of ike various Forms of Energy. 281 



The energy possessed by matter in motion is called Kinetic. 

 The energy possessed by matter exerting force is called Poten- 

 tial. It might with great propriety be called Dynamic energy; 

 and it has been very conveniently called Static energy*, in op- 

 position to kinetic. Of the two factors F and s, then, in the 

 product work, kinetic energy corresponds to s; there is motion 

 through space, but no force : potential energy corresponds to 

 F ; there is force, but no motion. 



11. Whenever work is being done, both factors must be 

 present — that is, both kinetic and potential energy ; and the 

 energy is always passing from one of these forms into the 

 other while the work is being done. For if the motion of a 

 body is with the force which acts upon it, its speed must in- 

 crease ; and if the motion is against the force, the speed must 

 decrease; while in the first case the available distance through 

 which the force can act, or the range of the force, is decreasing, 

 in the second increasing. 



12. The groups into which the forms of energy have been 

 arranged (§ 8) — viz. strain, rotation, translation, and vibra- 

 tion—may now be subdivided further, by considering how the 

 effects produced when work is done upon a body depend 

 upon its nature and size. 



A convenient division of bodies, according to size, will be — 

 1st. Masses comparable in size with the human body : 



which may be called ordinary masses. 

 2nd. Masses incomparably larger, as planets. 

 3rd. Masses incomparably smaller, as particles or mole- 

 cules. 

 4th. The ultimate atoms. 

 All these material bodies agree in general properties, and 

 differ only in size. But distinct apparently from these there 

 exists an unknown something, which is material enough to be 

 capable of possessing energy, to disturbances in which electrical 

 phenomena seem to be due, and of which probably an aspect 

 has been called aether. This must therefore constitute a 5th 

 group, differing from the others apparently in respect of na- 

 ture, not of size. 



13. All these groups of bodies may be strained or set in 

 motion in various ways when work is done upon them ; and 

 the groups into which the known forms of energy are thus 

 thrown are exhibited provisionally in the following Table. 



* The cause of the stress exerted by a strained body in any particular 

 case is not in general known, and it may easily turn out often to be ulti- 

 mately due to a kinetic phenomenon, as it certainly is in the case of the 

 stress exerted by a compressed gas ; nevertheless it may still be called 

 static energy so long as the cause of the stress is not under consideration. 



