_Frictional Resistance._—The resistance with which bodies oppose the
movement of one surface on another is termed _friction_. It depends
on the nature, and the roughness of the surfaces in contact; at the
commencement of the sliding, it is greater than when the motion is
continued.
_Friction is in effect an equivalent force exerted in a direction
opposite to that in which the sliding occurs._ Its whole amount is the
product of two factors: the first of these, which sums up the effect of
the nature and condition of the surfaces, is called _the coefficient
of friction_; the second, which is the sum of all pressures, as weight
strain, and the adhesion due to magnetism (when employed), which act
to urge the two bodies together, _i. e._, perpendicularly to the
surface of contact, is called _the normal pressure_. But this law holds
only where, with dry surfaces, the pressure is not enough to indent
or abrade either; or, with wet surfaces, not enough to force out the
unguent. In either of these cases, the friction increases more rapidly
than the ratio of normal pressure.
_No surfaces can be made absolutely hard or smooth_; when one
surface is made to slide over another, the slight roughness of the
one interlock with those of the other, so that the surfaces must be
separated or the points abraded to allow of the motion; but if one
surface roll upon another, the prominent points are successively
raised, without the need of complete lifting of the body or wearing off
those points. _Hence, there are two kinds of friction, the sliding and
the rolling._ The former of these in amount greatly exceeds the latter;
it is a leading element in the stability of structures and fabrics of
all lands, and the most important resistance and source of waste in all
machinery, and is therefore a chief object of regard in the arts of
construction and the science of engineering.
Sliding friction increases with the roughness of the surfaces in
contact; hence, it is in practice diminished as these surfaces become
worn, also by polishing, and by the use of lubricants, which smooth the
rubbing surfaces by filling their depressions. It increases, almost
universally, in exact proportion with the entire pressure, owing to
weight or other causes, with which the two surfaces are held together;
but at very great pressures, somewhat less rapidly. Consequently, in
all ordinary cases, so long as the entire weight or pressure remains
the same, the friction is, in general, entirely independent of the
extent of the surfaces in contact.
The exceptions are, some increase when the rubbing surfaces under the
same total pressure are very greatly extended, or when either surface
is comparatively soft; and considerable lessening of friction when
the bodies are very small, as in the runners of skates upon ice. For
ordinary rates of motion, the total friction within a given space
or distance is in like manner entirely independent of the velocity
with which one surface is caused to move over the other; but in very
slow motions it is increased, and in very rapid motions perceptibly
diminished.
_Friction is also increased in proportion to the tendency of the
surfaces to adhere_; hence, it is usually found greater between bodies
of the same kind (steel on steel proving almost an exception) than
between those of different kinds; it is usually greater when the
surfaces have been long in contact, and at the beginning of motion, and
always so, unless corrected by lubricants, between metallic surfaces so
highly polished that air may be excluded from between them.
The frictional resistance retarding the flow of _water_ is subject to
three laws: