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How does Newton’s third law apply when two bodies are accelerating?

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Title page of Isaac Newton's Philosophiae Naturalis Principia Mathematica, 1687, featuring large Latin text and publication details in a bordered layout. — Newton’s laws of motion The title page of Isaac Newton's *Philosophiae Naturalis Principia Mathematica* (1687; *Mathematical Principles of Natural Philosophy*), the work in which the physicist introduced his three laws of motion.

Newton’s third law of motion applies to all interactions between bodies, regardless of their state of motion. Whether the bodies involved are at rest, in uniform motion, or undergoing accelerated motion, the law holds that for every action, there is an equal and opposite reaction.1

When two bodies interact, they exert forces on each other that are equal in magnitude and opposite in direction. Crucially, these action and reaction forces always act on different bodies, meaning they never cancel each other out by acting on the same object.1

If the bodies are accelerating, it is because there is a net force acting on them in accordance with Newton’s second law. While the third law ensures the mutual forces between the two bodies remain equal and opposite, the resulting acceleration of each body depends on its individual mass and any other external forces acting upon it. The third law does not produce a net force on the two-body system as a whole; rather, it describes the fundamental nature of the interaction between the two objects during their motion.12