As motion, heat, and nervous power the body uses most of its energy.
The Source of Bodily Energy.—As already indicated, the energy of the body is supplied through the food and the oxygen. These contain energy in the potential form, which becomes kinetic (active) through their uniting with each other in the body. Somewhat as the power of the steam engine is derived from the combustion of fuel in the furnaces, the energy of the body is supplied through the oxidations at the cells. How the food and oxygen come to possess energy is seen by a study of the general methods by which energy is stored up and used.
Fig. 81—Simple device for storing energy through gravity.
Simple Methods of Storing Energy.—Energy is stored by converting the kinetic into the potential form. Two of the simplest ways of doing this are the following:
1. Storing of Energy through Gravity.—On account of the attraction between the earth and all bodies upon the earth, the mere lifting of a weight puts it in a position where gravity can cause it to move (Fig. 81). As a consequence the raising of bodies above the earth's surface is a means of storing energy—the energy remaining stored until the[pg 188] bodies fall. As they fall, the stored-up (potential) energy becomes kinetic and can be made to do work.
2. Storing of Energy through Elasticity.—Energy is stored also by doing work in opposition to elasticity, as in bending a bow or in winding a clock spring. The bending, twisting, stretching, or compressing of elastic substances puts them in a condition of strain which causes them to exert a pressure (called elastic force) that tends to restore them to their former condition. Energy stored by this means becomes active as the distorted or compressed substance returns to its former shape or volume.
These simple methods of storing energy will serve to illustrate the general principles upon which such storage depends:
1. To store energy, energy must be expended, or work done.
2. The work must be against some force, such as gravity or elasticity, which can undo the work, i.e., bring about an effect opposite to that of the work.