Principia Mathematica
Newton connects terrestrial and celestial motion through mathematical laws and universal gravitation.
The requested Newton work is the Mathematical Principles of Natural Philosophy, commonly called the Principia, not Russell and Whitehead's later book with a similar shortened title.
A falling object and an orbit look very different. Newton's achievement is to connect them through one mathematical account, making the heavens less separate from the world under our feet.
Three laws describe inertia, the relation between force and changing motion, and paired interactions. Mathematical reasoning connects these principles with trajectories under different forces.
Universal gravitation links the motions of planets and moons with familiar terrestrial effects. The same physical framework explains phenomena previously treated as belonging to separate earthly and heavenly domains.
The work develops mathematical treatments of motion, resistance, and the system of the world. Its theory remains extremely useful within appropriate conditions, while later relativity changes the account of gravity and its limits.
A force changes motion rather than being required to sustain uniform motion.
A powerful model still has conditions of use. Later physics does not make every ordinary Newtonian calculation worthless; it explains where the earlier picture stops being a sufficient account.
Mathematical idealisation isolates relationships that complex observations can conceal.
Orbital paths and falling bodies belong to one explanatory framework.
The treatment of resisting media examines departures from ideal motion.
Evidence constrains proposed laws even when mathematics expresses them elegantly.
Later theories preserve useful approximations while revising fundamental interpretation.
Later theories preserve useful approximations while revising fundamental interpretation.
How did one mathematical framework connect a falling body with the motion of a planet without becoming valid in every condition?