balance point. The clockwise and anticlockwise moments are calculated live.
- Moment = force × perpendicular distance
- Principle of moments
- Equilibrium
Anticlockwise moment
6.00N m
Clockwise moment
6.00N m
Difference
0.00N m
Predict
A 20 N weight sits 0.30 m to the left. Where must a 12 N weight go on the right to balance it?
Experiment
- 1Set the left side to 20 N at 0.30 m, then find the right distance that balances 12 N.
- 2Halve one force and see what has to happen to its distance.
- 3Try to balance a large force close to the pivot against a small force far away.
Explain
The moment of a force is force × perpendicular distance from the pivot, measured in N m. A small force a long way out can balance a large force close in — which is why a long spanner works better than a short one.
For a body in equilibrium two conditions hold at once: the resultant force is zero, and the sum of the clockwise moments equals the sum of the anticlockwise moments about any point. That second statement is the principle of moments.
In exam questions with a uniform rule, remember its own weight acts at the centre — that hidden moment catches people out whenever the pivot is not at the middle.