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Newton's third-law interaction pairs

Key idea: Balanced and unbalanced forces, two-dimensional free-body diagrams, F = ma, friction and action–reaction pairs.

  • SEC G3 Combined Science Physics component 2027
Syllabus and review details

Free-body diagrams may involve forces in at most two dimensions. This Science lesson excludes graphical three-force equilibrium, inertia as a separate extension and terminal-velocity treatment.

Retrieval check
Before starting, name the body you are analysing and distinguish velocity from acceleration. The answer depends on the selected body and the change in velocity, not on motion alone.

Build the force model

For one interaction, body A's force on body B and body B's force on body A are equal in magnitude, opposite in direction and act on different bodies. Equal opposite forces on one body may balance, but they are not a Newton III pair.

Worked reasoning

Modelled example 1

Model the decisive reasoning

Core

Problem

A swimmer pushes water backward with 80 N. State the Newton III partner.
Study the worked solution

  1. Select and apply the model

    Method

    Reverse both bodies and direction: the water pushes the swimmer forward with 80 N.

    Reason

    The forces are the two sides of the same swimmer-water contact interaction.

    Working

    Swimmer on water: 80 N backward; water on swimmer: 80 N forward.

Guided practice 2

Complete a guided force model

About 5 min

Problem

Earth pulls a book downward. Identify the partner force and the body on which it acts.

Write your reasoning before opening the solution

Hints

Hint 1: choose the decisive model
Keep the gravitational interaction but reverse agent and target.
View solution step by step

  1. Compare your model with the physical interactions

    Method

    The book pulls Earth upward with an equal gravitational force.

    Reason

    A Newton III pair has the same interaction type and acts on different bodies.

    Working

    Earth on book pairs with book on Earth.

Misconception contrast

The weight and normal force on a book are a Newton III pair because they are equal and opposite.
Both forces act on the book, so they may balance but cannot be one Newton III pair. Their partners act separately on Earth and on the table.

Changed-context transfer

Challenge 3

Transfer the force model

Minimal support

Problem

A truck collides with a lighter car. Compare the contact forces during the collision.

Commit to a model before using the hint

Hints

Hint 1: identify what changed
Mass changes acceleration, not the equality of a Newton III pair.
View solution step by step

  1. Apply the model in the new context

    Method

    The truck and car exert equal-magnitude, opposite-direction contact forces on each other.

    Reason

    They are the two forces of the same collision interaction and act on different vehicles.

    Working

    Different masses can have different accelerations even though the interaction forces are equal.

Independent check

A lamp hangs at rest from a cable. Classify (i) tension up and weight down on the lamp, and (ii) cable pulls lamp up and lamp pulls cable down.

Check the force-model reasoning

(i) The two forces act on the same lamp and balance; they are not a Newton III pair. (ii) The equal opposite tension forces are one cable-lamp interaction and act on different bodies, so they are a Newton III pair.

Start the Dynamics topic check. Use its feedback to practise the smallest weak step.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Combined Science Physics component
Edition
SEC G3 Combined Science Physics component 2027