[6] Thurston. "Friction and Lost Work in Machinery," from which excellent work much of the next chapter is adapted.
CHAPTER III.
FRICTION—ITS NATURE AND THEORY.
29. Friction. The relative motion of one particle or body in forced contact with another is always retarded, or prevented, by a resisting force called friction.
Friction manifests itself in three ways: Between solids it is called sliding and rolling friction; between the particles of liquids, or of gasses, when they move in contact with each other, or with other bodies, it is called fluid friction. Quite different laws govern these three kinds of friction, as they are quite different in character.
Friction can never of itself produce or accelerate motion, being always a resisting force, acting at the surfaces of contact of the two particles, or masses, between which it occurs, and in the direction of their common tangent, resisting relative motion in whichever direction it may be attempted to produce it. The greatest loss of energy in a timepiece in which all the parts are rigid enough to prevent permanent distortion, is that occurring through friction. Another source of loss of energy is the reduction in elasticity of springs caused by a rise of temperature.
30. The Cause of Sliding Friction is the interlocking of the asperities of one surface with those of another; and only by the riding of one set over the other, or by a rubbing down or tearing off of projecting parts, can motion take place. It follows, then, that roughness is conducive to friction; and that the smoother the surface the less the friction will be.
31. The Cause of Rolling Friction is the irregularity and lack of symmetry of the surfaces between which it occurs. It acts as a resisting, or retarding, force when a smoothly curved surface rolls upon another surface, plane or curved.