Friction of pine upon pine; the mean values of the
friction given in Table II. (corrected for the
friction of the pulley) compared with the formula
_F_ = 0·27 _R_.
+-----------+----------+----------+----------+---------------+
| Number of | R. |Corrected | F. | Discrepancies |
|Experiment.|Total load|mean value|Calculated| between the |
| | on slide | of | value of | observed and |
| | in lbs. | friction.| friction.| calculated |
| | | | | frictions. |
+-----------+----------+----------+----------+---------------+
| 1 | 14 | 4·7 | 3·8 | -0·9 |
| 2 | 28 | 8·2 | 7·6 | -0·6 |
| 3 | 42 | 12·2 | 11·3 | -0·9 |
| 4 | 56 | 15·8 | 15·1 | -0·7 |
| 5 | 70 | 19·4 | 18·9 | -0·5 |
| 6 | 84 | 23·0 | 22·7 | -0·3 |
| 7 | 98 | 25·8 | 26·5 | +0·7 |
| 8 | 112 | 29·3 | 30·2 | +0·9 |
+-----------+----------+----------+----------+---------------+
133. Hence the law _F_ = 0·27 _R_ represents the experiments with
tolerable accuracy; and the numerical ratio O·27 is called the
_coefficient of friction_. We may apply this law to ascertain the
friction in any case where the load lies between 14 lbs. and 112 lbs.;
for example, if the load be 63 lbs., the friction is 63 × 0·27 = 17·0.
134. The coefficient of friction would have been slightly different
had the grain of the slide been parallel to that of the plank; and it
of course varies with the nature of the surfaces. Experimenters have
given tables of the coefficients of friction of various substances,
wood, stone, metals, &c. The use of these coefficients depends upon the
assumption of the ordinary law of friction, namely, that the friction
is proportional to the pressure: this law is accurate enough for most
purposes, especially when used for loads that lie between the extreme
weights employed in calculating the value of the coefficient which is
employed.