Introduction
Newton's Third Law is a cornerstone of Mechanics at A-Level and appears whenever two objects interact — a particle resting on the ground, a book on a table, or two blocks in contact. In 9709 exam questions, it is most commonly tested through correct labelling of force diagrams, setting up equations of equilibrium, and reasoning about contact forces between surfaces or between connected particles. Misidentifying action–reaction pairs is one of the most penalised errors at this level, so a precise understanding is essential.
Core Concept
Newton's Third Law states:
When body A exerts a force on body B, body B exerts an equal in magnitude, opposite in direction force on body A.
These two forces are called an action–reaction pair (or a Newton's Third Law pair). They are a matching pair if and only if they satisfy all three of the following conditions:
| Condition | What it means |
|---|---|
| Equal in magnitude | Both forces have exactly the same size |
| Opposite in direction | They act in exactly opposite directions along the same line |
| Same type of force | Both are the same kind (e.g. both normal contact forces, or both gravitational) |
| Act on different bodies | One acts on A, the other acts on B — never both on the same object |
The Classic Example from the Syllabus
A particle rests on the ground. Two forces form a Newton's Third Law pair:
- The particle exerts a downward contact force on the ground.
- The ground exerts an upward normal reaction force on the particle.
These are equal in magnitude, opposite in direction, both contact forces, and act on different bodies (one on the ground, one on the particle). This is the defining example given in the syllabus objective.
⚠️ Critical distinction: The weight of the particle (gravitational pull of the Earth on the particle) and the normal reaction from the ground on the particle are not a Newton's Third Law pair — they act on the same body (the particle). They happen to be equal in magnitude only because the particle is in equilibrium.
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