Equilibrium of forces
Cambridge A-Level Physics (9702) · Unit 4: Forces, density and pressure · 7 flashcards
Equilibrium of forces is topic 4.2 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 4 — Forces, density and pressure , alongside Turning effects of forces and Density and pressure. In one line: The principle of moments states that for an object in equilibrium, the sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments about the same point. This implies no net rotational effect.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 7 flashcards — 3 definitions and 4 key concepts — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Some cards in this deck are on other syllabus topics: 4.1 Turning effects of forces. Each card is labelled with its topic.
Principle of moments
The principle of moments states that for an object in equilibrium, the sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments about the same point. This implies no net rotational effect.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- State the principle of moments and apply it.
- Understand that a system is in equilibrium when it has neither a resultant force nor a resultant torque acting on it.
- Represent coplanar forces that are in equilibrium with a vector triangle.
Also in this deck: 4.1 Turning effects of forces
- Understand that an object's weight can be treated as acting at one point, called its centre of gravity.
- Define a force's moment and apply this idea.
- Understand that a couple consists of two forces that together cause rotation and nothing else.
- Define the torque produced by a couple and apply it.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in these topics
- › State clearly that for an object in rotational equilibrium, the sum of clockwise moments equals the sum of anticlockwise moments about the same point.
- › Recall that the torque of a couple is the product of one of the forces and the perpendicular distance between the forces.
- › Ensure your free-body diagrams include the weight of uniform beams acting at their center of gravity in addition to external loads.
Topic 4.2 · Equilibrium of forces
State the principle of moments.
The principle of moments states that for an object in equilibrium, the sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments about the same point. This implies no net rotational effect.
Topic 4.2 · Equilibrium of forces
What two conditions must be met for an object to be in complete equilibrium?
For complete equilibrium, there must be no resultant force (translational equilibrium) and no resultant torque (rotational equilibrium). This means the vector sum of all forces is zero, and the sum of moments about any point is zero.
Topic 4.2 · Equilibrium of forces
Describe how a vector triangle can be used to represent three coplanar forces in equilibrium.
When three coplanar forces are in equilibrium, they can be represented by the sides of a closed triangle. The arrows representing the forces should follow head to tail, illustrating that their vector sum is zero.
Topic 4.2 · Equilibrium of forces
A beam is supported at two points. How do you determine the reaction forces at the supports when a load is placed on the beam?
Apply the principle of moments by taking moments about one of the supports. This allows you to calculate the reaction force at the other support. Then, use the condition that the sum of upward forces equals the sum of downward forces to find the remaining reaction force.
Topic 4.1 · Turning effects of forces
Explain the difference between 'centre of gravity' and 'centre of mass'.
The centre of gravity is the point where the entire weight of the object appears to act. The centre of mass is the point where the entire mass of the object appears to be concentrated. They are usually the same, but differ when gravity is non-uniform.
Topic 4.2 · Equilibrium of forces
How does increasing the base area of an object affect its stability?
Increasing the base area of an object increases its stability. A larger base area means the centre of gravity can be displaced further before the line of action of the weight falls outside the base, causing the object to topple.
Topic 4.2 · Equilibrium of forces
Describe the effect on equilibrium if the lines of action of all forces acting on a body pass through a single point.
If the lines of action of all the forces pass through a single point, the forces have no resultant moment about that point. If the vector sum of the forces is also zero (the force vectors form a closed polygon), there is no resultant force and no resultant moment about any point, so the body is in complete equilibrium.
Review the material
Read full revision notes on Equilibrium of forces — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 4 — Forces, density and pressure
Equilibrium of forces sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Equilibrium of forces deck
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