Every Action Has... MeNewton's Third Law

He doesn't strike first. He can't. In this universe nobody gets to.

Newton wrote the rule in Latin in the Principia of 1687: to every action there is always an equal and opposite reaction. The modern reading is the first line on this page. If body A pushes on body B, then B pushes on A with a force of exactly the same size in exactly the opposite direction, at the same moment.

Two things people get wrong. First, the pair never cancels, because the two forces act on different bodies. A horse pulls a cart forward; the cart pulls the horse back just as hard. The horse still moves, because the force that decides the horse's motion is the ground pushing its hooves forward, and that is bigger than the cart's pull. List the forces acting on one object, add only those, and the puzzle goes away.

Second, equal force is not equal effect. Stand on the floor: Earth pulls you down with your weight, and you pull Earth up with the same force. For a 70-kilogram person, Earth accelerates toward you at about 10⁻²² metres per second squared. It obeys. It just has 5.97 × 10²⁴ kilograms of reasons to barely move.

Add the two forces up over time and you get the second line: the total momentum of the pair cannot change by itself. That is how a rocket works in empty space. It has nothing to push against except its own exhaust, and that is enough. The exhaust leaves one way with some momentum; the rocket gains the same amount in the other direction.

He is not vindictive. He is exact. Push him and he pushes back with the same force, on you, at the same instant.