Three short rules from 1687 still run most of the engineering around us
Things keep doing what they are doing unless pushed. A push changes motion in proportion to its strength. Every push comes with an equal push back. Isaac Newton set out those three statements in 1687, and although relativity and quantum theory later found their limits, they remain the backbone of classical mechanics.
Newton published the laws in his Principia Mathematica, the Mathematical Principles of Natural Philosophy, and used them to explain the movement of a huge range of objects. Later physicists, drawing especially on the idea of energy, built all of classical mechanics on that base. Only in modern times did extreme cases, the very small and the very fast, reveal where the laws fail, prompting quantum mechanics and relativity.
The first law is the principle of inertia. Left alone, a body stays at rest or keeps moving straight at a constant speed; only an outside force disturbs that. Its modern reading is that no inertial observer is privileged. Someone standing still beside a railway and a passenger on a smoothly running train are equally entitled to call themselves at rest, since neither feels any effect of motion.
The second law links force and change. What Newton called motion is today's momentum, depending on how much matter a body holds, how fast it goes and in which direction. The net force equals the rate at which momentum changes, which for a body of fixed mass means mass times acceleration. The third law says forces come in pairs: when two bodies act on each other, the pushes are equal in size and opposite in direction.
Applying the laws requires careful words. Physics gives technical meanings to everyday terms, so force is not the same as power or pressure, and mass is not weight. The laws are usually stated for point masses, which works when a body's internal parts can be ignored and distances dwarf sizes. Earth counts as a point while orbiting the Sun, but certainly not for anything happening on its surface.
Source: Newton's laws of motion
