What mistakes do students make with Equilibrium?

Assuming that any three forces in equilibrium must form a triangle without checking that their lines of action are concurrent. The forces must be concurrent for the triangle of forces to apply. Drawing the triangle with forces not taken in order (head-to-tail). The forces must be arranged head-to-tail to form a closed triangle. Forgetting that the triangle of forces only applies to three forces. For more than three forces, a polygon of forces is used.

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

Sketch the triangle of forces clearly, labelling the forces and angles. Use the sine rule or cosine rule to solve for unknown magnitudes or angles. Check that the directions of the forces in the triangle correspond to the actual directions in the problem.

OCR · A-Level · Physics

Course version: H556 · Version from Sept 2015

(f) condition for equilibrium of three coplanar forces; triangle of forces.

Full specification wording

Equilibrium: (f) condition for equilibrium of three coplanar forces; triangle of forces.

For three coplanar forces to be in equilibrium, their lines of action must be concurrent (meet at a single point) and the vector sum of the forces must be zero.

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

Point 6 of 6
2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

State that for three coplanar forces to be in equilibrium, their lines of action must be concurrent.
State that the vector sum of the three forces must be zero.
Describe the triangle of forces: the three forces can be represented by the sides of a closed triangle taken in order.
Use the triangle of forces to find an unknown force or angle by applying trigonometry (e.g., sine rule, cosine rule).
Recognise that the triangle of forces is a consequence of the general equilibrium conditions.

Common mistakes and how to put them right

Assuming that any three forces in equilibrium must form a triangle without checking that their lines of action are concurrent.
Put it right: The forces must be concurrent for the triangle of forces to apply.
Drawing the triangle with forces not taken in order (head-to-tail).
Put it right: The forces must be arranged head-to-tail to form a closed triangle.
Forgetting that the triangle of forces only applies to three forces.
Put it right: For more than three forces, a polygon of forces is used.

Exam tips

Sketch the triangle of forces clearly, labelling the forces and angles.
Use the sine rule or cosine rule to solve for unknown magnitudes or angles.
Check that the directions of the forces in the triangle correspond to the actual directions in the problem.

Try the lesson

The start of the Equilibrium: (f) condition for equilibrium of three coplanar forces; triangle of forces. lesson, made from checked teaching notes for this specification point.

Listen

For three coplanar forces to be in equilibrium, their lines of action must be concurrent (meet at a single point) and the vector sum of the forces must be zero. This means the three forces can be represented in magnitude and direction by the sides of a closed triangle, taken in order. This is known as the triangle of forces. For example, if three forces act on an object and it remains in equilibrium, drawing the forces head-to-tail will form a closed triangle. The condition is a special case of the general equilibrium conditions: resultant force zero and resultant torque zero. It is used to solve problems involving three forces acting on a point or a rigid body. A strong answer earns marks for these points. State that for three coplanar forces to be in equilibrium, their lines of action must be concurrent. State that the vector sum of the three forces must be zero. Describe the triangle of forces: the three forces can be represented by the sides of a closed triangle taken in order. Use the triangle of forces to find an unknown force or angle by applying trigonometry (e.g., sine rule, cosine rule). A common slip: Assuming that any three forces in equilibrium must form a triangle without checking that their lines of action are concurrent. Instead, the forces must be concurrent for the triangle of forces to apply. Exam tip: Sketch the triangle of forces clearly, labelling the forces and angles.

See it

A flowchart starting from a node labelled 'Three coplanar forces', which branches to two conditions: 'Lines of action concurrent (meet at a single point)' and 'Vector sum of the forces is zero'. Both conditions lead to a node 'Triangle of forces applies'. From there, one path goes to 'Forces represented by sides of a closed triangle, taken in order', then to 'Draw forces head-to-tail to form a closed triangle', then to 'Use trigonometry (sine rule, cosine rule) to find unknown force or angle'. A second path from 'Triangle of forces applies' goes to 'Special case of general equilibrium conditions: resultant force zero and resultant torque zero'. — It shows the conditions for equilibrium of three coplanar forces leading to the triangle of forces and its use.

Key words

Say what each word means before you turn the card.

Concurrent forces

Try answering aloud or in your head before turning the card.

Triangle of forces

Try answering aloud or in your head before turning the card.

See every definition

Concurrent forces: Forces whose lines of action meet at a single point, as required for three coplanar forces to be in equilibrium.
Triangle of forces: The representation of three forces in magnitude and direction by the sides of a closed triangle, taken in order.

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 flowchart starting from a node labelled 'Three coplanar forces', which branches to two conditions: 'Lines of action concurrent (meet at a single point)' and 'Vector sum of the forces is zero'. Both conditions lead to a node 'Triangle of forces applies'. From there, one path goes to 'Forces represented by sides of a closed triangle, taken in order', then to 'Draw forces head-to-tail to form a closed triangle', then to 'Use trigonometry (sine rule, cosine rule) to find unknown force or angle'. A second path from 'Triangle of forces applies' goes to 'Special case of general equilibrium conditions: resultant force zero and resultant torque zero'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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