# 牛顿第三定律的疑惑与修正

Feynman 说：

“Newton told us only two things about the nature of forces. In the case of gravitation, he gave us the complete law of the force. In the case of the very complicated forces between atoms, he was not aware of the right laws for the forces; however, he discovered one rule, one general property of forces, which is expressed in his Third Law, and that is the total knowledge that Newton had about the nature of forces—the law of gravitation and this principle, but no other details.

This principle is that action equals reaction.

What is meant is something of this kind: Suppose we have two small bodies, say particles, and suppose that the first one exerts a force on the second one, pushing it with a certain force. Then, simultaneously, according to Newton’s Third Law, the second particle wnill push on the first with an equal force, in the opposite direction; furthermore, these forces effectively act in the same line. This is the hypothesis, or law, that Newton proposed, and it seems to be quite accurate, though not exact (we shall discuss the errors later). ”

“————–Let me make something clear. Isaac Newton had it tough. Here he is trying to make a model that describes what forces do to objects. However, he just doesn’t have all the words we have – like momentum, acceleration, velocity.Fortunately, there is an online translation of the original works of Newton. Here is an older translation into English of Newton’s The Mathematical Principles of Natural Philosophy (volume 1).The part about the Third Law says:To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.He also writes:
If you press a stone with your finger, the finger is also pressed by the stone.This above example could indicate that he did not mean the stone and the finger have to move. I would suggest that perhaps Newton meant the word “action” to really mean “force”. However, he later uses the word force so I’m not exactly sure of his intentions. Perhaps by “action” he means a force that actually does something. I don’t know.Oh, and it seems Newton does use the words momentum and velocity (if you read more of the passage). Maybe he was just trying to be poetic with his use of the words “action” and “reaction”.”

*为何写《行星齿轮的动力学》？