Smooth horizontal surface of pine 72" × 11"; slide
also of pine 9" × 9"; grain crosswise; slide
started; force applied is sufficient to maintain
uniform motion of the slide.
+-----------+-------------+--------------+--------------+-------+
| |Load on slide| Force | Force | |
| Number of | in lbs., | necessary | necessary | Mean |
|Experiment.| including | to maintain | to maintain |values.|
| | weight of | motion. | motion. | |
| | slide. | 1st Series. | 2nd Series. | |
+-----------+-------------+--------------+--------------+-------+
| 1 | 14 | 4·9 | 4·9 | 4·9 |
| 2 | 28 | 8·5 | 8·6 | 8·5 |
| 3 | 42 | 12·6 | 12·4 | 12·5 |
| 4 | 56 | 16·3 | 16·2 | 16·2 |
| 5 | 70 | 19·7 | 20·0 | 19·8 |
| 6 | 84 | 23·7 | 23·0 | 23·4 |
| 7 | 98 | 26·5 | 26·1 | 26·3 |
| 8 | 112 | 29·7 | 29·9 | 29·8 |
+-----------+-------------+--------------+--------------+-------+
127. Two series of experiments to determine the power necessary to
maintain the motion have been recorded. Thus, in experiment 7, the load
on the slide being 98 lbs., it was found that 26·5 lbs. was sufficient
to sustain the motion, and a second trial being made independently,
the power found was 26·1 lbs.: a mean of the two values, 26·3 lbs., is
adopted as being near the truth. The greatest difference between the
two series, amounting to 0·7 lb., is found in experiment 6; a third
value was therefore obtained for the friction of 84 lbs.: this amounted
to 23·5 lbs., which is intermediate between the two former results, and
23·4 lbs., a mean of the three, is adopted as the final result.
128. The close accordance of the experiments in this table shows that
the means of the fifth column are probably very near the true values of
the friction for the corresponding loads upon the slide.
129. The mean frictions must, however, be slightly diminished before
we can assert that they represent only the friction of the wood upon
the wood. The pulley over which the rope passes turns round its axle
with a small amount of friction, which must also be overcome by the
power. The mode of estimating this amount, which in these experiments
never exceeds 0·5 lb., need not now be discussed. The corrected values
of the friction are shown in the third column of Table III. Thus, for
example, the 4·9 lbs. of friction in experiment 1 consists of 4·7,
the true friction of the wood, and 0·2, which is the friction of the
pulley; and 26·3 of experiment 7 is similarly composed of 25·8 and 0·5.
It is the corrected frictions which will be employed in our subsequent
calculations.