Definition · Plain-language
Newton’s third law
Newton’s third law states that for every action there is an equal and opposite reaction: forces always occur in pairs acting on two different objects.
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Forces come in pairs
Newton’s third law states that forces never occur alone — they always come in pairs. If object A pushes on object B, then object B pushes back on object A with a force that is equal in magnitude and opposite in direction. These are often called the action and reaction forces, though neither comes first; they appear and disappear together. When you press on a wall, the wall presses back on your hand with exactly the same force. The pairing is universal: gravity, electric forces, contact pushes and pulls all obey it.
Why the pair does not cancel
A common puzzle is why, if every force has an equal and opposite partner, anything moves at all. The answer is that the two forces in a pair act on different objects, never the same one. When you jump, your feet push down on the Earth and the Earth pushes up on you; the upward force is what launches you, while the downward force on the Earth is the same size but, given the Earth’s enormous mass, produces no noticeable acceleration. Because the partner forces are on separate bodies, they do not combine and cannot cancel each other.
Propulsion and everyday motion
The third law explains how things move forward. A rocket pushes hot exhaust gas downward, and the gas pushes the rocket upward — it does not need anything to push against, which is why rockets work in the vacuum of space. Walking works the same way: your foot pushes backward on the ground, and the ground pushes forward on you. Swimming, rowing, the recoil of a fired gun and the lift of an aircraft wing are all manifestations of equal-and-opposite force pairs. Recognising the two objects involved is the key to applying the law correctly.
Key facts
At a glance
- Definition: for every action there is an equal and opposite reaction
- Force pair: A on B equals B on A, opposite in direction
- Key point: the two forces act on different objects, so they never cancel
- Equation: F(A on B) = −F(B on A)
- Propulsion: rockets work by pushing exhaust gas, which pushes back
- Universal: applies to gravity, electric, contact and all other forces
Common misconceptions
What people often get wrong
Often heard: Action and reaction forces cancel each other out, so nothing can move.
Actually: They act on two different objects, so they never combine on one body. Each object feels only its own force, which is why both can accelerate.
Often heard: The action force happens first and the reaction follows a moment later.
Actually: The two forces appear, change and vanish at exactly the same instant. Neither is the cause of the other; they are two sides of a single interaction.
Often heard: A rocket pushes against the air or the ground to move.
Actually: A rocket pushes against its own ejected exhaust gas, which pushes back. That is why rockets work in the vacuum of space, with no air to push on.
Going deeper
