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



Robert Cohen wrote:

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.

I wouldn't go near this with a 3m pole.

*ARE* there any laws that are "always true" ... so true that it
couldn't possibly be worth checking them?
*) Newton's laws? Depending on how you define F, m, and a,
the 2nd law is only approximately true, i.e. dependent on
the approximation that v << c
*) Special relativity? People still do ether-drift experiments.
They expect a null result, but they keep their eyes open anyway.
*) Equivalence of gravitational mass and inertial mass? People
still do Eötvös experiments. And the "equivalence" must be
verrry carefully stated to have any chance of withstanding
scrutiny (e.g. for rotating masses).
*) "2 and 2 makes 4"? Not true for the amplitude of waves.

Physics is a _natural science_, not an exact science. Get used to it.