112. A discussion of the force of friction is a necessary preliminary
to the study of the mechanical powers which we shall presently
commence. Friction renders the inquiry into the mechanical powers more
difficult than it would be if this force were absent; but its effects
are too important to be overlooked.
[Illustration: FIG. 31.]
113. The nature of friction may be understood by Fig. 31, which
represents a section of the top of a table of wood or any other
substance levelled so that C D is horizontal; on the table rests a
block A of wood or any other substance. To A a cord is attached, which,
after passing over a pulley P, is stretched by a suspended weight B.
If the magnitude of B exceeds a certain limit, then A is pulled along
the table and B descends; but if B be smaller than this limit, both A
and B remain at rest. When B is not heavy enough to produce motion it
is supported by the tension of the cord, which is itself neutralized
by the _friction_ produced by a certain coherence between A and the
table. Friction is by this experiment proved to be a force, because it
prevents the motion of B. Indeed friction is generally manifested as a
force by destroying motion, though sometimes indirectly producing it.
114. The true source of the force lies in the inevitable _roughness_
of all known surfaces, no matter how they may have been wrought. The
minute asperities on one surface are detained in corresponding hollows
in the other, and consequently force must be exerted to make one
surface slide upon the other. By care in polishing the surfaces the
amount of friction may be diminished; but it can only be decreased to
a certain limit, beyond which no amount of polishing seems to produce
much difference.
115. The law of friction under different conditions must be inquired
into, in order that we may make allowance when its effect is of
importance. The discussion of the experiments is sometimes a little
difficult, and the truths arrived at are principally numerical, but we
shall find that some interesting laws of nature will appear.