HIS 322D November 21, 2011
Today:
The Newtonian World System
Newton’s finding of these laws had an enormous impact on European thought since the 1680s.
His work gave a new refined meaning to natural law, and what laws of nature mean.
Newton divided his
Principia
into three parts. (PrinKipia)
In the first book, he laid out his general principles.
First Law: His first law of motion. A body in motion will stay in motion unless acted on by
something else. A body at rest will stay the same unless acted by another force.
Second law: F = ma. Force is defined as mass times acceleration.
Third law: there is an equal and opposite force in everything. If A exerts a force on B, there is an
equal and opposite force on A.
F1 = F2 = G (m1 X m2) all divided by r
2
. This gives you the gravitational forces.
Also in this first book, Newton gives abstract theories with how these laws will work out.
Newton works these problems in terms of geometry (and not in his new calculus). If he used his
new mathematical form, then people can question this math. People may not believe his calculus
really works. But by putting it in terms of geometry that people know, he avoids any controversy
over his mathematical techniques.
The second book is mostly aimed at refuting Descartes. It’s his treatment of fluid. He’s showing
Descartes vortices don’t work. He shows vortex theory is inconsistent and does not explain what
it seen. (not much else was said about this book)
The third book is the “payoff.” He put this is very rigorous mathematical form. This is where he
applies the laws of motion and force to the planetary system and the motion of things on earth.
He explains the observed motions of the planets. He derives very readily Kepler’s equal areas
law.
There were parts of all of this that were already in the air, and already thinking of these things.
But Newton carries this so much farther, particularly due to his treatment of force.
He states every particle is exerting a force on other particles. Weight is what’s exerted here on
earth. Mass does not change, though. Weight will change depending on where you are.
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 Fall '11
 HUNT
 Scientific Revolution, Sir Isaac Newton

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