The Traffic Accident Reconstruction Origin -ARnews-

Re: Is Drag Factor Velocity-Dependent or Not?

Bruno Schmidtt (SchmidtBF@aol.com)
Wed, 25 Jun 1997 07:22:32 -0400 (EDT)

Andrew, my point is this: When we invoke Coulomb's "law" (and Newton's "laws") we are using statements (or equations) that were developed from a series of experiments and observations that are ALWAYS limited, and in trying to apply them to all situations, some errors will undoubtedly occur. Some "laws" are better than others, and have more universal application. Neither Coulomb nor Newton did an experiment with a braking automobile or truck. It turns out, however, that Newton's "law" is, as you put it, a damn good one, because we have observed that his equations apply very, very well unless objects approach the speed of light, which automobiles usually don't do. So my point was identical to yours regarding Newton. Coulomb's law, however, is much more limited, and does not address the fact that friction has a much more complicated behavior than Coulomb's simple equation would indicate. Just because an equation is called a law does not mean that we can invoke it without criticall
y examining it. The same goes for Hooke's LAW--that the amount something stretches (or compresses) is proportional to the force applied to it. That is the basis for computer-based crush simulations. Yet, Hooke's law is limited in its application, and I can give you countless examples of where applying Hooke's "law" will lead to very erroneous results (even for things not traveling at the speed of light).

The bottom line: Virtually all physical laws, based on limited observations, are only approximate descriptions of the way the world as we know it behaves. Fortunately, the approximation is so good, that reconstructionists can invoke Newton as truly a law. Other "laws" must be examined carefully to see how well they apply to our situation. Don't say something is absolutely perfectly correct because people call it a law.

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