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

For about a week now, my mailbox has been filling up with messages from this listserve. I stopped subscribing to it a couple of years ago, but suddenly it has reappeared on my computer. I sent the NOMAIL message to L-Soft list server at LISTS.NAU.EDU (1.8d) [LISTSERV@list1.ucc.nau.edu] and the reply was that I am not subscribed to PHYS-L. Then why am I getting messages from it? Could some please tell me how to get off this listserv that I am apparently not on, yet I keep getting messages as though I am still on it?


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David D. Moore
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-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
Sent: Friday, February 17, 2006 3:21 PM
To: PHYS-L Maillist
Subject: 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.

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