IGCSEPhysicsCambridge (CIE)Revision Notes1. Motion, Forces & EnergyMomentsMoments

Moments (Cambridge (CIE) IGCSE Physics): Revision Note

Exam code: 0625 & 0972

Written by: Katie M

Reviewed by: Tim

Updated on 7 September 2026

Moments

A moment of a force is defined as:

The turning effect of a force about a pivot

Examples of the turning effect of a force are:
- A child on a see-saw
- Turning the handle of a spanner
- A door opening and closing
- Using a crane to move building supplies
- Using a screwdriver to open a tin of paint
- Turning a tap on and off
- Picking up a wheelbarrow
- Using scissors

Forces can cause the rotation of an object about a fixed pivot
This rotation can be clockwise or anticlockwise

Two clocks with one circular arrow around each clock face, one turning clockwise and one turning anticlockwise — **Consider the hands of a clock when deciding if an object will rotate in a clockwise or anticlockwise direction**

A force applied on one side of the pivot will cause the object to rotate

A bar resting on a pivot at the centre of the bar. A downward force is applied at the right end of the bar, making the bar rotate about the pivot. — **The force applied will cause the object to rotate clockwise about the pivot**

The moment equation

The size of a moment is defined by the equation:

$moment = force \times perpendicular distance from pivot$

The forces should be perpendicular to the distance from the pivot
- For example, on a horizontal beam, the forces which will cause a moment are those directed upwards or downwards

The turning effect of a force exerted on a spanner

moment-of-force, IGCSE & GCSE Physics revision notes — **The moment depends on the force and perpendicular distance to the pivot**

Increasing the distance at which a force is applied from a pivot decreases the force required
- If you try to push open a door right next to the hinge, it is very difficult, as it requires a lot of force
- If you push the door open at the side furthest from the hinge, then it is much easier, as less force is required

Forces required to open a door

Diagram of a hinged door, with a small force needed at the handle and a much larger force needed closer to the hinges to produce the same turning effect. — **A greater force is required to push open a door next to the hinges than at the door handle**

Worked Example

A carpenter attempts to loosen a bolt that has rusted. To turn the bolt, they exert a force of 22 N using a spanner of length 20 cm. The force is exerted 5 cm from the end of the spanner.

Calculate the turning effect of the force.

[4]

Answer:

Step 1: List the known quantities

Force, $F = 22 N$
Length of spanner, $l = 20 cm$

Step 2: Determine the perpendicular distance from the pivot

The force is exerted 5 cm from the end of the spanner
Therefore, the perpendicular distance from the force to the pivot, $d$ , is

$d = 20 - 5$

$d = 15 cm$ [1 mark]

Convert cm to m

$d = \frac{15}{100}$

$d = 0.15 m$ [1 mark]

Step 3: Write out the equation for moments

$moment = force \times perpendicular distance from pivot$

$M = F d$ [1 mark]

Step 4: Substitute in the known values to calculate

$M = 22 \times 0.15$

$M = 3.3 N m$ [1 mark]

Examiner Tips and Tricks

Always work out the perpendicular distance from the pivot.

The moment of a force is measured in newton metres (N m), but can also be newton centimetres (N cm) if the distance is measured in cm instead. If your IGCSE moments exam question doesn't ask for a specific unit, always convert the distance into metres.

The following two units should be avoided when calculating the moment of a force: N/m, which suggests you have divided rather than multiplied, and joules, which are never accepted for a moment even though one joule is equal to one newton metre.

Principle of moments (core)

The principle of moments states that:

If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot

The principle of moments means that for a balanced object (opens in a new tab), the moments on both sides of the pivot are equal

clockwise moment = anticlockwise moment

Examiner Tips and Tricks

You must include direction words when stating the principle of moments. Only saying "the moments are equal" will not be credited.

Diagram of a beam on a central pivot showing two forces causing clockwise moments and two forces causing anti-clockwise moments, labelled with arrows indicating rotation directions. — Imagine holding the beam about the pivot and applying just one of the forces. If the beam moves clockwise then the force applied is clockwise. This is how to decide whether a moment is clockwise or anticlockwise

Worked Example

An adult and child are at opposite ends of a playground see-saw.

