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Re: Friction



< Subject: Re: Friction
< Date: Thu, 19 Oct 2000 19:52:24 -0500
< From: Doug Craigen <dcc@ESCAPE.CA>

<In fact, it is not clear to me why one would even talk about "static"
<and "kinetic" cases for a boat

It's not clear, because one would not do it. Friction between solids
(w/ or w/o lubrication) is described by the equation f=kN where k is
the coefficient of friction and N is the normal force. Friction
(actually shear stress or skin friction per unit area) between a solid
and a fluid is described by f=mu(dU/dy) where mu is the viscosity and
dU/dy is the velocity gradient in the fluid.

Now, if we get our definition of friction from Merriam-Webster's
dictionary, we would think that f=kN=mu(dU/dy) (Static, kinetic,
skin? Hey, what's the diff? Friction is friction.), but as
physicists we see that the friction of a boat at rest relative to the
fluid is zero not because mu=0 (or, even, because k=0 ?!), but because
dU/dy = 0.

And, even if we could redefine ("generalize") f=kN to include solids
and fluids, what is N for a solid in a fluid? Is f proportional to N
for solids and fluids as it is for solids and solids?

The original question about coefficients of friction (static and
kinetic) was as clear as can be. This idea about coefficients of
friction and boats reminds me of the student who when he doesn't have
an answer for X, redefines the question to Y, and muddles through
that.

Glenn A. Carlson
gcarlson@mail.win.org