Fig. 33.
THE ANGLE OF FRICTION.
142. There is another mode of examining the action of friction besides that we have been considering. The apparatus for this purpose is shown in [Fig. 33], in which b c represents the plank of pine we have already used. It is now mounted so as to be capable of turning about one end b; the end c is suspended from one hook of the chain from the “epicycloidal” pulley-block e. This block is very convenient for the purpose. By its means the inclination of the plank can be adjusted with the greatest nicety, as the raising chain g is held in one hand and the lowering chain f in the other. Another great convenience of this block is that the load does not run down when the lifting chain is left free. The plank is clamped to the hinge about which it turns. The frames by which both the hinge and the block are supported are weighted in order to secure steadiness. The inclination of the plane is easily ascertained by measuring the difference in height of its two ends above the floor, and then making a drawing on the proper scale. The starting-screw D, whose use has been already mentioned, is also fastened to the framework in the position shown in the figure.
143. Suppose the slide a be weighted and placed upon the inclined plane b c; if the end C be only slightly elevated, the slide remains at rest; the reason being that the friction between the slide and the plane neutralizes the force of gravity. But suppose, by means of the pulley-block, c be gradually raised; an elevation is at last reached at which the slide starts off, and runs with an accelerating velocity to the bottom of the plane. The angle of elevation of the plane when this occurs is called the angle of statical friction.
144. The weights with which the slide was laden in these experiments were 14 lbs., 56 lbs., and 112 lbs., and the results are given in [Table VII].
We see that a load of 56 lbs. started when the plane reached an inclination of 20°·1 in the first series, and of 17°·2 in the second, the mean value 18°·6 being given in the fifth column. These means for the three different weights agree so closely that we assert the remarkable law that the angle of friction does not depend upon the magnitude of the load.
Table VII.—Angle of Statical Friction.
A smooth plane of pine 72" × 11" carries a loaded slide of pine 9" × 9"; one end of the plane is gradually elevated until the slide starts off.