At Principia's heart were Newton's three laws of motion (which state, very baldly, that a thing moves in the direction in which it is pushed; that it will keep moving in a straight line until some other force acts to slow or deflect it; and that every action has an opposite and equal reaction) and his universal law of gravitation. This states that every object in the universe exerts a tug on every other. It may not seem like it, but as you sit here now you are pulling everything around you—walls, ceiling, lamp, pet cat—toward you with your own little (indeed, very little) gravitational field. And these things are also pulling on you.
It was Newton who realized that the pull of any two objects is, to quote Feynman again, "proportional to the mass of each and varies inversely as the square of the distance between them." Put another way, if you double the distance between two objects, the attraction between them becomes four times weaker. This can be expressed with the formula F = Gmm/R^2 which is of course way beyond anything that most of us could make practical use of, but at least we can appreciate that it is elegantly compact. A couple of brief multiplications, a simple division, and, bingo, you know your gravitational position wherever you go. It was the first really universal law of nature ever propounded by a human mind, which is why Newton is regarded with such universal esteem.
是牛顿认识到，任何两个物体的引力，再用费曼的话来说，“与每个物体的质量成正比，以两者之间距离的平方反比来变化”。换一种说法，要是你将两个物体之间的距离翻一番，两者之间的引力就弱4倍。这可以用下面的公式来表示： F=Gmm/(R^2) 这个公式对我们大多数人来说当然是根本没有实际用途的，但至少我们欣赏它的优美，它的简洁。无论你走到哪里，只要做两个快速的乘法，一个简单的除法，嘿，你就知道你的引力状况。这是人类提出的第一个真正有普遍意义的自然定律，也是牛顿到处深受人们尊敬的原因。