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Re: Friction



Hi all.
I couldn't resist jumping into this. I co-founded a center at our
university to work on problems of applied friction (brakes, transmissions,
etc.). Part of my research has been on measuring surfaces (unworn and
worn) and determining a relationship between surface roughness and friction
coefficient. Contact area between any two surfaces is linearly
proportional to applied load (or simply the weight in the case of the
block). In experiments conducted by Bowden and Tabor in the late 1950s,
they found that the number of contact points is incredibly small, even for
two ground metal plates in contact. The minimum number of contacts is
three; and that's the number of contacts they found for simply laying one
plate on top of the other. If you apply a downward force to the top plate,
the number of contacts increases in proportion to the force. With an
applied force of 5000 N, the number of contacts was about 125 (if my memory
is correct). The area of each contact region increases linearly with the
load. However, the average area of the contacts remains constant (since
new contacts are made as the force is increased).
Now, applying this to the question at hand. If a block is used as
described and it has been prepared such that the roughness on all sides is
the same, then the classic law of friction concerning area does hold. The
reasoning is that that number of contacts and the contact area is the same
whether the nominal area of contact is large or small (on the macroscopic
scale). My suggestion would be to use a metal block (such as aluminum)
because it is easier to prepare the surfaces to be indentical. This
classic law of friction is indeed valid at the macroscopic scale, it is not
just an approximation as some have suggested.

David T. Marx, PhD
Associate Scientist
Center for Advanced Friction Studies
Southern Illinois University
Carbondale, Illinois 62901-4343
web: http;//www.siu.edu/~cafs