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Newton's Laws

Page history last edited by Joe Redish 8 years, 8 months ago

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Newton's theory of motion was the first of the broad, successful, and powerful scientific theories that organize our knowledge of the physical world.  In this sense it is comparable to Darwin's theory of evolution by natural selection in biology.  It provides a language to describe the motion of essentially all matter from the molecular scale to the scale of the solar system -- a range of more than 20 orders of magnitude! (At the sub-molecular scales, the quantum theory rules, and at supra-galactic scales, something is going on that we don't quite understand. We call it "dark matter" -- and it may be -- but it could also be a failure of Newton's theory of motion at very large distances or very small accelerations.)

 

You would think that a theory of motion would be easy.  After all, we all have experience with motion. We can throw objects and catch them, and we know how to walk, jump, and drag objects along.  But most of our knowledge about motion is in bits and pieces.  We know that when we push a box along a concrete floor, if we stop pushing, the box seems to want to stop.  But we also know (if we've ever played baseball, football, lacrosse, or field hockey) that a rapidly moving ball doesn't seem to want to stop at all. In lots of other places we find that our "folk theory of motion" may be locally consistent -- that is, it describes a few similar situations -- but different situations seem to require different principles. And though we know what to do in lots of situations, we don't have any idea of what governs which of our many principles we should apply. When we look at a paramecium under a microscope or consider the motion of a bullet shot from a gun, how do they compare with our everyday experiences?  Why do planets and moons seem to move differently from our everyday experiences?

 

The amazing thing about Newton's theory of motion is that with a few simple ideas it gives us a structure with which we can model and understand a huge range of phenomena. And one of the exciting things about it is that we can figure out from some of our simplest experiences with motion what these principles are. The hard part is that the unexpected champion concept that makes this all possible is acceleration -- a concept that we don't really use much in our everyday lives (at least we didn't before trains and cars were invented). That's perhaps a prime reason why it took people thousands of years to decide that Aristotle's theory, while it described some everyday phenomena reasonably well, was not correct. When we got sufficient technology to produce more motions and to observe things more quantitatively we discovered that Newton's theory was far more productive.

 

To begin, read the following items that specify the conceptual content that underlies Newton's theory of motion.

 

Follow-ons

 

Joe Redish 9/12/11

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