280 Dr. 0. J. Lodge on a Systematic Classification 



applied to it; and these constitute the different forms of 

 energy. 



8. We can proceed to classify the forms of energy by first 

 of all considering how the effects produced depend upon the 

 forces applied to the body. . 



If these forces have no resultant (i. e. if they are in equili- 

 brium), the body will be strained, and will exert a correspond- 

 ing stress. 



If the forces have a resultant, the body will be moved*; 

 and the motion will be either a translation or a rotation, or 

 both, according as the forces can be reduced to a single finite 

 resultant, a resultant zero at infinity, or to both combined. 



Similarly, the strain may be analyzed into compression, elonga- 

 tion, and shear, or a combination of them, according to the way 

 the forces act ; but this division does not appear to be of much use 

 for our present purpose. 



All these effects are forms of energy, because the working- 

 power of the body in which they are produced is in general 

 increased; i. e. the body is rendered capable of doing work as 

 soon as the proper condition is supplied. (See § 4.) 



Thus a steadily strained elastic body is exerting force or 

 pressure; but its point of application is stationary: allow it to 

 move, and work is immediately done. A body in free motion 

 is passing through space, but it is not exerting any force ; 

 supply a resistance, and work is immediately done. 



9. Energy, therefore, has two principal forms : — 



1) The free motion of bodies relatively to one another ; 



2) The separation of bodies from one another against stress. 

 And to these may be added for convenience the rapid alter- 

 nation from one form to the other, called vibration. 



10. The two fundamental forms of energy correspond to 

 the two factors in the product workf . A body exerting force 

 possesses energy, and a body moving through space possesses 

 energy ; but a body is not doing work unless it is both exert- 

 ing force and moving through space. 



* And possibly strained as well. It is only forces which like gravity 

 act uniformly on every particle of a body that can move an ordinary elastic 

 solid without straining it. 



t Energy and work are not to be confounded together ; and all such 

 phrases as " accumulated work," " conservation of work," "conversion of 

 heat into work," " work consumed," &c, should be eschewed. Energy 

 is not work, but work can be got out of it if the proper condition be sup- 

 plied. It is in fact iiossible work. 



The expression %)ossible energy, however, is meaningless; so also is the 

 expression actual energy. All energy is actual and real — potential just as 

 much as kinetic; and all represents possible ivork — that is, work that 

 will become actual as soon as the other factor is supplied. 



