Turning effects of forces
Cambridge A-Level Physics (9702) · Unit 4: Forces, density and pressure · 7 flashcards
Turning effects of forces is topic 4.1 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 4 — Forces, density and pressure , alongside Equilibrium of forces and Density and pressure. In one line: The centre of gravity is the single point through which the entire weight of an object appears to act, regardless of the object's orientation.
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 — 4 definitions, 1 key concept and 2 calculations — covering the precise wording mark schemes reward. Use the 4 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.
'centre of gravity'
The centre of gravity is the single point through which the entire weight of an object appears to act, regardless of the object's orientation.
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.
- understand that the weight of an object may be taken as acting at a single point known as its centre of gravity
- define and apply the moment of a force
- understand that a couple is a pair of forces that acts to produce rotation only
- define and apply the torque of a couple
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 Turning effects of forces
- › 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.
- › Recall that the torque of a couple is the product of one of the forces and the perpendicular distance between the forces.
Define 'centre of gravity'.
The centre of gravity is the single point through which the entire weight of an object appears to act, regardless of the object's orientation.
Define the 'moment of a force'.
The moment of a force is the turning effect of a force about a pivot. It is calculated as the product of the force and the perpendicular distance from the line of action of the force to the pivot: Moment = Force × Perpendicular Distance.
State the formula to calculate the moment of a force.
Moment of a force = Force × Perpendicular Distance from the line of action of the force to the pivot. Units are Newton-metres (Nm).
What is a 'couple'?
A couple is a pair of equal and opposite forces acting on a body, but not along the same line. The net force is zero, so it produces rotation only, without translation.
Define the 'torque of a couple'.
The torque of a couple is the product of one of the forces and the perpendicular distance between the lines of action of the forces. Torque = Force × Perpendicular Distance.
State the formula to calculate the torque of a couple.
Torque of a couple = Force × Perpendicular Distance between the forces. The unit is the Newton-metre (Nm).
Explain how to determine the centre of gravity of an irregularly shaped object experimentally.
Suspend the object from a point and hang a plumb line. Draw a line along the plumb line. Repeat from another point. The intersection of the lines indicates the centre of gravity.
Review the material
Read full revision notes on Turning effects of forces — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 4 — Forces, density and pressure
Turning effects 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 Turning effects 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 Turning effects of forces deck
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