116. Friction is present between every pair of surfaces which are in
contact: there is friction between two pieces of wood, and between a
piece of wood and a piece of iron; and the amount of the force depends
upon the character of both surfaces. We shall only experiment upon the
friction of wood upon wood, as more will be learned by a careful study
of a special case than by a less minute examination of a number of
pairs of different substances.
117. The apparatus used is shown in Fig. 32. A plank of pine
6' × 11" × 2" is planed on its upper surface, levelled by a
spirit-level, and firmly secured to the framework at a height of about
4' from the ground. On it is a pine slide 9" × 9", the grain of which
is crosswise to that of the plank; upon the slide the load A is placed.
A rope is attached to the slide, which passes over a very freely
mounted cast iron pulley C, 14" diameter, and carries at the other end
a hook weighing one pound, from which weights B can be suspended.
118. The mode of experimenting consists in placing a certain load upon
A, and then ascertaining what weight applied to B will draw the loaded
slide along the plane. As several trials are generally necessary to
determine the power, a rope is attached to the back of the slide, and
passes over the two pulleys D; this makes it easy for the experimenter,
when applying the weights at B, to draw back the slide to the end of
the plane by pulling the ring E: this rope is of course left quite
slack during the process of the experiment, since the slide must not
be retarded. The loads placed upon A during the series of experiments
ranged between one stone and eight stone. In the loads stated the
weight of the slide itself, which was less than 1 lb., is always
included. A variety of small weights were provided for the hook B; they
consisted of 0·1, 0·5, 1, 2, 7, and 14 lbs. There is some friction to
be overcome in the pulley C, but as the pulley is comparatively large
its friction is small, though it was always allowed for.
[Illustration: FIG. 32.]
119. An example of the experiments made is thus described. A weight of
56 lbs. is placed upon the slide, and it is found on trial that 29 lbs.
on B (including the weight of the hook itself) is sufficient to start
the slide; this weight is placed upon the hook pound by pound, care
being taken to make each addition gently.
120. Experiments were made in this way with various weights upon A, and
the results are recorded in Table I.