Newton's third law

High School

Definition

The principle that for every action force there is an equal and opposite reaction force. When one object pushes on a second, the second pushes back with equal strength in the opposite direction, as when a rocket expels gas and is pushed forward.

Worked examples

A swimmer pushes water backward with their hands; the water pushes the swimmer forward with equal force.

The action force (hands on water) and reaction force (water on hands) are equal in strength but opposite in direction.

A rocket expels \(1000\,\)N\(\) of gas downward; the gas exerts \(1000\,\)N\(\) upward on the rocket, propelling it.

The rocket and expelled gas form an action-reaction pair, allowing thrust even in the vacuum of space.

Common mistakes

The action and reaction forces act on the same object and cancel each other out. → Action and reaction forces act on different objects, so they do not cancel. The swimmer's hand pushes water (action); water pushes the swimmer (reaction) — two separate objects, so motion occurs.
The reaction force is always smaller or happens after the action force. → Action and reaction forces are simultaneous and equal in magnitude. Newton's third law guarantees the forces exist at the same instant with identical strength, only opposite directions.
If I push a wall and it doesn't move, there is no reaction force. → The wall pushes back with equal force; it doesn't move because other forces (friction, foundation) balance yours. Reaction forces always exist; whether an object accelerates depends on the net force from all sources.

Where you'll use it next

You'll apply Newton's third law when analyzing collisions and momentum conservation, designing vehicles and propulsion systems, and studying forces in statics and dynamics in physics and engineering courses.

Found in 1 StudyPug lesson

Newton's Third Law: Equal and Opposite Forces Explained

AS-Level Maths

Dive into Newton's Third Law of Motion. Understand action-reaction pairs, explore real-world applications, and enhance your physics knowledge. Perfect for students seeking clarity on this fundamental principle.

11th Grade11th