GCSE Physics revision

Moments, levers and gears (physics only)

AQA 4.5.4

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AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Describe examples in which forces cause rotation.

MOST 🎯🎯

Apply the scientific explanation of moments, levers and gears to a relevant example.

SOME 🎯🎯🎯

Analyse a new situation involving moments, levers and gears and explain the scientific reasoning.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

What Is a Moment?

A moment is the rotational or turning effect of a force acting around a fixed point called the pivot.
When a force is applied to a spanner, instead of moving in a straight line, it rotates around the pivot — this rotation is the moment.
Moments explain everyday situations, such as why a longer spanner is easier to use or how two people of different weights can balance on a seesaw.

1. What Is a Moment?AQA

Notes

A moment is the rotational or turning effect of a force acting around a fixed point called the pivot.
When a force is applied to a spanner, instead of moving in a straight line, it rotates around the pivot — this rotation is the moment.
Moments explain everyday situations, such as why a longer spanner is easier to use or how two people of different weights can balance on a seesaw.

The Moment Equation

The moment of a force is calculated using:
Moment = Force × Perpendicular Distance
The unit of moment is the newton metre (N m).
To maximise a moment, you should apply a large force as far from the pivot as possible.

Perpendicular Distance Explained

Perpendicular distance is the distance measured at right angles (90°) to the direction of the force.
If a force is applied at an awkward angle, the perpendicular distance is smaller, which reduces the size of the moment.
Using the full length of a spanner as the perpendicular distance gives the greatest possible moment for a given force.

3. Perpendicular Distance ExplainedAQA

Notes

Perpendicular distance is the distance measured at right angles (90°) to the direction of the force.
If a force is applied at an awkward angle, the perpendicular distance is smaller, which reduces the size of the moment.
Using the full length of a spanner as the perpendicular distance gives the greatest possible moment for a given force.

Calculating a Moment: Worked Example

Always convert distances into metres before substituting values into the equation (e.g.
20 cm = 0.2 m).
Applying an 80 N force at 0.2 m from the pivot gives a moment of 80 × 0.2 = 16 N m.
To find the force needed to produce the same 16 N m moment at only 0.1 m from the Moment 16 pivot, rearrange to Force = Distance = 0.1
= 160 N.

4. Calculating a Moment: Worked ExampleAQA

Notes

Always convert distances into metres before substituting values into the equation (e.g.
20 cm = 0.2 m).
Applying an 80 N force at 0.2 m from the pivot gives a moment of 80 × 0.2 = 16 N m.
To find the force needed to produce the same 16 N m moment at only 0.1 m from the Moment 16 pivot, rearrange to Force = Distance = 0.1
= 160 N.

Clockwise and Anticlockwise Moments

Because moments are turning effects, they are described as either clockwise or anticlockwise relative to the pivot.
A force applied to the right side of a seesaw's pivot creates a clockwise moment, whilst a force on the left side creates an anticlockwise moment.
The direction of the moment depends on which side of the pivot the force is applied.

5. Clockwise and Anticlockwise MomentsAQA

Notes

Because moments are turning effects, they are described as either clockwise or anticlockwise relative to the pivot.
A force applied to the right side of a seesaw's pivot creates a clockwise moment, whilst a force on the left side creates an anticlockwise moment.
The direction of the moment depends on which side of the pivot the force is applied.

The Principle of Moments

When an object is in equilibrium (balanced and not rotating), the total clockwise moment equals the total anticlockwise moment.
This rule is known as the principle of moments:
Total Clockwise Moment = Total Anticlockwise Moment
If the moments are unequal, the object will rotate towards the side with the larger moment.

6. The Principle of MomentsAQA

Notes

When an object is in equilibrium (balanced and not rotating), the total clockwise moment equals the total anticlockwise moment.
This rule is known as the principle of moments:
Total Clockwise Moment = Total Anticlockwise Moment
If the moments are unequal, the object will rotate towards the side with the larger moment.

Balancing a Seesaw: Worked Example

If a 600 N force acts 2 m from the pivot, the clockwise moment is 600 × 2 = 1200 N m.
To balance the seesaw, the anticlockwise moment must also equal 1200 N m.
An 800 N force must be placed 1200
800
= 1.5 m from the pivot to achieve balance.

Real-World Applications of Moments

A longer spanner is easier to use because the greater perpendicular distance produces a larger moment for the same applied force.
Two people of different weights can balance on a seesaw by adjusting their distances from the pivot to equalise the clockwise and anticlockwise moments.
Understanding moments is essential in engineering and design, from bridges to door handles, wherever rotational forces are involved.

8. Real-World Applications of MomentsAQA

Notes

A longer spanner is easier to use because the greater perpendicular distance produces a larger moment for the same applied force.
Two people of different weights can balance on a seesaw by adjusting their distances from the pivot to equalise the clockwise and anticlockwise moments.
Understanding moments is essential in engineering and design, from bridges to door handles, wherever rotational forces are involved.