Which law states that for every action there is an equal and opposite reaction?

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The main idea here is action-reaction forces that arise when two objects interact, described by Newton's third law. For every force one object applies to another, the second object applies an equal in magnitude but opposite in direction force back on the first one. These two forces form a pair, act on two different bodies, and occur simultaneously. That’s why they don’t cancel each other out when you look at the whole system—they each cause accelerations according to the mass of the object they act on.

A familiar example is pushing against a wall: your hand pushes the wall, and the wall pushes your hand back with the same force. Another example is a rocket rushing exhaust gases backward—the gas feels a backward force while the rocket feels a forward force, propelling it ahead.

This differs from Newton’s first law, which talks about motion continuing unless a net external force acts, and from inertia, which is the resistance to changes in motion. Conservation of momentum is about the total momentum of a system remaining constant in the absence of external forces, and while it relates to interactions, it isn’t the statement about the paired forces themselves.