Newton’s Third Law — Action and Reaction: Every force has a counterforce

How do you walk? You put your foot on the ground and push backward. But what moves you forward — you didn’t apply any force to yourself!

Actually, you did — you pushed the ground backward. And the ground pushed you forward. That’s Newton’s Third Law.

The Law

Every force that object A exerts on object B is matched by an equal and opposite force that B exerts on A.

The Key Point Everyone Gets Wrong

The two forces act on two different objects. So don’t think they cancel each other out! Forces only cancel when they act on the same object.

Real-World Examples

🚀 Rocket: Gases shoot downward out of the rocket (rocket’s force on gas). The gas pushes the rocket upward with an equal force. The rocket climbs.

🏊 Swimming: Your hand pushes water backward. The water pushes you forward.

🔫 Gun: The bullet flies forward. The gun kicks backward — the reaction force.

🌍 Earth and the apple: When an apple falls, Earth pulls it (gravity). The apple also pulls Earth — with the same force! But because Earth is 10²⁵ times heavier, its acceleration is imperceptible.

References

• Halliday, Resnick & Krane — Physics, Ch. 5
• Feynman Lectures on Physics — Vol. 1, Ch. 10