O-Level and SEC G3 Physics K323
P4: Turning Effect of Forces
Choose a pivot, use perpendicular distance, test equilibrium, and relate centre of gravity to stability.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Turning questions depend on the force and its perpendicular distance from a chosen pivot. Stability questions then use centre of gravity, base width and the line of action of weight.
Moment of a force
A moment describes the turning effect of a force about a pivot. The distance in the equation is the shortest perpendicular distance from the pivot to the force line of action, not simply the length of a lever or a sloping separation shown in the diagram.
State clockwise and anticlockwise senses before summing moments. If a force passes through the pivot, its perpendicular distance is zero and it produces no moment about that pivot.
Equilibrium and the principle of moments
For rotational equilibrium, total clockwise moment equals total anticlockwise moment about the same pivot. Full equilibrium also requires zero resultant force. Choosing a pivot through an unknown support force can remove that force from a moments equation.
Keep units consistent, commonly newton metres. A moment is not itself a force, so do not add moments directly to forces in one equation.
Centre of gravity and stability
The weight of a body can be treated as acting at its centre of gravity. An object remains stable while the vertical line of action of its weight falls within its base. It topples when that line moves beyond the edge of the base.
A lower centre of gravity and a wider base generally increase the angle through which an object can tilt before toppling. Explain the change through the line of action, not by saying only that the object is heavier.
Formulae and relationships
| Relationship | Use |
|---|---|
| Calculate a moment about a pivot. | |
| Apply rotational equilibrium. |
Worked examples
Example 1: A force acts from a pivot. What force at balances it on the opposite side?
- First moment: .
- For equilibrium, .
- Therefore .
Answer: .
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Choose a pivot and use the perpendicular distance from the force line of action.
- Apply clockwise and anticlockwise moments consistently when testing equilibrium.
- Relate centre of gravity, base width, and the line of action of weight to stability.
Official outcome coverage
K323 P4: 6 mapped outcomes, references P4(a), P4(b), P4(c), P4(d), P4(e), P4(f). Check the official K323 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Balance a rule with known loads, measure perpendicular distances, and compare clockwise and anticlockwise moments within experimental uncertainty.
Exam traps and retrieval check
Avoid these traps
- Using the sloping distance instead of the perpendicular distance.
- Equating moments taken about different pivots.
- Explaining stability without referring to the weight line of action and base.
Check from memory
When is a moment zero?
When the force line of action passes through the pivot or the force is zero.
Does equal clockwise and anticlockwise moment guarantee no translation?
No. The resultant force must also be zero for full equilibrium.
How does a lower centre of gravity improve stability?
The object can tilt further before the weight line of action leaves the base.
Pure versus Combined scope
Combined Physics revisits part of this core under CP5 Turning Effect of Forces, but with reduced outcome scope. Combined students should follow the component checklist rather than assume every K323 outcome is assessable.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K323. Open the related IP explanation.