ENERGY AND ITS TRANSFORMATIONS. 



19 



given moving mass, in which case it measures the work 

 which the moving body is capable of doing upon that 

 mass, when brought by it to its own speed. 



Potential Energy is the capacity for doing work pos- 

 sessed by a body in virtue of its position, of its condi- 

 tion, or of its intrinsic properties. Thus, a weight 

 suspended at a given height possesses the potential 

 energy, in consequence of its position, E — W/i, and 

 may do work to that amount while descending through 

 the height, //, under the action of gravity. A bent 

 bow or coiled spring has potential energy, which 

 becomes actual in the impulsion of the arrow or is ex- 

 pended in the work of the mechanism driven by the 

 spring. A mass of gunpowder or other explosive has 

 potential energy in virtue of the unstable equilibrium 

 of the chemical forces affecting its molecules. Food 

 has potential energy in proportion to the amount of 

 vital and muscular energy derivable by its consumption 

 and utilization in the human or animal system. These 

 potential energies are not measured by the observed 

 actual energies derived from these substances in any 

 case, but are the maximum quantities possibly obtain- 

 able by any perfect system of development and utiliza- 

 tion. In practical application, more or less waste is 

 always to be anticipated. 



Every instance of disappearance of actual energy in- 

 volves the performance of work, and the production of 

 potential or of some new form of actual energy in 

 precisely equal amount. A stone thrown vertically up- 

 ward loses kinetic energy as it rises, in precisely the 

 amount — resistance of the air being neglected — by 

 which it gains potential energy. A falling mass striking 

 the earth surrenders the actual energy acquired by loss 



