What mistakes do students make with Equilibrium?

Using the distance along the line of action instead of the perpendicular distance from the pivot. Use the perpendicular distance from the pivot to the line of action. Forgetting to convert distances to metres before calculating moments. Convert all lengths to metres to keep units consistent (e.g., 50 cm = 0.50 m). Assuming the principle of moments only applies when the pivot is at the centre of the object. The principle holds for any chosen pivot point when the object is in equilibrium.

What are the exam tips for Equilibrium in Physics OCR A-Level?

Always identify the pivot point clearly before calculating moments. Check that all distances are perpendicular to the force; if not, resolve the force or use the perpendicular component. Verify that the sum of clockwise moments equals the sum of anticlockwise moments as a final check.

OCR · A-Level · Physics

Course version: H556 · Version from Sept 2015

Equilibrium: (c) the principle of moments

The principle of moments states that for an object in rotational equilibrium, the sum of clockwise moments about any pivot equals the sum of anticlockwise moments.

Checked against the OCR H556 specification (opens in a new tab) on 10 November 2026.

Point 3 of 6

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

State that the principle of moments applies to an object in rotational equilibrium.
Define moment as force multiplied by the perpendicular distance from the pivot to the line of action of the force.
Use the correct unit for moment: newton metre (N m).
Apply the principle by equating the sum of clockwise moments to the sum of anticlockwise moments about a chosen pivot.
Calculate individual moments correctly using the perpendicular distance, not the distance along the force direction.

Common mistakes and how to put them right

Using the distance along the line of action instead of the perpendicular distance from the pivot.
Put it right: Use the perpendicular distance from the pivot to the line of action.
Forgetting to convert distances to metres before calculating moments.
Put it right: Convert all lengths to metres to keep units consistent (e.g., 50 cm = 0.50 m).
Assuming the principle of moments only applies when the pivot is at the centre of the object.
Put it right: The principle holds for any chosen pivot point when the object is in equilibrium.

Exam tips

Always identify the pivot point clearly before calculating moments.
Check that all distances are perpendicular to the force; if not, resolve the force or use the perpendicular component.
Verify that the sum of clockwise moments equals the sum of anticlockwise moments as a final check.

Try the lesson

The start of the Equilibrium: (c) the principle of moments lesson, made from checked teaching notes for this specification point.

Listen

The principle of moments states that for an object in rotational equilibrium, the sum of clockwise moments about any pivot equals the sum of anticlockwise moments. A moment is the product of a force and the perpendicular distance from the pivot to the force's line of action: moment = F × d, measured in newton metres (N m). For example, a 10 N force acting 0.50 m from a pivot produces a moment of 5.0 N m. When several forces act, calculate each moment separately, then equate the totals. This principle applies to any pivot point, provided the object is in equilibrium. It is used to solve problems involving seesaws, beams and balanced rulers, and is fundamental to understanding static equilibrium. A strong answer earns marks for these points. State that the principle of moments applies to an object in rotational equilibrium. Define moment as force multiplied by the perpendicular distance from the pivot to the line of action of the force. Use the correct unit for moment: newton metre (N m). Apply the principle by equating the sum of clockwise moments to the sum of anticlockwise moments about a chosen pivot. A common slip: Using the distance along the line of action instead of the perpendicular distance from the pivot. Instead, use the perpendicular distance from the pivot to the line of action. Exam tip: Always identify the pivot point clearly before calculating moments.

See it

A top-down flowchart. The first box reads 'Identify the pivot point' and points to a second box reading 'Find the perpendicular distance from the pivot to the line of action of each force'. This points to a third box reading 'Calculate each moment: moment = F × d', which points to a fourth box reading 'Use the correct unit: newton metre (N m)'. From this fourth box two branches lead to two boxes side by side: 'Sum the clockwise moments' and 'Sum the anticlockwise moments'. Both of these point to a box reading 'Equate clockwise moments to anticlockwise moments', which finally points to a box reading 'Object in rotational equilibrium'. — The steps for applying the principle of moments: identify the pivot, use perpendicular distances, calculate each moment, then equate the clockwise and anticlockwise totals for an object in rotational equilibrium.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first box reads 'Identify the pivot point' and points to a second box reading 'Find the perpendicular distance from the pivot to the line of action of each force'. This points to a third box reading 'Calculate each moment: moment = F × d', which points to a fourth box reading 'Use the correct unit: newton metre (N m)'. From this fourth box two branches lead to two boxes side by side: 'Sum the clockwise moments' and 'Sum the anticlockwise moments'. Both of these point to a box reading 'Equate clockwise moments to anticlockwise moments', which finally points to a box reading 'Object in rotational equilibrium'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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