When a body is raised from the earth. work is done equal to the product of the weight of the body and the height it is raised. moll. 'Ibis amount of energy is gained by the system consisting of the 1111111 and the body whine mass is HI: lint until gravitation is understood it will be impossible to locate the energy in any definite place or pinees, If a body falls through a height 11, it awl the earth lose potential energy. mob, which is gained in the form of kinetie energy by the falling hotly and the earth, principally by the former. since the charge in the speed of the earth occasioned by the body as it falls toward it is so infinitesinml, If, after the body falls a dis afwe, 11. its speed is its kinetle energy is L.nrs', and therefore nigh = ma' or 2gh.
formula shows that the speed of a falling Lute depends upon the rcrtical height traversed, not on the slope or length of the path itself: it may fall vertically, or down an inclined plane, or down a spiral, etc.
The cases of work being done against cal and inagmetie forces are discussed under ELECTRICITY and MAGNETISM. Whenever work is done in overcoming friction, it is observed that heat-effects :ire prthInced, which can be traced to the fact that the minute portions of the body on which the work is done gain energy. This question is hilly dismissed under
HEAT. Since, when any inelastic body is de• formed in any way, there is internal friction, part of the energy gained by such a body when it strikes another body goes into producing heat effects.
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It is a general property of motion, III f 1 lows at once from the definition of potential energy, that all motions take place of themselves in such a manner as to make the potential en ergy of the system decrease, and that equilibrium is not reached until the potential energy has reaehed a value such that it is a is. is as small as is possible under existing condi tions.
The unit of work or energy is that correspond ing to a unit force acting through a distance of a unit length. On the C. l:. S. system this unit is. then, that corresponding to a force of I dyne acting through I CID.; it is called an An erg is, however, such a small unit that ergs— a as it is used as the practical unit. The amount of work dune in it unit interval of time by any agency is called its or (9,y„). On the C.C.S. sys• tom the unit is. then, 1 erg per second. The praetieal unit is. however, I joule persecond; this is called a