IGCSE Physics revision · Mechanics
Moments and turning effects
A short topic with one equation and one principle, commonly tested through calculations. The marks are often lost in the distances: they must be perpendicular, and they must be measured from the pivot.
Dr Desouky Physics Academy · Meet your physics tutor
What the syllabus demands
- —Define the moment of a force and calculate it
- —Apply the principle of moments to balanced beams and levers
- —Describe an experiment to verify the principle of moments
- —Define centre of gravity and describe how to find it for a plane lamina
- —Relate the position of the centre of gravity to stability
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Moment of a force
- The turning effect of a force: moment = force × perpendicular distance from the pivot.
- Principle of moments
- For an object in equilibrium, the sum of clockwise moments about any point equals the sum of anticlockwise moments about that point.
- Centre of gravity
- The point through which the whole weight of an object appears to act.
- Equilibrium
- The state in which there is no resultant force and no resultant moment on an object.
The equations
More equations to practise: the IGCSE formula sheet.
Where the marks die
Common mistakes to check
- 01
Measuring distances from the wrong point. Every distance in a moments calculation is measured from the pivot — not from the end of the beam, not from the other force.
- 02
Forgetting the weight of the beam itself in Extended questions. A uniform beam's weight acts at its centre — include it as a force at the midpoint.
- 03
Defining equilibrium with only 'no resultant force'. Full marks require both: no resultant force and no resultant moment.
- 04
Mixing units — a distance given in centimetres is fine as long as every distance is in centimetres. Consistency, not conversion, is what matters here.
One worked example, done properly
Question
A uniform seesaw is pivoted at its centre. A child of weight 400 N sits 1.5 m from the pivot. Where must a 600 N adult sit to balance it?
Method
- 1.Balance: clockwise moment = anticlockwise moment.
- 2.400 × 1.5 = 600 × d.
- 3.d = 600 ÷ 600 = 1.0 m.
d = 1.0 m from the pivot, on the other side