The weight force acting on the adult is 690 N and the weight force acting on the child is 140 N.

The adult sits at a distance of 0.3 m from the pivot.

A see-saw balanced on a central pivot, with an adult of weight 690 N sitting 0.3 m from the pivot on the left and a child of weight 140 N sitting at an unknown distance on the right.

Calculate the distance the child must sit from the pivot for the see-saw to be balanced.

Use the principle of moments in your calculation.

[4]

Answer:

Step 1: List the known quantities

Clockwise force (child), $F_{c h i l d} = 140 N$
Anticlockwise force (adult), $F_{a d u l t} = 690 N$
Distance of adult from the pivot, $d_{a d u l t} = 0.3 m$

Step 2: Write down the moment equation and the principle of moments

Moment equation:

$moment = force \times perpendicular distance from pivot$

$M = F d$

Principle of moments:

$total clockwise moments = total anticlockwise moments$ [1 mark]

Step 3: Calculate the total clockwise moments

The clockwise moment is from the child

$M_{c h i l d} = F_{c h i l d} \times d_{c h i l d}$

$M_{c h i l d} = 140 \times d_{c h i l d}$

Step 4: Calculate the total anticlockwise moments

The anticlockwise moment is from the adult

$M_{a d u l t} = F_{a d u l t} \times d_{a d u l t}$

$M_{a d u l t} = 690 \times 0.3$

$M_{a d u l t} = 207 N m$ [1 mark]

Step 5: Substitute into the principle of moments equation

$total clockwise moments = total anticlockwise moments$

$M_{c h i l d} = M_{a d u l t}$

$140 \times s_{c h i l d} = 207$

Step 6: Rearrange for the distance of the child from the pivot

$d_{c h i l d} = \frac{207}{140}$ [1 mark]

$d_{c h i l d} = 1.5 m$ (to 2 significant figures)

The child must sit 1.5 m from the pivot to balance the see-saw [1 mark]

Examiner Tips and Tricks

Make sure that all the distances are in the same units and that you’re considering the correct forces as clockwise or anticlockwise

If you are studying the core tier for IGCSE Physics, you will only be expected to apply the principle of moments to a situation where one force acts on either side of the pivot

Principle of moments (extended)

Extended Tier Only

In the example below, the forces and distances of the objects on the beam are different, but they are arranged in a way that balances the whole system

A horizontal beam on a central pivot, with a force F₁ acting downwards on the left at a distance d₁ from the pivot, a force F₂ acting downwards on the right at a distance d₂ from the pivot, and a force F₃ acting upwards on the right at a distance d₃ from the pivot. — **The clockwise and anticlockwise moments acting on a beam are balanced**

In the above diagram:
- Force $F_{1}$ causes an anticlockwise moment of $F_{1} \times d_{1}$ about the pivot
- Force $F_{2}$ causes a clockwise moment of $F_{2} \times d_{2}$ about the pivot
- Force $F_{3}$ causes an anticlockwise moment of $F_{3} \times d_{3}$ about the pivot

Collecting the clockwise and anticlockwise moments:
- Sum of the clockwise moments = $F_{2} \times d_{2}$
- Sum of the anticlockwise moments = $(F_{1} \times d_{1}) + (F_{3} \times d_{3})$

Using the principle of moments, the beam is balanced when:

sum of the clockwise moments = sum of the anticlockwise moments

$F_{2} \times d_{2} = (F_{1} \times d_{1}) + (F_{3} \times d_{3})$

Examiner Tips and Tricks

Note that when a beam is pivoted at its centre of mass, it contributes no moment because the perpendicular distance from the pivot to its weight force is zero. If the pivot was not placed at the centre of mass of the beam, the beam would contribute a moment and would need to be considered in calculations.

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Moments (Cambridge (CIE) IGCSE Physics): Revision Note

Moments

The moment equation

The turning effect of a force exerted on a spanner

Forces required to open a door

Worked Example

Examiner Tips and Tricks

Principle of moments (core)

Examiner Tips and Tricks

Worked Example

Examiner Tips and Tricks

Principle of moments (extended)

Extended Tier Only

Examiner Tips and Tricks

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

Author: Katie M

Reviewer: Tim