SEC G3 Combined Science Physics component K326/K327
CP5: Turning Effect of Forces
Use moments, perpendicular distance, equilibrium, and centre of gravity within component scope.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
A force turns an object according to both its size and its perpendicular distance from the pivot.
Moment and leverage
The moment of a force describes its turning effect about a pivot. Increasing the force or applying it farther from the pivot increases the moment.
Measure the shortest perpendicular distance from the pivot to the force's line of action. The length of a sloping handle is not automatically the correct distance.
Equilibrium by moments
For a body in rotational equilibrium, the total clockwise moment about a chosen pivot equals the total anticlockwise moment about that pivot.
Choose a pivot that removes an unknown force where possible, assign each force to a turning direction, and compare complete moment totals.
Centre of gravity
For turning calculations, the whole weight of a body can be treated as acting at its centre of gravity.
The centre of gravity is a modelling point. It does not mean all the material or mass is physically concentrated there.
Formulae and relationships
| Relationship | Use |
|---|---|
| Calculate a moment about a pivot. |
Worked examples
Example 1: A force acts from a pivot. What force at the other side balances it?
- Calculate the first moment: .
- Set the opposing moment equal to .
- Solve , giving .
Answer: Place the force from the pivot.
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 each force line of action.
- Apply clockwise and anticlockwise moments consistently when testing equilibrium.
- Place the body's weight through its centre of gravity before calculating its moment.
Official outcome coverage
K326 CP5: 5 mapped outcomes, references CP5(a), CP5(b), CP5(c), CP5(d), CP5(e). Check the official K326 syllabus.
K327 CP5: 5 mapped outcomes, references CP5(a), CP5(b), CP5(c), CP5(d), CP5(e). Check the official K327 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Test the principle of moments with a balanced rule and known loads, then locate the centre of gravity of a plane lamina by suspending it from more than one point. Measure perpendicular distances, compare clockwise and anticlockwise moments, and identify parallax or balance-point uncertainty.
Exam traps and retrieval check
Avoid these traps
- Using a non-perpendicular distance.
- Adding clockwise and anticlockwise moments instead of equating totals.
- Ignoring the body's own weight when its centre of gravity matters.
Check from memory
What two factors set a moment?
Force and perpendicular distance from the pivot.
What balances in rotational equilibrium?
Total clockwise and anticlockwise moments.
Where may weight be modelled as acting?
At the centre of gravity.
Official Combined Science scope
This shared Combined Physics owner serves both K326 and K327. Moments, perpendicular distance, equilibrium by moments, and centre of gravity.