From 1665 to 1667, Newton was already thinking about gravity. One night, he was sitting under an apple tree to enjoy the cool, and an apple fell from the tree. It suddenly occurred to him: Why do apples only fall to the ground and not fly to the sky? He analyzed the three laws of Copernicus' Heliocentrism and Kepler, and then thought: Why do planets go around the sun without leaving? Why are planets fast when they are near the sun and slow when they are far away from the sun? The farther the planet is from the sun, why does it run longer? Newton believed that their fundamental reason was that they were too attractive.
After a series of experiments, observations and calculations, Newton found that the gravity of the sun is closely related to its huge mass. Newton further revealed the universal law of the universe: everything is attractive; The greater the quality, the greater the attraction; The greater the distance, the less attractive it is. This is the famous law of universal gravitation in classical mechanics.