Newton's third law
Whenever one object exerts a force on a second object, the second exerts an equal and opposite force back on the first.
Practise this conceptEvery force comes as part of a pair: if object A pushes on object B, then B pushes back on A with a force of exactly the same size, in exactly the opposite direction. This holds all the time, not just in collisions — standing still on the floor means the floor is pushing up on you as hard as you're pushing down on it.
The pair of forces never cancels out, because the two forces act on different objects. A force on you and a force on the floor can't add up to zero for you specifically; only forces acting on the same object can do that. This is the single most common trap with this law.
Rockets are the classic illustration: the engine pushes exhaust gas backwards, and the gas pushes the rocket forwards with equal force — no air or ground to push against required, which is exactly why rockets work in the vacuum of space.
Key ideas
Requires: Newton's first law
Unlocks: Conservation of momentum
Common misconceptions
- The two forces in an action-reaction pair act on the same object, so they cancel out.
- The reaction force is slightly smaller than the action force because some energy is lost.
- A heavier object exerts a bigger force on a lighter one than the lighter one exerts back.