ENERGETICS. 70 from their product; if we appreciate the work in the first place, we recognize the force as the relation of the work to the space iu which it W was performed, since F = — . Experiment shows that a constant force applied to a body move varies as the mass, m, of the body. and the rate of change, - , in its velocity, and by a suitable choice of units, the relation may be stated F—m-, if v is the velocity pro- duced in time t by the constant force, I '; , the space s, traversed in time t, while its velocity is increasing from ENERGETICS. to s = i(-j t'. Combining these two equations we find Fsrr^rrev 1 . That i>. a bodyoi mass in, free to move by the application of a force F, would require a velocity r. win n it had been moved over a distance s, such that Fs ~ The expression Ys is the measure of the action viewed with regard to the agent, femv', that with regard to the body; the former is called work, the latter energy. In this particu lar case, however, the only change that, has been produced in the body is a change in its motion, but Hie body is now possessed of more ability nge other bodies than it possessed before, and, in being brought back to its original state of rest or motion it i- found to be able either to do mechanical work equal to Fs, or to confer on other bodies a motion such that the total value of i._.„u : lor them i- exactly equal to that for the given body. This is a simple form 'if conversion or transference of energy. Energy due t tion only i- called kinetic energy. The same ai ml nt work migl » ■ a done a- before, resulting in :. tin, condition of the body upon which work ha- been d< a-, e _ iii compressing or ing an elastic body, but the body would now lie iu condition to do work on it- own tie i.e. it v.'iul, i posse ss energy, though not in motion. Bere a strain has been pro duced by changing the position by some or all parts of the body. Similarly a body might have rred upon it power to do work by changing ,i one place to anot her, as w hen a bodj i lifted to a [>oint above the earth. It should not i„ i bought . ho rever, I hat I hi i hange in posit ion is 1 1"' ■■ , nt el' imenon : it is ,i ,,iih I ndoubtedly, the rea l -,,,i of the energy i- connected with thi hidden mechanism which renders it i e force in order to secure the strain ,oi ppi posit i"ii ; e.g. th I gravi due to strain or to posit ion is (The due to 'I'll, ,ui.i N ,,ii'i" potential Rankine, static energy to I This affoi li Snit ion of energy i'i.I t he amount of it capai ihi for Work, too, may now be n nergy fnpni one boil- other. ■ t in conae bendii nit -elf ..nt in part, th. i.-i. moving some other body, it 1 . ,i I if the to give it a certain velocity, the kinetic energy the body acquires will exactly equal the worK done by' the spring. But the spring will now possess less energy, less capacity for doing work, tp the amount it has done; the energy it loses equals that gained by the other. The total energy of the two, however, is the same as be- fore. If the two bodies are viewed as comprising a system, the total energy of the system is not altered by any exchange of energy between its parts. We may extend this consideration in- definitely. Given a system of bodies arranged in a definite configuration, and with certain stresses between them: if in obedience to these stresses a rearrangement of the bodies takes place, a change of configuration ensues, the energy of the parts may be altered, but the total is not changed. The energy is conserved, and such a system is called a conservative system. So far as known, all material systems are con- servative. The energy of such a s stem is a quantity that can neither be increased nor dimin- ished by any action between the bodies them- selves, though the form of energy maybe changed If the total energy is to be increased, it can only be done by work expended upon the system by some externa] agent, and then the agent loses energy by the amount it expends upon the sys- tem. By further extending the same considera- tions we reach the view that energy cannot be created or destroyed, and thai the total energy of the universe is a constant quantity. This is the doctrine of the conservation of energy. Kinetic
i- constantly being changed into potential,
and vica versa; but. besides the forms in which energy lias been mentioned, it exists in a variety of other forms which are not so obviously of a mechanical nature. Both potential and kinetic energy may be classified, as follow-: Potential. Kinetic Strain (extension, compres- Motion (translation or ro- si. .n or distortion). tation). i irtcation. Vioratii in. Magnetization. Electricity in motion, ical separat ion, Heat. it i ,- >,|,:ir,.tion. Radiation. In all wave-motions, i.e. in radiation, energy is transferred from one point to another; and at any instant put of the energy of the medium carrying the waves is kinetic and part potential. In beat phenomena, all heat-effects are due to the a,l, lit ion or withdrawal of energy from the molecules or small portions of the bodies expe- riencing the effects; the energy of the minute parts is both kinetic anil potential in general The energy , >t I lie parts of a gas is almost entirely kinetic. Motion ami strain are the obvious me- chanical forms of energy, and their equivalence wascarh i d, but energetics to-day involves the statements (a) that energy in any form may .I into energy of any other form, which i- a declaration of the correlation and transfor- on >i energy; (b) that when energy in any form disappears, an exact equivalent of some .(h.-r form or forms tal place, which is a declaration of the conservation of energy, and (e) that when energy undergoes transformation, ,,r transference from one body to another, the pletely reversible, but that it some "f the energy is recovered in its original form ;i residual portion reappears in what i- called .1 lower form. This i, the degradation and . of en. i ■•;. Both ( ■< ) ami ( b)
