What Is Newton's Third Law?
Newton's third law of motion states: "For every action, there is an equal and opposite reaction." When object A exerts a force on object B, object B simultaneously exerts a force equal in magnitude and opposite in direction on object A. These forces act on different objects and are called action-reaction pairs.
This law is fundamental to understanding how forces work in nature. It explains how rockets propel themselves in space, how we walk on the ground, why a gun recoils when fired, and how fish swim through water. The third law applies to all types of forces including gravitational, electromagnetic, and contact forces.
Action-Reaction Pairs
| Action | Reaction |
|---|---|
| Earth pulls you down (gravity) | You pull Earth up (gravity) |
| Foot pushes ground backward | Ground pushes foot forward |
| Rocket pushes exhaust down | Exhaust pushes rocket up |
| Hammer hits nail forward | Nail pushes hammer backward |
Real-World Examples
- Swimming: You push water backward with your hands; the water pushes you forward.
- Walking: Your foot pushes the ground backward; friction pushes you forward.
- Jet engines: Exhaust gases are expelled backward; the reaction force propels the aircraft forward.
- Sitting in a chair: Your weight pushes the chair down; the chair pushes you up (normal force).
Frequently Asked Questions
If forces are equal and opposite, why do objects accelerate?
Action-reaction forces act on different objects. When you push a wall, the wall pushes back on you. But the net force on you (from the wall, friction, etc.) and the net force on the wall (from you, its foundation, etc.) are analyzed separately. Objects accelerate based on the net force on that specific object.
Does Newton's third law apply to non-contact forces?
Yes. Gravitational, electric, and magnetic forces all obey Newton's third law. The Earth pulls the Moon with gravity, and the Moon pulls the Earth with exactly the same magnitude of force. This is why the Earth wobbles slightly as the Moon orbits it.