# Interlude 2 - Dynamics: Causes of motion

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 and stopping it might be painful.

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?

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.

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 a productive way forward. When we got sufficient technology to produce more motions and to observe things more quantitatively we discovered that Newton's theory was far more useful.

In many ways, Newton's laws can be seen as the principles that serve as conceptual basis for many of results that are the basis of our understanding of the physical world, ranging from figuring out how to send a rocket past Pluto, to making sense of what work and energy really mean, to understanding the electrical signals on an axon, to the ideal gas law, and to the HP equation. In a real sense, just as Dobzhansky said, "Nothing in biology makes sense without evolution," once might say, "Nothing in physics makes sense without Newton's laws." They are worth the effort to learn them well and to learn to reach for them in any situation that involves motion (and in many that don't).

The next few chapters build Newton's laws, how to models of interactions (forces), and how to apply them to describe physical motions.

• Newton's laws — deciding on and understanding the basic principles governing motion, both conceptually and mathematically
• Modeling forces — building models of the different kinds of interactions that occur between objects that lead to changes in motion, ranging from crude models (Hooke's law for spring forces) to powerful wide ranging principles (Coulomb's law for electric forces)

Joe Redish 9/12/11 and 1/28/19

Article 301