MOTION AND FALLING-FOKCE. 343 



quainted with the modern literature of science that a modifi- 

 cation of scientific language in the sense of my views is act- 

 ually beginning to take place. But in matters of this kind 

 the chief part of the work must be left to time. 



According to what has been said thus far, the vis viva of 

 motion must be called a force. But since the expression vis 

 viva denotes in mechanics, not only a magnitude which is 

 proportional to the mass and to the square of its velocity, but 

 also one which is proportional to the mass and to the height 

 from which it has fallen, force thus conceived naturally di- 

 vides itself into two very easily distinguished species, each of 

 which requires a distinct technical name, for which the words 

 motion (Bewegung) and falling-force (Fallkraft) seem to me 

 the most appropriate.* 



Hence, according to this definition, " motion " is always 

 measured by the product of the moved mass into the square 

 of the velocity, never by the product of the mass into the 

 velocity. 



By " falling-force" we understand a raised weight, or still 

 more generally, a distance in space between two ponderable 



[* The distinction here drawn between "motion" and " falling-force " 

 is the same as that made by Helmholtz (J)ie Erhaltung der Kraft, 1847) 

 between " vis viva " (leberidige Kraft] and " tension " (Spankraft). The Eng- 

 lish expressions " dynamical energy " and " statical energy " were used by 

 Prof. W. Thomson (Phil. Mag. S. vol. iv. p. 304, 1852) in the same sense, 

 but were afterwards abandoned by him in favour of the terms " actual en- 

 ergy" and "potential energy " introduced by Prof. Rankine. More re- 

 cently (" Good Words " for October, 1862) Professors Thomson and Tait 

 have employed the expression "kinetic energy" in place of " actual ener- 

 gy." The German word Kraft in the text has been uniformly translated 

 force, to which term the ambiguity of the German original has thus been 

 transferred. This ambiguity, however, may be avoided in English by al- 

 lowing the word " force " to retain the meaning which it bears in common 

 language, that is, to denote all resistances which it requires the exertion of 

 a power to overcome (whence the expressions gravitating force, cohesive 

 force, &c.), and by using the word " energy " to denote force as denned by 

 Mayer. G. C. R] 



