Imagine you are standing on a skateboard and you push against a wall. The wall does not move, but you roll backward. You pushed the wall, and the wall pushed back on you — at the very same instant, with the very same strength.
This is the core idea behind Newton's Third Law: whenever one object exerts a force on a second object, the second object exerts an equal and opposite force back on the first. These two forces are called an action-reaction pair (or "Third Law pair").
Note
The two forces in a Third Law pair act at the same instant — there is no delay between "action" and "reaction." They are two sides of a single interaction, not a cause followed by an effect.
The Precise Statement
Newton's Third Law: If object A exerts a force on object B, then object B exerts an equal and opposite force on object A.
FA on B=−FB on A
Both forces have the same magnitude and act along the same line, but point in opposite directions — and, crucially, they act on two different objects.
The Key Rule for Spotting a Third Law Pair
A genuine Third Law pair must satisfy two conditions:
The two forces act on different objects (never on the same object).
They arise from the same interaction (the same pair of bodies in contact, or the same gravitational/electric attraction).
If a pair of forces acts on the same object, they are not a Third Law pair — even if they happen to be equal in magnitude and opposite in direction.
Why Equal and Opposite Forces Don't Cancel
This confuses many students: if the two forces are always equal and opposite, why does anything ever move?
Because the two forces act on different objects — they never appear together in the same free-body diagram. When you push against a wall, the wall pushes back on you. That reaction force acts on you, not on the wall, so it can accelerate you backward even though the wall itself does not move (the wall is anchored to the Earth, which is far too massive to notice the force).
A Careful Worked Example: A Book on a Table
A book sits at rest on a table. Two different force pairs are involved here, and it is important not to mix them up:
Pair 1 (gravitational interaction): Earth pulls the book downward with a force equal to the book's weight, mg. By the Third Law, the book pulls the Earth upward with the same magnitude, mg. This pair acts on two different bodies — the book and the Earth.
Pair 2 (contact interaction): The table pushes up on the book with a normal force N. By the Third Law, the book pushes down on the table with a force of the same magnitude, N. This pair also acts on two different bodies — the book and the table.
Watch out
A very common mistake is to say "the normal force equals the weight because of Newton's Third Law." This is wrong. The book's weight and the table's normal force both act on the same object (the book), so they cannot be a Third Law pair — they merely happen to be equal in magnitude here because the book is in equilibrium (Newton's second law with zero acceleration, N−mg=0). The true Third Law partner of the book's weight is the pull the book exerts on the Earth; the true Third Law partner of the normal force is the push the book exerts on the table.
More Everyday Examples
Walking: Your foot pushes backward on the ground; the ground pushes forward on your foot. That forward push is what propels you ahead. …
Pushing water backward is the "action"; by Newton's third law, the water simultaneously pushes the swimmer forward with an equal and opposite "reaction" — that reaction is what actually moves the swimmer through the water.
Newton's third law of motion states that for every action there is an equal and opposite reaction, and these two forces act on two different bodies (never cancelling each other out on the same body).
When a person swims, they use their arms and legs to push a mass of water backward relative to their body. This backward push on the water is the action force (exerted by the swimmer's hands/feet on the water).
By Newton's third law, the water exerts an equal and opposite reaction force on the swimmer's hands/feet — and since the swimmer pushed the water backward, this reaction force on the swimmer is directed forward.
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2026Set ANNUAL1 markMCQ
Q.Swimming is possible on account of:
(a) First Law of Motion
(b) Second Law of Motion
(c) Third Law of Motion
(d) Newton's Law of Gravitation
›Reveal solutionSolution
Swimming works because of the action-reaction pair described by Newton's third law.
Newton's third law states that for every action there is an equal and opposite reaction, acting on the other body. When a swimmer pushes water backward with their limbs (the action force on the water), the water simultaneously pushes the swimmer's body forward with an equal-magnitude, oppositely-directed reaction force. It is this …
Q.The forward movement in swimming takes place because of:
(A) First law of motion
(B) Second law of motion
(C) Third law of motion
(D) Fourth law of motion
›Reveal solutionSolution
Swimming works by Newton's third law: the swimmer pushes water backward, and the water pushes the swimmer forward.
While swimming, a person exerts a backward force (action) on the water using arms and legs.
By Newton's third law of motion, the water exerts an equal and opposite forward force (reaction) on the swimmer. …
Q.When we Kick a stone, we get hurt due to which property of the stone if happens?
(a) Inertia
(b) Velocity
(c) Reaction
(d) Momentum
›Reveal solutionSolution
By Newton's third law, every action has an equal and opposite reaction -- when your foot exerts a force on the stone, the stone simultaneously exerts an equal and opposite reaction force on your foot, and it is this reaction force that hurts.
Newton's third law states that for every action force exerted by body A on body B, there is a simultaneous, equal-in-magnitude and opposite-in-direction reaction force exerted by body B on body A.