Force and Energy

Abstract

SINCE a year or two back, when Herbert Spencer started, in the columns of NATURE, a discussion as to the real meaning of the word “force,” most careful thinking students of mechanics have probably come to the conclusion that either the use of the word “force” must be discontinued as a physical scientific term, or that it must be defined in a different manner from that adopted almost universally by those “doctors” whose writings seemed to weigh so heavily on the brain of “poor Publius.” They all agree in saying that in its physical application the word “force” means that which produces i.e., the CAUSE of change of momentum. It is needless to give quotations. They are all, except one, curiously explicit. Germans, French, and English agree. “So sehen wir diese Aenderung als Wirkung irgend einer in demselben thätigen Ursache an; diese URSACHE nennen wir Kraft.” (Ritter's “Mechanik,” p. 36). “On donne, en general, le nom de FORCE à la CAUSE quelconque qui met un corps en maouvement, ou seulement qui tend à le mouvoir.” (Poisson: “Traité de Méchanique,” Introduction, p. 2.) Although in Tail's “Recent Advances” we find on p. II “that we have not yet quite cast off that tendency to so-called metaphysics which has So often blasted,” &c., &c.; yet on p. 16 of the same book there is reproduced the fine old crusty Newtonian maxim to which Thomson and Tait and Tait and Steele cling with such fond reverence: “force is any CAUSE which,” &c. Clerk Maxwell gives no formal definition of force in his “Electricity and Magnetism.” On p. 5 he simply gives its dimensions. On p. 83 of his invaluable “Theory of Heat” he defines, “force is WHATEVER changes or tends to change,” &c. This is a very ingenious mode of escaping the difficulty by simply giving no definition at all. We are told what the result of force is, but not what force itself is. We are told that force is “whatever,” which is not very clear. Jeames would hardly think that justice was done him if we asserted that the complete definition of him was “whatever opens a door,” and made no mention of the fact of his humanity or of his grand plush breeches. It is, in fact, a confusion between a statement of the mode of measuring quantitatively the force, and the definition of the force itself. A physical definition should certainly show clearly what the proper way of measuring the quantity is; but this latter is not the definition itself. Moreover, there may be different almost equally good modes of measurement, all leading to the same numerical result. Clerk Maxwell's definition is clear as to α mode of measuring force, but furnishes absolutely no information as to the nature of the thing intended to be defined. It, therefore, differs from the others in that they are real meta-physical definitions, presumably comprehensible to those who understand metaphysics, while his is no definition at all. Prof. John Perry, in his book on “Steam,” adopts the same device as Prof. Clerk Maxwell, substituting the word “anything” for “whatever.” Rankine forms a remarkable exception. He says that “force is an action between two bodies either causing or tending to cause change in their relative rest or motion.” Here the word “cause” is used in such a sound, practical, common-sense way that no one could take exception to such use of it, even in a physical definition, and probably “action,” as here used, might be explained clearly enough for all useful purposes as “a changing relation” or “a change of relation.” Rankine, however, does not take the trouble to do this last.