What is Newton's Third Law?
Newton's third law says forces always come in pairs: whenever one object pushes or pulls another, the second object pushes or pulls back with equal strength. It explains everything from walking to rocket launches.
For every action force, there is an equal and opposite reaction force: if object A exerts force F on object B, object B exerts force −F on object A, at the same instant.
- •Exerted by object A on object B
- •Has magnitude F
- •Acts on object B
- •Exerted by object B on object A
- •Has the same magnitude F
- •Acts on object A
- •Points in the opposite direction
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Step-by-step worked examples
A swimmer pushes water backward with 300 N. What force does the water exert on the swimmer?
By Newton's third law, the reaction force is equal and opposite. Water pushes swimmer forward with 300 N.
A rocket expels exhaust gas downward with 5000 N. What force propels the rocket upward?
Action: rocket pushes gas down with 5000 N. Reaction: gas pushes rocket up with 5000 N.
While walking, a foot pushes the ground backward with 150 N. What pushes the person forward?
Action: foot pushes ground backward, 150 N. Reaction: ground pushes foot forward, 150 N — this friction reaction propels the walker.
Flashcards
Quick quiz
Q1.Newton's third law states that every action has...
Q2.Action and reaction force pairs act on...
Q3.Do action-reaction pairs cancel each other out?
Q4.A box rests on a table. What is the reaction to the table pushing up on the box?
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Common mistakes
Thinking action-reaction forces cancel each other out. — Correct: They act on different objects, so they never cancel — only forces on the same object can cancel.
Confusing a third-law pair with two balanced forces on one object (like gravity and normal force on a resting book). — Correct: Those act on the same object and are just balanced, not a third-law action–reaction pair.
Believing the reaction force happens slightly after the action. — Correct: Action and reaction occur at exactly the same instant, with no delay.
Assuming a larger or heavier object exerts a bigger force in the pair. — Correct: The action and reaction forces are always exactly equal in magnitude, regardless of mass.
FAQ
What is Newton's third law?
It states that for every action force, there is an equal and opposite reaction force acting on a different object.
What is an example of Newton's third law?
A swimmer pushing water backward is pushed forward by the water with equal force — that's an action–reaction pair.
Do action and reaction forces cancel each other?
No — because they act on two different objects, so each object still experiences a net force.
How is Newton's third law used in rocket propulsion?
A rocket pushes exhaust gas backward (action); the gas pushes the rocket forward with equal force (reaction), propelling it.