Re: Coefficient of Friction

["John S. Denker" <jsd@MONMOUTH.COM>]

Abul Kalam wrote:
>     In measurement of coefficient of friction on an inclined plane, the
> equation for the coefficients of both static and kinetic friction have
> the same form, that is, (mu) = tan A, where A is the angle of incline.
> Now, theoretically, this mu can have a value anywhere from 0 to
> infinity, but it hardly exceeds 1. I saw only one value that was greater
> than 1 in the table of coefficients. So, I imagine it is the physical
> properties of matter that set this limit. I guess my question is: What
> sets this limit?

It depends on details.  There will never be a simple
theory to predict static friction.

I just now did an experiment:  fuzzy sock resting on piece
of sandpaper.  I observed the angle of incline A = 90 degrees
or more.  Go figure out what mu is.  It's a big number.
Even if you think this example is too artificial, you can
pretty easily come up with other examples of seriously
large static friction.

If you want to restrict it to "smooth" surfaces, I'll
say that there's no such thing as a completely smooth
surface.  And if you get two metal surfaces sufficiently
clean and smooth, they will cold-weld.  Then you'll get
A = 180, even more extreme than my example.

Regarding the general topic of strength of materials,
this is very, very complicated.  If you want 1% of
the tip of the iceberg, read section 30-7 in chapter
30 of volume II of _The Feynman Lectures on Physics_.