MECHANICS AND PHYSICS 187 



celestial and terrestrial (light and heavy) bodies, and 

 between natural and violent movements and their 

 kinds, Aristotle defined the conditions and laws of all 

 motion. 



In his eyes any body which is moved is necessarily 

 subjected to two influences, a force and a resistance ; 

 without the force it would not be able to move, but 

 without the resistance, its movement would be accom- 

 plished in an instant, and it would immediately reach 

 the point to which it is impelled by the force ; the 

 velocity with which a body moves depends therefore 

 both on the magnitude of the force and the magnitude 

 of the resistance. 1 



This being so, if bodies of different weights, balls, 

 for instance, of the same material and of various sizes, 

 are placed on a plane horizontal surface, and if each 

 of them is pushed at the same time with the same force, 

 the lighter balls will roll more quickly and further than 

 the heavy ones. From this, and other analogous facts, 

 Aristotle deduces the following law which he considers 

 the basis of mechanics. 



The force F which moves a body is equal to the 

 resistance R which acts on this body, multiplied by 

 the velocity V imparted to it by the force 



F = RV. 



This law of mechanics excludes the possibility of empty 

 space in nature, for if empty space existed anywhere, 

 bodies would not be subject to any resistance when 

 passing through it, and the ratio F/R which expresses 

 the velocity would lose all numerical significance 

 (Phys., 216 b). Thus, the existence of empty space 

 is far from being that which rendered movement pos- 

 sible, as the atomists pretended ; on the contrary it 

 is inconceivable that a body may move in empty 

 space with a local movement. 2 



1 13 Duhem, Systeme, I, p. 192. a Ibid., p. 197. 



