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## The motion according to Aristotle

One of the fundamental propositions of Aristotelian philosophy is that there is no effect without a cause. Applied to moving bodies, this proposition dictates that there is no motion without a force. Speed, then is proportional to force and inversely proportional to resistance

force=(resistance)×(speed)

(though none of these quantities were unambiguously defined). This notion is not at all unreasonable if one takes as one's defining case of motion, say, an ox pulling a cart: the cart only moves if the ox pulls, and when the ox stops pulling the cart stops.

Qualitatively this implies that a body will traverse a thinner medium in a shorter time than a thicker medium (of the same length): things will go faster through air than through water. A natural (though erroneous) conclusion is that there could be no vacuum in Nature, for if the resistance became vanishingly small, a tiny force would produce a very large motion''; in the limit where there is no resistance any force on any body would produce an infinite speed. This conclusion put him in direct contradiction with the ideas of the atomists such as Democritus (see Sect. 2.3.2). Aristotle (of course) concluded the atomists were wrong, stating that matter is in fact continuous and infinitely divisible.

For falling bodies, the force is the weight pulling down a body and the resistance is that of the medium (air, water, etc.). Aristotle noted that a falling object gains speed, which he then attributed to a gain in weight. If weight determines the speed of fall, then when two different weights are dropped from a high place the heavier will fall faster and the lighter slower, in proportion to the two weights. A ten pound weight would reach the Earth by the time a one-pound weight had fallen one-tenth as far.

Next: Ptolemy Up: Aristotle and Ptolemy Previous: Aristotelian Cosmology

Jose Wudka
9/24/1998