The outlines of physics: an elementary text-book . g friction. 83. Experiment 17. — Laws of Sliding Friction. Apparatus : (1) An inclined plane of soft wood, about 100 cm. long. At oneend this plane is hinged to a substantial base. The other end playsbetween two stiff uprights to which it can readily be clamped at anydesired angle. The upper surface of the plane is to be freshly sand-papered before the beginning of the measurements. (2) A wooden block 20 cm. x 10 cm. x 5 cm., also freshly sand-papered. In the middle of one of the broader sides is bored a hole, COHESION, ADHESION, AND FRICTION
The outlines of physics: an elementary text-book . g friction. 83. Experiment 17. — Laws of Sliding Friction. Apparatus : (1) An inclined plane of soft wood, about 100 cm. long. At oneend this plane is hinged to a substantial base. The other end playsbetween two stiff uprights to which it can readily be clamped at anydesired angle. The upper surface of the plane is to be freshly sand-papered before the beginning of the measurements. (2) A wooden block 20 cm. x 10 cm. x 5 cm., also freshly sand-papered. In the middle of one of the broader sides is bored a hole, COHESION, ADHESION, AND FRICTION 91 into which fits a tall wooden peg. The same peg may also be usedin a similar hole in the middle of one of the narrower sides. (3) Several 5 kg. iron disk weights, a meter scale, some joinersclamps, and a wedge. Procedure : (a) The inclined plane is clamped to a table, as in Fig. 70. Thewooden block is placed upon the plane and the free end is lifted untilthe block, when tapped into motion with the finger, shows a tendencyto continue Fig. 70. (J) Fasten two joiners clamps to the uprights just below the plane,to afEord a support for the latter, insert the wedge (lo), which must bebroad enough to reach across between the clamps. By means of the wedge adjust the pitch further, untU the criticalangle is found, the angle at which the block, once started, will con-tinue to slide with a uniform motion. That this angle measures the coefficient of friction is evident froma consideration of forces acting upon the sliding body. The forcemg between the earth and a body on an inclined plane (as in some TO, \y^ \v ^^rnQ \ ^ 92 THE OUTLINES OF PHYSICS previous examples) is resolved into p and / (Fig. 71), the functions of which are to produce pressure against the plane and motion along /the same respectively. The ratio - is therefore the coefficient of friction. Since the triangle of forces in this case, however, is similarto ABC, the ti-iangle made by the plane, its base,
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