Newton's mathematical principles of natural philosophy

Some people say that Newton realized the law of gravitation because of the apple hitting his head and thinking of why the moon didn't fall. Actually, Newton thought about the problem that "the moon is like an apple falling down". The moon moves at the point of A along the direction of A. If it is not affected by the attraction of the ground ball, it will fly away along AB due to inertia. It is because of the attraction of the earth that the moon will eventually fall below B. Qi OB', the moon's curvilinear motion is the same as that of the earth's surface, so the moon will never fall to the ground. Newton realized at this point that the attraction between the celestial bodies and the earth's attraction to the earth's surface is the same force.

In the 20 years from 1665 to 1685, Newton expanded the idea of gravitation. In the end, he thought that all things in the universe had gravitational effects on each other, so called "gravitation". The law of universal gravitation: there is an interaction gravity between any two objects, and the direction of the force is along the line of two objects. The magnitude of the force is directly proportional to the product of the mass of two objects, and is inversely proportional to the square of the distance between them.

Newton finally realized the true meaning of gravitation, and unified the mechanics of the ground and the mechanics of the sky in harmony, forming a mechanical system based on the three laws of motion. This is the first great synthesis in the history of physics, and also a great leap in the history of human cognition.

More than 100 years later, in 1798, the British scientist Cavendish used the torsion balance experiment to determine the gravitational constant G value accurately. This experiment is one of the "most beautiful ten experiments". The data measured at that time were G 6.754, 10, 1, m3/ (kg? S). With constant G, the quality of the earth can be easily calculated. Cavendish is the first person to call out the earth's quality, and he has opened up a new channel for the determination of the quality of the stars.