In the following years, Newton claimed that this happened. Sitting in his sister's orchard, Newton heard a familiar voice, and an apple fell on the grass with a bang. He turned quickly to watch the second apple fall to the ground. The second apple fell from the outstretched branch, bounced on the ground and lay quietly on the grass. This apple is definitely not the first apple Newton saw fall to the ground. Of course, the second apple is no different from the first one. Although the apple landing did not provide Newton with an answer, it inspired the young scientist to think about a new problem: the apple will land, but the moon will not. What's the difference between an apple and the moon?

The next morning, it was sunny, and Newton saw his little nephew playing with a ball. He has a rubber band tied to his hand, and a ball tied to the other end of the rubber band. He swung the ball slowly first, then faster and faster, and finally the ball was thrown straight.

Newton suddenly realized that the movements of the moon and the ball were very similar. There are two forces acting on the ball, one is the outward driving force, and the other is the tension of the rubber band. Similarly, there are two forces acting on the moon, namely, the driving force of the moon's movement and the pulling force of gravity. It is under the action of gravity that the apple will fall to the ground.

Newton believed for the first time that gravity is not only a force between planets and stars, but also a universal force. He believed in alchemy and thought that matter attracted each other, which led him to assert that mutual attraction applied not only to huge celestial bodies, but also to objects of various sizes. Apples fall, raindrops fall, and planets revolve around the sun, all of which are the results of gravity.

It is generally believed that the laws of nature applicable to the earth are completely different from those applicable to space. Newton's law of universal gravitation gave this view a heavy blow, telling people that the laws governing nature and the universe are very simple.

Newton promoted the development of the law of universal gravitation, pointing out that universal gravitation is not only the characteristic of stars, but also the characteristic of all objects. As one of the most important scientific laws, the law of gravity and its mathematical formula have become the cornerstone of the whole physics.

Of course, Newton put forward the theory of universal gravitation at that time, but failed to get the formula of universal gravitation, because the "G" in the formula was too small, so he put forward: f ∝ mm/r 2. It was not until 1798 that Cavendish, a British physicist, used the famous Cavendish torsion balance (Cavendish experiment) to accurately measure the value of the gravitational constant.