Page:Elementary Text-book of Physics (Anthony, 1897).djvu/102

 theory of the vortex ether (§ 323), gives an almost complete model of the essential features of the physical universe; it does not, however, explain gravitation, nor does it without some addition explain the inertia of a body, and not until it is shown that these characteristic features of matter are explained by it can it be adopted as a final theory of matter.

74. General Statements.—When the surface of one body is made to move over the surface of another, a resistance to the motion is set up. This resistance is said to be due to friction between the two bodies. It is most marked when the surfaces of two solids move over one another. It exists, however, also between the surfaces of a solid and of a liquid or a gas, and between the surfaces of contiguous liquids or gases. When the parts of a body move among themselves, there is a similar resistance to the motion, which is ascribed to friction among the molecules of the body. This internal friction is called viscosity.

The forces to which friction gives rise do not conform to the conditions of conservative forces. They are not uniquely dependent on the position of the moving body, and are exerted only when the body is in motion, and always in such a sense as to oppose the motion. The work done on a body in moving it against friction does not give the body potential energy, and the sum of the kinetic and potential energies in a system, the parts of which exert friction on one another, continually diminishes. Most of the departures from the law of the conservation of mechanical energy exhibited in the ordinary operations of Nature are due to friction. The mechanical energy lost is for the most part transformed into heat.

75. laws of Friction.—Owing to our ignorance of the arrangement and behavior of molecules, we cannot form a theory of friction based upon mechanical principles. The laws which have been found are almost entirely experimental, and are only approximately true even in the cases in which they apply.