The same force that makes an apple fall from a tree keeps the Moon in orbit around the Earth, and the Earth around the Sun. This unifying insight was Newton’s great triumph: for the first time, a single mathematical law described bodies on Earth and celestial bodies in the heavens. The chapter starts from the inverse-square law, derives its potential energy, classifies orbits by total energy, and closes with escape velocity and satellites. It brings Part I, devoted to mechanics, to a close.
Sections
- The law of universal gravitation
- Universal gravitational potential energy
- Orbital trajectories
- Escape velocity
- Satellites and circular orbits
Reflections
- From Kepler to Newton: universal gravity
- The apple and the Moon
- Newton, Hooke and priority for the inverse square
- An apple and a moon: the invention of the earthly sky