4.1MechanicsCore

Moments

A moment measures the turning effect of a force about a point. It depends on the size of the force and its perpendicular distance from the pivot — the basis of all rigid-body statics.

24 min Video by Zeeshan Zamurred Moments

Edexcel A Level Maths: 4.1 Moments (Forces)Watch the full walkthrough before the notes below.

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What you'll be able to do

Define the moment of a force
Use moment = force × perpendicular distance
Identify clockwise and anticlockwise moments
Work in newton-metres (N m)

Moment of a force

The moment of a force about a point is $moment = F \times d$ , where $d$ is the $perpendicular$ distance from the point to the line of action of the force. The unit is the newton-metre (N m).

moment = F \times d

Force × perpendicular distance.

moment = 10 \times 0.5

= 5

N m.

Answer

5

N m

Tip — Always use the PERPENDICULAR distance from the pivot to the force’s line of action.

Direction

Moments are either $clockwise$ or $anticlockwise$ about the pivot. State the sense, as it matters when combining moments.

Formula recap

moment = F \times d

Perpendicular distance.

units: N m

Newton-metres.

Common mistakes to avoid

Using the slant distance instead of the perpendicular distance.

Use the perpendicular distance to the line of action.

Ignoring the sense (clockwise/anticlockwise).

State the direction of each moment.

Key takeaways

Moment = F × perpendicular distance.
Unit is the newton-metre (N m).
Moments are clockwise or anticlockwise.

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