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XXXVI. On the Meaning of " Force.' 3 

 By the Rev. Maxwell H. Close, M.A. 



To the Editors of the Philosophical Magazine and Journal, 



University Club, Dublin, 



Gentlemen, March 9, 1883. 



PROFESSOR Lamb's communication, on the Basis of 

 Statics, in the current number of the Philosophical 

 Magazine, is intended to promote the interests of the learner 

 in dynamics. Will you permit me to point out, with the same 

 object in view, that the word " Force " has not yet acquired a 

 definite invariable meaning in the science of d3 r namics ? Until 

 lately the learner was very seriously incommoded by the fact 

 that the name " Force " was applied, not only to force proper, 

 or F, but also to energy, or \~F ds, these two being, as is uni- 

 versally recognized, totally disparate. This practice has been 

 happily discontinued, or almost entirely so ; but it is not, as 

 far as I know, acknowledged, though it must have been per- 

 ceived by some, that a similar one still very generally obtains. 

 In accordance with Newton's rather free usage of "ms," the 

 name " Force" is still applied not only to force proper, or F, 

 but also to what we shall call on the present occasion Impul- 

 sion, or \~Fdt, these two being as completely disparate as 

 force proper and energy. 



Clerk Maxwell designates J Fd£ by Impulse ; which name, 

 however, usually implies suddenness, intensity, and shortness 

 of action, like that of impact. To avoid this association we 

 shall now vary this name to Impulsion. It will conduce to 

 brevity and convenience, and will answer the present purpose 

 perfectly well, to suppose F constant and write Ft for fF dt. 



Of course in the first place " force " is applied to force 

 proper. This is simple pressure or tension, which can be 

 measured in pounds weight. It is represented by F in 

 Yt = mv ; kinetically, therefore, it is the time-rate of change of 

 momentum (energetically it is F in ¥s = ^mv\ and the space- 

 rate of change of energy). But it will be seen from the fol- 

 lowing that " force " is often used for impulsion, or F^, the 

 correlate and numerical equivalent of momentum. 



(a) Whoever translates Newton's Second Law of Motion 

 into English, rendering " vis raotrix " by " force," and adopt- 

 ing the statement as from Newton for himself, is using " force" 

 for impulsion. From the exposition appended to Law II. it 

 will be seen that " vis motrix " means, not F, but Ft ; for it is 

 something which may be " sive simul et semel, sive gradatim et 

 successive impressa." These words would be without meaning 



