Moments | CIE AS Physics Revision Notes 2025

What is a moment?

A moment is the turning effect of a force
Moments occur when forces cause objects to rotate about a pivot

The moment of a force is given by the equation:

$moment = F d$

Where:
- moment = turning effect in newton metres (N m)
- F = force in newtons (N)
- d = perpendicular distance from the pivot in metres (m)

The force may not always be applied perpendicularly to the pivot
However, the distance used in the equation must be the perpendicular distance from the pivot
In these cases, the component of the distance that is perpendicular to the pivot must be used

Force applied at an angle to the pivot

Perpendicular-distance, downloadable AS & A Level Physics revision notes

When the force is applied at an angle to the pivot, the component of the distance that is perpendicular must be used

An example of moments in everyday life is opening a door
- The door handle is placed on the opposite side of the door to the hinge (the hinge is the pivot)
- This maximises the distance for a given force
- Which provides a greater moment (turning effect)
  - This makes it easier (requires less force) to push or pull the door open

Worked example

A uniform metre ruler is pivoted at the 50 cm mark.

A 0.5 kg weight is suspended at the 80 cm mark, causing the ruler to rotate about the pivot.

Calculate the turning moment about the pivot. Assume the weight of the ruler is negligible.

WE - Mass on ruler question image, downloadable AS & A Level Physics revision notes

Answer:

Step 1: List the known quantities and convert to SI units

Pivot at 0.50 m
Mass, m = 0.5 kg
Force applied at 0.80 m

Step 2: State the equation for moment

$moment = F d$

Step 3: Identify the force applied

The only force acting is the force of weight on the mass

$W = m g$

$W = 0.5 \times 9.81$

$W = 4.905 N$

Step 4: Identify the perpendicular distance from the pivot

$d = 0.80 - 0.50 = 0.30 m$

Step 5: Substitute the known values into the moment equation to calculate

$moment = 4.905 \times 0.30$

$moment = 1.4715 N m$

$moment = 1.5 N m (2 s . f .)$

Examiner Tip

It is better to only round your answer right at the end of your calculation. Always work with at least one, preferably two more significant figures than your final answer requires. This will help you to avoid compound rounding errors that could lose you marks.

Sketching a quick diagram of all the forces acting on the object will help you visualise which forces are perpendicular to the distance from the pivot. Not all forces will provide a turning effect and it is common for questions to provide more forces than required to increase the level of difficulty of the question.

Moments (CIE AS Physics)

Revision Note

What is a moment?

Force applied at an angle to the pivot

Worked example

Examiner Tip

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Author: Leander