Turning Effect of Forces: Complete O Level Physics Cheatsheet

What is a moment?

A moment is the turning effect of a force about a pivot (also called a fulcrum). It is not the same as the force itself, the same force can produce a large or small turning effect depending on how far it acts from the pivot.

Formula: M = F × d, where d is the perpendicular distance from the pivot to the line of action of the force, not just any distance to where the force is applied.

Unit: newton metre (N m).

Direction: every moment is either clockwise or anticlockwise about the chosen pivot, state which one when you calculate it.

Trick: If a question gives you a distance that clearly isn't perpendicular to the force (e.g. measured along a slope), you need to find the perpendicular distance first, usually with basic trigonometry or geometry, before applying M = F × d. At O Level this is uncommon, most questions already give you the perpendicular distance directly, so check the diagram before assuming you need extra steps.

Trap: Students often forget that a longer distance from the pivot does not automatically mean a bigger effect if the force itself is small, and vice versa. Always calculate both F and d, don't estimate visually from the diagram.

Principle of Moments

When a body (like a beam, plank or seesaw) is balanced and not rotating, the turning effects on each side of the pivot cancel out: Sum of clockwise moments = Sum of anticlockwise moments

How to apply it, step by step:

1. Choose a pivot. If the object already rests on a pivot or hinge, use that point, it usually eliminates one unknown force from the equation entirely.

2. List every force acting on the body and find its perpendicular distance from the pivot.

3. Work out whether each force's moment is clockwise or anticlockwise.

4. Set (sum of clockwise moments) = (sum of anticlockwise moments) and solve.

Trap: If the pivot is not at the object's centre of gravity, the object's own weight also produces a moment and must be included. This is the single most common mark lost on this topic, students remember the people/objects placed on a beam but forget the beam's own weight acts at its centre of gravity and has a turning effect too, unless the pivot happens to be exactly at the centre of gravity.

Centre of gravity and stability

Centre of gravity (C.G.): the single point through which the entire weight of an object appears to act. For a uniform (symmetrical, evenly-weighted) object, this is its geometric centre.

Stability depends on two things: a wider base area makes an object harder to topple, and a lower height of centre of gravity makes an object harder to topple.

How to reason about a tilting object (a common diagram-based question): if the vertical line through the centre of gravity still falls within the base area, the weight creates a restoring moment and the object falls back to its original position. If that vertical line falls outside the base area, the weight creates a toppling moment and the object falls over.

Everyday examples worth quoting in an exam: racing cars are built low and wide for stability, a Bunsen burner has a heavy, wide base for the same reason, double-decker buses are designed with a low centre of gravity despite their height.

Worked Example

A non-uniform plank AB, 5.0 m long and of weight 150 N, rests on a pivot 2.0 m from end A. The plank's centre of gravity is at its midpoint (2.5 m from A). A boy of weight 400 N stands at end A.

Why this question is a good test of the topic: it's easy to set up the boy's moment and stop there. The mark scheme awards a mark specifically for including the plank's own weight in part (b), that's the trap flagged above in action.

Frequently Asked Questions