Physics for Geologists, Second edition

90 Stress and strain
Friction is involved in sliding, which we shall consider in some detail a little
later. You can place a book on a bench or any other planar surface and lift
one end of the bench. There is some angle of slope of the bench at which
the book will just begin to slide, so the frictional force can be measured. It
is equal to the maximum component of weight down the slope just before
the object begins to slide, W sin0 (where 0 is the angle of slope below the
horizontal). This is sometimes called the limiting or static friction.
The coefficient of friction, b, is the ratio of the limiting friction to the
component of weight normal to the sliding surface (dimensionless, of course):
W sin 0
p=- = tan0
W cos 0
and is independent of the size or weight of the object. For solids, this is
determined for dry materials without any form of lubricant at the sliding
surface. For porous materials it can be shown that the coefficient of friction
is the same for dry materials in air as wet materials in water (the requirement
of non-lubricating fluids remains). It is the ambient fluid that matters, as long
as it is not a lubricant.
The coefficient of friction is a material constant, for practical purposes.
Once an object is sliding, it requires less force to keep it sliding, so the
coefficient of friction is smaller. This is called the kinetic coefficient of
friction.
Sliding down a slope is not the only form of sliding: you can push a block
along a surface. If you push a block along a horizontal surface, you find that
the force required to start movement has a fairly constant relationship to the
normal reaction (i.e. the normal force between the two surfaces, W),
Note: If dense materials are given highly polished surfaces, sliding does not
readily take place because of the molecular attraction between the surfaces.
Two highly polished flat surfaces of metal are not easily separated, and you
may be able to lift both when you lift the upper one.
When we strain a solid to the point of fracture, one surface slides over
another within the material. Clearly there is a cohesive strength that must
be overcome before fracture occurs; then there is frictional resistance to be
overcome. The angle of fracture is a function of both these components. This
topic will be taken up in Fracture on page 97.
Viscosity
Viscosity is the property of internal friction in fluids. There are two kinds of
viscosity, so care must be taken to distinguish between them. We can also
talk of the effective viscosity of rocks because, in the immense time-scale
of geology, rocks can behave much like fluids - but it would be unwise to
assume that the internal motion of rocks and fluids are strictly comparable.
Copyright 2002 by Richard E. Chapman