Lesson focus: The turning effect of a force depends on force and perpendicular distanceThe shortest distance from a pivot to the line a force acts along, measured at right angles. from the pivotThe point that something turns around, like the middle of a seesaw..
Calculate clockwise and anticlockwise momentThe turning effect of a force about a pivot: moment = force × perpendicular distance from the pivot. and apply balance to levers.
Build on earlier learning
Forces in real situationsYear 7 combined forces along a line; this lesson considers turning about a pivot.
Practical
Balancing moments
- Balance a metre rule on a pivot at its centre.
- Hang a weight on one side and balance it with another weight on the other side.
- Calculate each moment: force × perpendicular distance from the pivot.
- Use the model on this page to test other positions.
Record: A table showing that clockwise and anticlockwise moments are equal when balanced.
Balancing moments
Hang weights on each side of the pivot. Moment = force × distance from the pivot. The rule balances when the two moments are equal.
Left side (anticlockwise)
Right side (clockwise)
You will be successful when you can…
- Identify pivot and line of action.
- Calculate F × perpendicular distance.
- Assign turning direction.
- Apply moment balance.
Words that carry the science
- moment
- Turning effect of a force about a pivot.
- pivot
- Point or axis about which an object turns.
- line of action
- Straight line through a force arrow.
- perpendicular distance
- Shortest pivot-to-line-of-action distance.
- equilibrium
- Zero resultant force and zero resultant moment.
Force and distance set the moment
01Force and distance set the moment
Moment = force × perpendicular distance from pivotThe point that something turns around, like the middle of a seesaw. to line of actionThe straight line along which a force acts., measured in N m.
A force through the pivot has zero momentThe turning effect of a force about a pivot: moment = force × perpendicular distance from the pivot. because that distance is zero.
02Direction matters
Moments act clockwise or anticlockwise. State a sign convention when combining them.
For rotational equilibriumWhen the forces on an object are balanced, so it stays still or keeps moving at a steady speed., total clockwise moment equals total anticlockwise moment. Forces must also balance for complete equilibrium.
03Levers trade force for distance
Effort farther from a pivot creates more moment, allowing a smaller force to balance a nearby load.
An ideal lever does not create energy: the smaller effort moves farther, and real devices lose energy.
Balance a seesaw
A 300 N child sits 1.2 m left. Where should a 240 N child sit right?
- Left moment = 300 × 1.2 = 360 N m.
- Right moment must equal 360 N m.
- Distance = moment ÷ force.
- 360 ÷ 240 = 1.5 m.
How to make the reasoning visible: The second child sits 1.5 m from the pivot.
Moments
Use the lever data to predict rotation.
| Case | Anticlockwise | Clockwise | Prediction |
|---|---|---|---|
| A | 20 N × 2 m | 40 N × 1 m | ? |
| B | 30 N × 1.5 m | 20 N × 1 m | ? |
| C | 50 N × 0.4 m | 10 N × 2 m | ? |
| D | 12 N × 3 m | 18 N × 1 m | ? |
Calculate and predict.
Where should D’s 18 N force act to balance?
Check your understanding
Answer from memory, then check against the suggested answer.
State the moment equation.
State the unit.
Moment through pivot?
Why is a door handle far from its hinges?
Why does a balanced lever still need an upward pivot force?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Explain the advantage of using a 0.80 m spanner rather than a 0.20 m spanner to turn a nut.
(4)A moment of 120 N m is needed to turn the nut. Calculate the force needed with the 0.80 m spanner.
(2)Total for this question: 6 marks
Command word: Connect perpendicular distance to moment before calculating.