Newtonian

Newtonian refers to concepts associated with Isaac Newton or to a broad class of theories and models known as Newtonian physics. In common usage it often denotes Newtonian mechanics, the framework for understanding motion and forces in everyday scales, before the advent of relativity and quantum mechanics.

Newtonian mechanics is based on Newton’s three laws of motion, the concept of inertial frames of reference, and the law of universal gravitation. Objects resist changes to their motion (inertia), forces cause accelerations proportional to mass (F = ma), and every interaction has an equal and opposite reaction. Gravitational attraction between masses follows an inverse-square law, allowing precise predictions of planetary motion and terrestrial dynamics. The theory is generally deterministic: given initial conditions, future states can be calculated. It also relies on the notions of absolute space and absolute time, though these ideas are refined in more modern treatments.

The reach of Newtonian physics extends to engineering, astronomy, and everyday mechanics, particularly for macroscopic objects moving well below the speed of light and in weak gravitational fields. It provides practical approximations for fluids, structures, projectiles, and orbital dynamics.

Limitations and evolution: Newtonian mechanics is an approximation. At high velocities, near-light speeds, special relativity and general relativity provide more accurate laws. In microscopic systems, quantum mechanics governs behavior, while gravity in strong curvatures is described by general relativity. Some modern formulations, like Newton–Cartan theory, connect Newtonian ideas with relativistic concepts in certain limits.

Other uses of the term include Newtonian fluids, whose viscosity is independent of shear rate (unlike non-Newtonian fluids), and Newton's broader contributions to optics and mathematics that underpin the classical view of physics.