146 THE REALITIES OF MODERN SCIENCE 



play only when changes occur in the energy of a system, 

 either in its total amount or in its kind or location. 

 From this viewpoint a table does no work in keeping 

 a book from falling, for there is no change in the energy 

 of the book and the ground and hence no force is 

 exerted by the table. The difficulty in which this 

 definition of force involves us is obvious. The trouble, 

 however, is not due to our present definition but to 

 the fact that our scientific language lacks as yet ade- 

 quate terminology. What we actually do is to use 

 the word in the above rigorous sense and also in the 

 less technical sense of our earlier chapters. 



We recognize that if the supporting table is with- 

 drawn the book will fall and a force will be exerted in 

 accelerating it. But the book falls not because of a 

 force but rather because of the partial conversion of 

 its potential energy into kinetic energy. We do not 

 as yet know why such a change in energy occurs but 

 it is a phenomenon of nature that it does unless some 

 obstacle like the table intervenes. We have no word 

 to express this, idea, although the word " tractate" 

 has been proposed. 1 Similarly "pellate" has been 

 suggested to describe the natural motion of bodies 

 away from each other. For example, like electrical 

 charges pellate, unlike tractate. 



In the case of a falling body mg dynes is the force 

 exerted when it falls freely. (This follows at once from 

 equation (8) by substituting g for a.) If the energy 

 released as it falls is not available for acceleration 

 then the space rate of change of the energy available 



1 By Frederick Soddy in "Matter and Energy," Holt, Home 

 Univ. Library, pp. 110-111. 



