Aristotle: Some Passages on Natural Motion


De Caelo 273b30-274a2: A given weight moves a given distance in a given time; a weight which is as great and more moves the same distance in a less time, the times being in inverse proportion to the weights. For instance, if one weight is twice another, it will take half as long over a given movement.


De Caelo 290a1-2: Whenever bodies are moving with their proper motion, the larger moves quicker.


Physics 216a13-16: We see that bodies which have a greater impulse either of weight or of lightness, if they are alike in other respects, move faster over an equal space, and in the ratio which their magnitudes bear to each other.


De Caelo 308a29-33: By absolutely light, then, we mean that which moves upward or to the extremity, and by absolutely heavy that which moves downward or to the center. By lighter or relatively light we mean that one, of two bodies endowed with weight and equal in bulk, which is exceeded by the other in the speed of its natural downward movement .


De Caelo 277b4-5: The greater the mass of fire or earth the quicker always is its movement towards its own place.


De Caelo 309b11-15: For any two portions of fire, small or great, will exhibit the same ratio of solid to void; but the upward movement of the greater is quicker than that of the less, just as the downward movement of a mass of gold or lead, or of any other body endowed with weight, is quicker in proportion to its size.


De Caelo 294a13-15: A little bit of earth, let loose in mid-air, moves and will not stay still, and the more there is of it the faster it moves....


Cf. Hero of Alexandria, Mechanics II. 34d: Why do heavier bodies fall to the ground in shorter time than lighter bodies? The reason is that, just as heavy bodies move more readily the larger is the external force by which they are set in motion, so they move more swiftly the larger is the internal force within themselves. And in natural motion this internal force and downward tendency are greater in the case of heavier bodies than in the case of lighter.