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

["R. McDermott" <rmcder@peoplepc.com>]

----- Original Message ----- 
From: "Robert Cohen" <Robert.Cohen@po-box.esu.edu>
To: "PHYS-L Maillist" <phys-l@carnot.physics.buffalo.edu>
Sent: Friday, February 17, 2006 3:05 PM
Subject: [Phys-l] Is the friction relationship a model or a law?


> 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

<cutting>

> 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 raises some questions for me as well.  Firstly, isn't the model of 
"rolling friction" simply a means of making sense of a mechanism that 
results in a loss of energy?  Isn't this really a compression-expansion of 
the flexible tire material that gradually results in a loss of energy to the 
surroundings?  A rolling tire is stationary at the point where it contacts 
the roadway, so is classical "friction" acting?  As to "a whole range of 
accelerations", this is limited by the available static friction of tire on 
roadway, is it not?  Generating a torque exceeding that available due to 
static friction results in the tire "peeling out".  The actual static 
friction produced is constrained by Newton's 3rd Law, and can "adjust" up to 
its maximum value depending on the force being applied to the surface. 
Rolling friction, if it IS a compression-expansion effect, should also be 
related to the force applied  to the surface (and thus to the static 
friction), so I'm inclined to agree that "rolling friction" should also be 
variable up to some maximum value and not "always the same"?  Does the text 
imply that it IS "always the same"?