Moments and stability explain why a long spanner loosens a stubborn nut, why a see-saw can balance a heavy adult against a light child, and why a tall bus tips more easily than a low car. The module joins one calculation idea (force times perpendicular distance) to one reasoning idea (where the centre of mass sits compared with the base).
Check the current Cambridge IGCSE Physics 0625 syllabus for your exam year to see exactly which statements are listed under this topic.
What should you know before starting?
You should be comfortable with force in newtons and weight as a force, from forces and momentum and mass, weight and density. You also need to rearrange a simple equation and keep units consistent, for example converting centimetres to metres.
An orienting example
A light plank is pivoted at its middle. A 40 N weight hangs 1.5 m to the left of the pivot. Where must a 60 N weight hang on the right for the plank to balance?
The anticlockwise moment is 40 × 1.5 = 60 N m. For balance the clockwise moment must also be 60 N m, so 60 × d = 60 and d = 1.0 m.
Check by working forwards: 60 N at 1.0 m gives 60 N m, which matches. The heavier weight sits closer to the pivot, which is what your intuition about a see-saw expects.
In what order should you study the lessons?
- Calculate a moment about a pivot: this gives you the core formula and the N m unit.
- Use clockwise and anticlockwise balance: this turns the formula into the principle of moments.
- Locate a centre of mass conceptually: this explains where “the weight acts” and how to find it for an irregular shape.
- Explain stability using a base and centre of mass: this applies the centre of mass to toppling and equilibrium.
- Check perpendicular distance in a turning problem: this sharpens the one step that most wrong answers share.
What traps catch students in this topic?
- Using the wrong distance. The distance is measured to the pivot, along a line at right angles to the force, not to the nearest end of the object.
- Mixed units. A force in N and a distance in cm gives N cm. Convert first, or say clearly that the answer is in N cm.
- Adding forces instead of moments. Balance is about turning effects, not about equal forces.
- Saying heavier means more stable. Stability depends on the base and the height of the centre of mass.
- Forgetting a force. The weight of a uniform beam acts at its centre, and it counts too unless the question says the beam is light.
How should you use the practice set?
Work through the lessons first, then try the moments and stability practice set on paper. Open each answer only after you have a full attempt, and use the table at the end to route each slip back to a lesson. The mistake log and retest queue helps you record which kind of slip repeated.
For more of the subject, see the Physics learning guide. Students who can follow a worked example but freeze on a fresh diagram often benefit from online one-to-one Physics tuition, where a teacher watches how you set the problem up.