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.
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
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- state and apply the principle of moments
- understand that, when there is no resultant force and no resultant torque, a system is in equilibrium
- use a vector triangle to represent coplanar forces in equilibrium
- www.cambridgeinternational.org/alevel
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 Equilibrium of forces
- › Always specify 'resultant force is zero' and 'resultant torque is zero' when defining equilibrium conditions.
- › Define upthrust strictly as the force due to the pressure difference (pb – pt)A between the top and bottom surfaces.
- › Use the formula Upthrust = ρgV, where V is the volume of the submerged part of the object and ρ is the density of the fluid.
- › Ensure the 'total weight' in buoyancy calculations includes every part of the submerged or floating system.
- › Recognize that scale readings indicate the normal contact force; use R - mg = ma to solve for acceleration.
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.
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.
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.
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.
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.
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.
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 forces pass through a single point, there is no net torque about that point. The body is in translational equilibrium if the vector sum of the forces is also zero, but may still rotate if an external torque is applied.
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
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
How to study this Equilibrium of forces deck
Start in Study Mode, attempt each card before flipping, then rate Hard, Okay or Easy. Cards you rate Hard come back within a day; cards you rate Easy push out to weeks. Your progress is saved in your browser, so come back daily for 5–10 minute reviews until every card reads Mastered.
Study Mode
Rate each card Hard, Okay, or Easy after flipping.