Newton's Laws of Motion
The three laws of classical mechanics — inertia, F = ma, and action–reaction — that form the basis of classical physics.
Newton's three laws of motion, published in the Principia in 1687, describe how forces change motion and form the foundation of classical mechanics. They are statements about idealized objects — point masses moving in inertial frames — yet they accurately describe everything from falling apples to planetary orbits and spacecraft trajectories.
The first law (inertia) states that an object at rest stays at rest, and an object in motion stays in motion with constant velocity, unless acted on by a net external force. It defines inertial frames: reference frames in which the law holds. Its content is subtle — it asserts the existence of frames where free objects move uniformly — and it implies that a force is not needed to keep something moving, only to change its motion, which contradicted the everyday intuition that motion requires continuous pushing.
The second law quantifies the first: the net force on an object equals the time rate of change of its momentum, which for constant mass reduces to the familiar F = ma, with force measured in newtons (1 N = 1 kg·m/s²). Because force and acceleration are vectors, the law applies component by component — the vertical and horizontal motions of a projectile are independent. The third law (action–reaction) states that forces always come in pairs: if body A exerts a force on body B, then B exerts an equal and opposite force on A. A rocket works not by pushing on the air but by expelling exhaust backward, and the reaction pushes the rocket forward; the pair of forces always acts on different bodies, which is why they do not cancel.
Newton's laws fail in two regimes. At speeds approaching the speed of light, special relativity replaces them with relativistic dynamics; on atomic scales, quantum mechanics governs. The laws also make precise predictions only in inertial frames, requiring fictitious forces (centrifugal, Coriolis) in rotating frames. Historically, Newton's laws unified terrestrial and celestial mechanics: Kepler's laws of planetary motion, which had been empirical, were derived from the inverse-square law of gravitation and the second law — the first great unification of physics.
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forces inertia mechanics physics
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