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Re: [Phys-l] Is the friction relationship a model or a law?

As to the meaning of the word "law" I would not agree with Knight. The usage is that a law is an equation or relationship between variables. At one time the idea of a law as being a proven theory was being taught in school, but that was a complete misrepresentation of usage. It would seem that Knight is partially using that idea.

I would also not agree with his usage of model. A model is usually used in a similar fashion to theory. But as it is used by "Modeling" it is a coherent body of ideas that can be used to "understand" a set of phenomena. So they have the constant velocity model, the constant acceleration model...

Now the U. Mass Amherst group has an interesting take on some of these ideas. They use the term "empirical force law" for Hooke's law, friction, F_g = m g ... They do distinguish beween a limited usage empirical force law and a more general law such as Newton's gravitational law.

Unfortunately there is not yet a text that I would call best in every way. So you always have to fight or supplement some parts of almost any text.

John M. Clement
Houston, TX


1. Knight writes that these equations are a "model" of friction, not a
"law" of friction because while they are reasonably accurate
descriptions of how friction forces act, they are not perfect. Because
they are simplifications of reality that work reasonably well, they are
more appropriately called "models", rather than laws.

I can see why Knight makes these points - he wants to distinguish
between "laws of nature" that are "always true" and empirical
relationships that are "mostly true". However, the way he is using the
terms "model" and "law" are not the way I would. Rather, I'd call these
laws (or, at least, empirical relationships) which, like Hooke's law or
Ohm's law, describe an observed relationship. [By the way, Knight makes
the same comment about Hooke's law and Ohm's law.]