Newton's Third Law — Action and Reaction, Clearly Explained
For every action there is an equal and opposite reaction. What action-reaction pairs really are, why they don't cancel, and classic examples.
Quick answer
When object A exerts a force on object B, object B simultaneously exerts a force on A that is equal in strength and opposite in direction.
Example: A rocket pushes exhaust gases downward; the gases push the rocket upward with exactly equal force — that push is what lifts the rocket.
What you will learn
- State Newton's third law precisely
- Identify action-reaction pairs in real situations
- Explain why the pair doesn't cancel out
- Apply the law to walking, rockets and collisions
The law, precisely stated
Newton's third law: for every action there is an equal and opposite reaction. More carefully: forces always come in pairs — if body A exerts a force on body B, then body B exerts a force on body A, equal in magnitude and opposite in direction.
Two words matter: equal (same size) and opposite (exactly reversed direction). And the two forces are simultaneous — there is no delay while the universe 'responds'.
Action-reaction pairs: the rules
A genuine third-law pair always satisfies three rules:
- Same type of force — both gravitational, both contact, both electric. Weight's partner is another gravitational pull, not the normal force.
- Different objects — one force acts on A, the other on B. Never both on the same object.
- Same line, opposite direction — along the line connecting the interaction.
Test yourself: a book rests on a table. The book's weight (Earth pulling the book down) pairs with the book pulling Earth up. The table pushing the book up (normal force) pairs with the book pushing the table down. Weight and normal force are not a third-law pair — a classic trap.
Why don't they cancel out?
The number-one confusion: if every force has an equal opposite partner, why does anything ever move? Because the two forces act on different objects.
When you push a shopping cart forward, the cart pushes you backward with equal force. The cart accelerates because your push is (nearly) the only horizontal force on it. You don't fly backward because friction between your shoes and the floor anchors you. Cancellation only happens for forces on the same object — and third-law partners never share an object.
Walking, swimming, flying
- Walking: your foot pushes backward on the ground; the ground pushes you forward. Try walking on frictionless ice to feel how essential that reaction is.
- Swimming: hands push water back; water pushes you forward.
- Birds and planes: wings push air down and back; air pushes wings up and forward.
- Recoil: a rifle pushes the bullet forward; the bullet pushes the rifle back into your shoulder — same force, but the rifle's large mass means small acceleration.
The horse-and-cart puzzle
A horse says: 'If I pull the cart forward, the cart pulls me back equally — so we can never move!' The flaw: the horse's pull on the cart and the cart's pull on the horse act on different bodies.
What moves the system is external: the horse pushes backward on the ground, and the ground's forward reaction exceeds the cart's backward pull. Internal pairs cancel; external forces decide the motion.
Third-law pairs never cancel because they act on different objects. To find motion, list all forces on ONE object — that's where the second law takes over.
Variables and units
| Symbol | Name | Unit | Meaning |
|---|---|---|---|
| Force of A on B | N | Action force exerted by object A on object B | |
| Force of B on A | N | Reaction force exerted by object B on object A |
Common mistakes
They act on different objects, so they can't cancel on either one. Each object responds to the single force acting on it.
The forces are exactly equal — always. A truck hitting a mosquito exerts the same force on the mosquito as the mosquito exerts on the truck. The effects differ because accelerations (a = F/m) differ wildly.
The pair is simultaneous. When your hand pushes the wall, the wall pushes back at the very same instant — that's why you feel it immediately.
Real-world applications
Rocket and jet propulsion are pure third law: throw mass one way fast, get pushed the other way.
Firearm and artillery design must manage recoil — the equal-and-opposite momentum given to the gun.
Swim technique coaching focuses on maximizing the backward push of water to maximize the forward reaction on the swimmer.
Frequently asked questions
Do action and reaction act on the same object?
Never. That's the whole point of the law. If both forces acted on the same object they'd cancel, and the law would predict nothing ever moves. One force per object, always.
If the forces are equal, why does the lighter object accelerate more?
Because acceleration is force divided by mass (a = F/m). Same force, smaller mass, bigger acceleration. The mosquito and the truck feel equal forces; only the mosquito notices.
Is the normal force the reaction to weight?
No — this is the most common textbook trap. Weight (Earth pulling you down) pairs with you pulling Earth up. The normal force (floor pushing you up) pairs with you pushing the floor down. They're equal only in simple resting cases, by coincidence of the second law, not the third.
Does the third law apply to magnetic and electric forces too?
Yes. It applies to all fundamental interactions, including fields. The law is really a statement of momentum conservation: whatever momentum A gains, B loses.