Isaac Newton (1642-1727) discovered the three laws of motion and the law of gravity. He wrote about it in his book, the “Principia” (1687). It laid the foundation of a new and better physics in the West after the fall of Aristotle’s physics in the early 1600s. It made physical sense out of the theories of Copernicus and Kepler.
The old physics of Aristotle only told you what happened in general. With Newton you could work out the numbers in detail for yourself and predict what would happen. It was that good.
Both Newton and Kepler were numerologists: like
Pythagoras, they played with numbers endlessly, believing they held the key to the mysteries of the universe. For them it worked: They both succeeded in discovering laws of nature where numbers played an important part. This gave science in the West its great faith in numbers and laws of nature.
Newton built on the physics of Galileo, but he did two things that Galileo did not do:
- Newton took Kepler seriously.
- Newton added the idea of a force of nature acting at a distance: one thing could affect another across empty space. Magnets are just one example of this.
Legend has it that Newton once sat under an apple tree thinking about the motion of the moon. Just then an apple struck him on the head. In that moment he saw how both the moon and the apple followed the same law of nature: the law of gravity.
Gravity is the force that on earth causes things to fall.
Anything made of matter has gravity, but the larger it is the more gravity it has. Even a stone has gravity – but it is too weak to notice. The earth, however, is so large that it pulls everything near it to itself. That is why things fall.
In the case of the moon, it does not fall to earth because it is going so fast that it goes in a circle round the earth, attempting to break loose but never quite succeeding. In a sense the moon is falling forever.
But the force of gravity falls off quickly – by the square of the distance. For example, it is only a fourth as powerful at two times the distance; a ninth as powerful at three times the distance, and so on.
Newton’s three laws of motion:
- Absent any force, an object at rest remains at rest; an object in motion remains in motion – in the same straight line, going neither faster nor slower.
- F = ma. Force (F) is measured by the amount of mass (m) something has (on earth this is the same as its weight) times the acceleration (a) (how much faster the mass is forced to go).
- Every action has an opposite and equal reaction.
The first comes from Galileo.
Applying these three laws to those of Kepler, Newton worked out the law of gravity.
Physics was largely unchanged until the theories of Einstein and quantum physics in the 1900s.
See also:
Abagond 
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