# 3 Laws of Motion The three laws of motion formulated by [[Isaac Newton]] in *Philosophiæ Naturalis Principia Mathematica* (1687). They describe the relationship between a body and the forces acting on it, and the body's motion in response. They are the foundation of classical mechanics. ## First Law — Inertia An object at rest stays at rest, and an object in motion stays in motion with the same speed and direction, unless acted upon by an external force. Inertia is the tendency of a body to resist changes in its state of motion. Without a net force, velocity does not change. ## Second Law — F = ma The net force on an object equals its mass times its acceleration: $F = m \cdot a$ More precisely, force equals the rate of change of momentum. Heavier objects need more force to accelerate; the same force produces less acceleration on a larger mass. ## Third Law — Action & Reaction For every action there is an equal and opposite reaction. Forces always come in pairs. If body A exerts a force on body B, then B exerts an equal force in the opposite direction on A. This is why rockets work, why you can walk, and why a gun recoils. ## Why It Matters - Explains everyday motion — walking, driving, throwing, collisions. - Foundation of engineering — from bridges to rockets to orbital mechanics. - A model for how a small set of principles can describe a vast range of phenomena — a template for how to think in systems. - Limits: Newton's laws break down at very high speeds (relativity) and very small scales (quantum mechanics), but remain an excellent approximation for most human-scale problems. ## References - *Philosophiæ Naturalis Principia Mathematica* — [[Isaac Newton]] (1687) ## Related - [[Isaac Newton]]