the Foundations of Dynamics. 21 



may be perceived to result from some kind of motion. Thus 

 a reservoir of compressed air is a store of potential energy, 

 though it is well known that al] the pressure is due to the 

 motion of unstressed particles, and is really just as much 

 kinetic as any of the energy of planets and comets. A 

 powder-magazine is a store of potential energy, though quite 

 possibly the forces awaiting a trigger to liberate them into 

 activity are due to atomic or etherial motions ; and the same 

 may be said of a charged Leyden jar and of a bent bow. 



Briefly the idea of potential energy corresponds to the idea 

 of elastic force when the cause of that force is not sought. 

 The idea of kinetic energy corresponds to the idea of sustained 

 motion. Each corresponds to one of the factors in the product 

 " activity," or Fv. 



In order that a body may possess energy it must be 

 capable of exerting force and also of moving ; but it need be 

 doing only one of these things, and so long as it merely stores 

 energy it must be doing only one of them. Both factors 

 must concur before it can be active, both factors must be 

 possible before it can be said to possess energy. The only 

 question is, which factor does it possess meanwhile ? If it is 

 exerting force, but stationary, then its energy is potential 

 or static; if it is moving quite freely, then its energy is 

 kinetic. 



So long as the other factor is absent no activity manifests 

 itself, no work is done, and the energy is merely stored. 

 Directly the other factor is supplied transference begins, 

 energy leaves the body storing it, and passes to the body 

 supplying the other factor. 



Now to make a moving body do work, a resisting obstacle 

 must be supplied, and this second body thus exerting force 

 receives a due proportion of the energy ; and it receives it 

 by reason of the force it exerts, that is, it receives static or 

 potential energy from a body which possessed kinetic. 



To make a strained body do work, motion must be per- 

 mitted ; the thing which is moved gains energy, and it gains 

 it in the kinetic form from a body possessing potential. 



Of course in both cases the body immediately receiving the 

 energy need not retain it, but may rapidly or instantaneously 

 pass it on to something else — in which case it resumes its first 

 form ; by simple alternation of transformations. 



Also it is not essential that the potential energy of the 

 particles of a spring shall be communicated to any foreign 

 body : if it is transferred to the spring as a whole, considered 

 as a mass possessing inertia, that is sufficient ; in other words, 

 it may be transferred from one set of particles to another set 



