Smooth horizontal surface of pine 72" × 11"; slide
also of pine 9" × 9"; grain crosswise; slide is
not started; force acting on slide is gradually
increased until motion commences.
+-----------+-------------+---------------+---------------+-------+
| Number of |Load on slide|Force necessary|Force necessary| Mean |
|Experiment.| in lbs., |to move slide. |to move slide. |values.|
| | including | | | |
| | weight | 1st Series. | 2nd Series. | |
| | of slide. | | | |
+-----------+-------------+---------------+---------------+-------+
| 1 | 14 | 5 | 8 | 6·5 |
| 2 | 28 | 15 | 16 | 15·5 |
| 3 | 42 | 20 | 15 | 17·5 |
| 4 | 56 | 29 | 24 | 26·5 |
| 5 | 70 | 33 | 31 | 32·0 |
| 6 | 84 | 43 | 33 | 38·0 |
| 7 | 98 | 42 | 38 | 40·0 |
| 8 | 112 | 50 | 33 | 41·5 |
+-----------+-------------+---------------+---------------+-------+
In the first column a number is given to each experiment for
convenience of reference. In the second column the load on the slide
is stated in lbs. In the third column is found the force necessary to
overcome the friction. The fourth column records a second series of
experiments performed in the same manner as the first series; while the
last column shows the mean of the two frictions.
121. The first remark to be made upon this table is, that the results
do not appear satisfactory or concordant. Thus from 6 and 7 of the 1st
series it would appear that the friction of 84 lbs. was 43 lbs., while
that of 98 lbs. was 42 lbs., so that here the greater weight appears to
have the less friction, which is evidently contrary to the whole tenor
of the results, as a glance will show. Moreover the frictions in the
1st and the 2nd series do not agree, being generally greater in the
former than in the latter, the discordance being especially noticeable
in experiment 8, where the results were 50 lbs. and 33 lbs. In the
final column of means these irregularities are lessened, for this
column shows that the friction increases with the weight, but it is
sufficient to observe that as the difference of the 1st and the 2nd is
9 lbs., and that of the 2nd and the 3rd is only 2 lbs., the discovery
of any law from these results is hopeless.
122. But is friction so capricious that it is amenable to no better
law than these experiments appear to indicate? We must look a little
more closely into the matter. When two pieces of wood have remained
in contact and at rest for some time, a second force besides friction
resists their separation: the wood is compressible, the surfaces become
closely approximated, and the coherence due to this cause must be
overcome before motion commences. The initial coherence is uncertain;
it depends probably on a multitude of minute circumstances which it is
impossible to estimate, and its presence has vitiated the results which
we have found so unsatisfactory.
123. We can remove these irregularities by _starting_ the slide at the
commencement. This may be conveniently effected by the screw shown at
F in Fig. 32; a string attached to its end is fastened to the slide,
and by giving the handle of the screw a few turns the slide begins to
creep. A body once set in motion will continue to move with the same
velocity unless acted upon by a force; hence the weight at B just
overcomes the friction when the slide moves uniformly after receiving a
start: this velocity was in one case of average speed measured to be 16
inches per minute.
124. Indeed in no case can the slide _commence_ to move unless
the force _exceed_ the friction. The amount of this excess is
indeterminate. It is certainly greater between wooden surfaces than
between less compressible surfaces like those of metals or glass. In
the latter case, when the force exceeds the friction by a small amount,
the slide starts off with an excessively slow motion; with wood the
force must exceed the friction by a larger amount before the slide
commences to move, but the motion is then comparatively rapid.
125. If the power be too small, the load either does not continue
moving after the start, or it stops irregularly. If the power be too
great, the load is drawn with an accelerated velocity. The correct
amount is easily recognised by the uniformity of the movement, and even
when the slide is heavily laden, a few tenths of a pound on the power
hook cause an appreciable difference of velocity.
126. The accuracy with which the friction can be measured may be
appreciated by inspecting Table II.