Re: Friction !!!

From: <Christopher>
Date: Tue Apr 02 2002 - 11:06:00 EST

>kindly tell me whether we can consider friction as a
>non continuous force

Three rules--

1. Friction is always acting. In static situations. It will vary in direction and magnitude so as to resist movement up to a threshhold value equal to the coefficient of static friction times the instantaneous normal force.
2. Friction on bodies in motion act opposite to the direction of relative motion. The frictional force equals the coefficient of dynamic friction times the normal force.
3. The coeficient of friction isn't constant--it varies with surface condition and surface pressure. The one comprehensive study I know about puts the variation for stainless on stainless and between 0.2 and 0.8, depending on surface condition and various surface films.

So-
friction is non-continuous because it is bi-linear, but its effects don't simply disappear.

> knife edge support or a support with a finite area , in my opinion
>the friction force should remain same

No. friction coefficients vary with contact pressure. Plastic flow under a knife edge may aslo produce a notch which acts as a lateral detent.

>Can friction generate such
>a magnitude of force couple which will make the joint rotationally rigid
I doubt it. I suppose a pipe clamp can force a pipe run onto a surface with enough built-in normal force to prevent twisting or lengthwise motion for a time, but it shouldn't be considered a real fixed support. Friction is a very unreliable way to keep something in place. You can never depend on it.

Christopher Wright P.E. |"They couldn't hit an elephant from <a href="/group/PipingDesign/post?postID=xD3ebgvPdHNni6n4ONUWv1NVdBPIcwPoFfg64JfbxxooT8d4n8Yncjhnu9h1ZFDlkyYODF68BxlDae8UTQ">chrisw@skypoint.com</a> | this distance" (last words of Gen.

___________________________| John Sedgwick, Spotsylvania 1864)

<a href="http://www.skypoint.com/~chrisw">http://www.skypoint.com/~chrisw</a> Received on Tue Apr 02 11:06:00 2002