Internal and External Resistance.—All bodies are subject to internal, as well as external resistance. The stone on the cliff resisted the movement to push it over. Weight was the resisting internal force, but when the stone was moving through the air, the friction with the air created external resistance.
Energy Indestructible.—There is another thing which should be understood, and that is the absolute indestructibility of energy. Matter may be changed in form, or in the direction of its motion, by the change of kinetic into potential energy, or vice versa, but the sum total of the energy in the world is unalterable or constant.
The tremendous power developed by the stone when it plunged through space and struck the rocks below, developed a heat at its impact. Thus the moving force which was a motion in one direction was converted into another form of energy, heat. The expansion of the material exposed to the heat also represented energy.
When powder explodes and absolutely changes the form of the substance, its volume of expansion, if it should be retained within a vessel, would perform a certain amount of work, and the energy is thus transferred from one form to another without ceasing.
Wind Power.—Primitive man also saw and felt the winds. He noted its tremendous power, but he could not see how a force moving in one direction only could be utilized by him.
Rectilinear Motion.—This movement of the wind in one direction, like the water flowing along the bed of the river, is called rectilinear motion. It required invention to convert rectilinear into circular motion.
Oscillating Motion.—When he threshed his grain and winnowed it by shaking it to and fro, to rid it of the chaff, the idea of using the wind to produce an oscillating motion did not occur to him. After circular motion was produced, the crank was formed and thus the oscillating movement was brought about.
Movements in Nature.—All movements in nature are simple ones, of which the following are illustrations:
1. Rectilinear, which, as stated, means in a straight line.