Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
9.8P Recall and use the principle of moments in situations where…
Full specification wording
Topic 9 – Forces and their effects: 9.8P Recall and use the principle of moments in situations where rotational forces are in equilibrium: the sum of clockwise moments = the sum of anti-clockwise moments for rotational forces in equilibrium
When a body is balanced and not rotating, the turning effects cancel.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 8 of 9
Study notes
From the checked teaching notes for this specification point.
The idea
When a body is balanced and not rotating, the turning effects cancel. The principle of moments states that, for rotational forces in equilibrium, the sum of the clockwise moments about a pivot equals the sum of the anti-clockwise moments about the same pivot. To use it, choose a pivot, identify each force and its perpendicular distance from that pivot, decide whether each moment is clockwise or anti-clockwise, then set the two totals equal.
For example: A 4 N weight 0,30 m to the left of a pivot gives an anti-clockwise moment of 1,2 N m; a balancing weight 0,20 m to the right must give a clockwise moment of 1,2 N m, so that weight is 1,2 N m ÷ 0,20 m = 6 N. Always measure distances from the same pivot and normal to each force.
What earns the marks
- States that for rotational equilibrium the sum of clockwise moments equals the sum of anti-clockwise moments.
- Chooses a single pivot and measures every perpendicular distance from that pivot.
- Classifies each moment as clockwise or anti-clockwise about the chosen pivot.
- Sets the total clockwise moment equal to the total anti-clockwise moment and solves for the unknown.
- Calculates an unknown force or distance correctly, for example 1,2 N m ÷ 0,20 m = 6 N.
Common mistakes and how to put them right
Measures distances from different points for different forces.
Put it right: Use one pivot for the whole calculation.
Mixes up clockwise and anti-clockwise moments.
Put it right: Label each moment's direction before adding.
Uses the slant distance rather than the perpendicular distance.
Put it right: Measure normal to each force.
Adds moments on the same side instead of equating opposite sides.
Put it right: Set clockwise total equal to anti-clockwise total.
Exam tips
- Mark the pivot clearly on a diagram before writing any distances.
- Write a clockwise column and an anti-clockwise column, then set them equal.
- Keep distances in metres and check the final unit matches the quantity you are finding.
Try the lesson
The start of the Topic 9 – Forces and their effects: 9.8P Recall and use the principle of moments in situations where rotational forces are in equilibrium: the sum of clockwise moments = the sum of anti-clockwise moments for rotational forces in equilibrium lesson, made from checked teaching notes for this specification point.
Have a go first
A see-saw is balanced on a pivot. A downward force of 30 N acts 2.0 m from the pivot on the left side. What downward force must act 3.0 m from the pivot on the right side to keep the see-saw in equilibrium?
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