Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Recall and apply Newton’s third law both to equilibrium situations and to…
Full specification wording
Topic 2 – Motion and forces: 2.23 Recall and apply Newton’s third law both to equilibrium situations and to collision interactions and relate it to the conservation of momentum in collisions
Newton's third law states that when two bodies interact, each exerts a force on the other; the two forces are equal in magnitude, opposite in direction and of the same type, and they act on…
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 22 of 32
Study notes
From the checked teaching notes for this specification point.
The idea
Newton's third law states that when two bodies interact, each exerts a force on the other; the two forces are equal in magnitude, opposite in direction and of the same type, and they act on different bodies. In equilibrium, a book resting on a table pushes down on the table while the table pushes up on the book with an equal force; these are a third-law pair acting on different objects. In a collision, body A pushes on body B while B pushes back on A with an equal and opposite force for the same contact time, so the impulses are equal and opposite. Because impulse equals change in momentum, the total momentum gained by one body equals that lost by the other, so total momentum is conserved. Distinguish a third-law pair (on two different bodies) from balanced forces on one body.
What earns the marks
- States that the two forces in a Newton's third-law pair are equal in magnitude and opposite in direction.
- Identifies that the two forces of a third-law pair act on two different bodies, not on the same body.
- Applies the third law to an equilibrium case by naming the two bodies and the equal and opposite forces between them.
- Explains that equal and opposite impulses during a collision give equal and opposite momentum changes, so total momentum is conserved.
- Distinguishes a third-law pair acting on two bodies from balanced forces acting on a single body.
Common mistakes and how to put them right
Treats the two forces of a third-law pair as acting on the same body.
Put it right: State that each force acts on a different body.
Claims the forces cancel to give zero net force on one object.
Put it right: Explain that they act on different objects so they do not cancel on either one.
Describes the reaction force as a delayed or subsequent force.
Put it right: State that the two forces occur simultaneously during the interaction.
Omits the equal contact time when linking forces to momentum change.
Put it right: Note that equal force over equal time gives equal and opposite impulses.
Exam tips
- Name the two bodies involved whenever you describe a third-law pair, so the examiner can see the forces act on different objects.
- Link force to momentum change through impulse (force × time) rather than asserting conservation without reasoning.
- Check whether the question asks about equilibrium or a collision, and choose the matching example of the pair.
Try the lesson
The start of the Topic 2 – Motion and forces: 2.23 Recall and apply Newton’s third law both to equilibrium situations and to collision interactions and relate it to the conservation of momentum in collisions lesson, made from checked teaching notes for this specification point.
Have a go first
A book rests on a table. According to Newton's third law, which pair of forces forms the action–reaction pair?
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