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

[Bernard Cleyet <bernardcleyet@redshift.com>]

I think rolling friction comes not from sliding and crushing / breaking 
of the "rough edges", but from climbing out of a hill.  e.g. inflated 
hard tire vs. soft one *, rail vs. road, etc.  The model, at least in 
the case of steel on steel, is Hooke's while the auto is less elastic, 
both in the tire and the air.

I was about to write Ahh, so it's Hooke's model  (never perfect w/ 
different degrees of perfection, e.g. fused quartz vs. a rubber band), 
but ...

"[By the way, Knight makes
the same comment about Hooke's law and Ohm's law.]"


I don't understand; pse. elaborate.


bc, who never thought friction followed a law;  "maybe a rule?", but it 
can be about as good as Hooke's, n'est pas?

* An expt. to check how good the rule, would be to use a bicycle cart 
(garden, etc.)  w/ various loads pulled w/ a spring scale.  Various 
inflations would give a family of curves.



Robert Cohen wrote:

> I am using Knight's text for scientists and engineers (copyright 2004)
> and I have a couple of questions on what he says about friction.  On
> page 134 he gives three equations for friction:
>
> static: f_s <= mu_s n
> kinetic: f_k = mu_k n
> rolling: f_r = mu_r n
>
> I have several questions:
>
> 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.]
>
> Later on, Knight provides an explanation for friction based on how
> surfaces are "jagged" on the microscopic scale.  I would call that a
> model of friction (regardless of whether I agree with it or not).  Is my
> usage consistent with the way everyone else does or should I change to
> how Knight uses the terms.
>
> 2. Would mu_r (the coefficient for rolling friction) be any different
> than, say, mu_s?  And, why would rolling friction always be the same?
> In my mind, I can accelerate a car with a whole range of accelerations
> so the rolling friction must be able to vary also.  What am I missing?
>
> This is the first time I'm using Knight so there might be subtleties
> that I am missing.
>
> ____________________________________________________
> Robert Cohen, Chair, Department of Physics
> East Stroudsburg University; E. Stroudsburg, PA 18301
> 570-422-3428; www.esu.edu/~bbq 
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