Newton’s Third Law

Identify force pairs acting on different objects.

Lesson 3 of 53 parts Practice Flashcards

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Action–Reaction Pairs

Objective: Match equal and opposite interaction forces correctly.

Newton’s third law states that when object A exerts a force on B, B exerts an equal-magnitude force in the opposite direction on A, at the same time. These forces act on different objects and therefore do not cancel in a free-body diagram for either one. For a hand pushing a wall, the wall pushes the hand back. The two forces are the same type of interaction and have the same magnitude even when the objects have different masses; their accelerations can differ because net forces and masses differ. Weight and the normal force on a resting book are not a third-law pair because both act on the book.

Two skaters push each other; equal-length opposite arrows show that each exerts an equal and opposite force on the other.

Worked examples

Try itA hand pushes a wall to the right. What is the paired force?Show answer

The wall pushes the hand to the left. The two objects exert equal opposite interaction forces.

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Action–Reaction Pairs

Objective: Match equal and opposite interaction forces correctly.

Worked examples

Try itA hand pushes a wall to the right. What is the paired force?Show answer

The wall pushes the hand to the left. The two objects exert equal opposite interaction forces.

Walking, Tires, and Propulsion

Objective: Explain motion through interactions with surroundings.

A walker pushes the ground backward with a foot; the ground exerts a forward static-friction force on the foot. A tire likewise pushes a road backward and receives a forward force when it has traction. A rocket pushes exhaust gases one way and receives an opposite force, so it does not require air to push against and can operate in a vacuum. In each case the third-law pair acts on different objects; acceleration of the person, vehicle, or rocket depends on that object’s full net force. Friction need not always oppose the direction of an object’s overall motion: static friction can propel it forward.

Worked examples

Try itWhen a person walks forward, what external horizontal force can propel them?Show answer

Forward static friction from the ground. Ground acts on foot after the foot pushes backward.

Third-Law Pairs Versus Balanced Forces

Objective: Separate interaction pairs from forces that cancel on one target.

Balanced forces in a net-force calculation act on the same object and sum to zero. Third-law forces act on different objects and are paired by one interaction. A resting book has Earth’s downward gravity and table’s upward normal force balanced on the book. The third-law partner of table-on-book is book-on-table, while the partner of Earth-on-book is book-on-Earth. Naming the source and target of each arrow prevents the common error of calling two opposing arrows on one diagram an action–reaction pair. Balance describes acceleration of one object; the third law describes reciprocity of an interaction.

Worked examples

Try itA book rests on a table. Which two forces can balance on the book?Show answer

Earth-on-book downward and table-on-book upward. Balanced forces must act on the same target.

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