Long before Newton discovered the law of gravity, many scientists had seriously considered this problem. For example, Kepler realized that there must be a force at work that makes the planet move along an elliptical orbit. He thinks this force is similar to magnetic force, just as a magnet attracts iron. 1659, Huygens found that a centripetal force was needed to keep the object moving in a circular orbit by studying the movement of the pendulum. Hooke and others thought it was gravity, and tried to deduce the relationship between gravity and distance.

1664, Hooke found that when comets approached the sun, their orbits were curved due to the sun's gravity. 1673, huygens deduced the law of centripetal force; 1679, Hooke and Halley deduced from centripetal force law and Kepler's third law that the gravitational force for maintaining planetary motion is inversely proportional to the square of distance.

Newton himself recalled that around 1666, he had considered the problem of gravity when he lived in his hometown. The most famous saying is that Newton often sits in the garden for a while during holidays. Once, as it happened many times before, an apple fell from the tree. ...

The accidental landing of an apple is a turning point in the history of human thought, which opens the mind of the person sitting in the garden and causes him to ponder: What is the reason why almost all objects are attracted by the center of the earth? Newton mused. Finally, he discovered the gravity which is of epoch-making significance to mankind.

Newton's genius lies in that he solved the mathematical argument problem that Hooke and others could not solve. 1679, Hooke wrote to Newton and asked him if he could prove that the planet moves in an elliptical orbit according to the law of centripetal force and the law that gravity is inversely proportional to the square of distance. Newton didn't answer the question. 1685, when Harley visited Newton, Newton had discovered the law of universal gravitation: there is gravitation between two objects, which is inversely proportional to the square of the distance and directly proportional to the product of the masses of the two objects.

At that time, accurate data such as radius of the earth and the distance between the sun and the earth were available for calculation. Newton proved to Harley that the gravity of the earth is the centripetal force that makes the moon move around the earth, and also proved that the planetary motion conforms to Kepler's three laws of motion under the action of solar gravity.

At the urging of Harley, at the end of 1686, Newton wrote an epoch-making masterpiece, Mathematical Principles of Natural Philosophy. The Royal Society is short of funds to publish this book. Later, one of the greatest works in the history of science was published in 1687 with Harley's support.

In this book, from the basic concepts of mechanics (mass, momentum, inertia, force) and basic laws (three laws of motion), Newton not only demonstrated the law of universal gravitation mathematically, but also established classical mechanics as a complete and rigorous system, unified celestial mechanics with ground object mechanics, and realized the first large-scale synthesis in the history of physics.

Standing on the shoulders of giants