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I was studying Aristotle's laws of motion and comparing them to Newton's. He states that heavier bodies fall faster than lighter ones. I really can't understand how he could have committed such a mistake. It seems obvious that two stones with different weights would reach the ground at the same time.

Furthermore, he believed that after applying a force to an object that object will move with a constant speed. But in the falling experiment we see that the speed will increase for normal objects like stones (not something like leather whose speed is influenced by air resistance).

How did Aristotle make these mistakes?

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    $\begingroup$ I don't think it's accurate to refer to "Aristotle's laws of motion." Aristotle never conceived of universal, mathematical laws that in theory would predict the outcome of any set of initial conditions. $\endgroup$ – Ben Crowell Jun 23 '15 at 22:58
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    $\begingroup$ @Ben Crowell I agree that "Aristotle's laws of motion" is a modernism, but I am not sure why laws are supposed to be universal or mathematical, some are qualitative and all have a range of applicability. Ironically, Newton's formulation of his laws is quite Aristotelian, he even distinguishes rest and motion as qualitatively different states, just like Aristotle. Predicting from a set of initial conditions is even more of a modernism that comes from working with differential equations. $\endgroup$ – Conifold Jun 24 '15 at 20:59
  • $\begingroup$ Formerly on Physics. $\endgroup$ – HDE 226868 Jun 26 '15 at 22:36
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Air or more generally medium resistance was not yet treated as a separate effect in Aristotle's time. Nor was there a clear idea of motion in a vacuum, in fact most ancient Greek philosophers, including Aristotle, did not believe that vacuum exists. So he had to explain phenomena as they are observed, resistance and all, and without the benefit of experiments where different factors can be controlled for. His prototypical example was a rock and a feather, which do not fall at the same speed. That different weights would fall simultaneously if effects of resistance can be neglected is not obvious at all unless you perform an experiment, and that did not become common practice in science until 17th century, two millennia after Aristotle.

In Aristotle’s Physics: a Physicist’s Look Rovelli explains that in a resisting medium the steady speed attained by a body after a transient phase is indeed proportional to the force causing the motion, so his "law of motion" does hold in the limit of high friction/resistance. As for the free fall, it is not that easy to notice the increase in speed over the short periods of time that everyday objects take to fall from small heights (people did not deliberately drop stones from tall towers in his time). Aside from that, Aristotle distinguished between violent motions, caused by a force, and natural motions, that do not require a force, free fall is one of the natural ones. Another one is the circular motion of stars in the heaven. Aristotle's "law" therefore applies to neither.

This being said, Aristotle's theories of free fall and projectile motion were criticized already in late antiquity, e.g. by John Philoponus (c. 490 – 570 AD), who anticipated modern empirical approach:"But this [view of Aristotle] is completely erroneous, and our view may be completely corroborated by actual observation more effectively than by any sort of verbal argument. For if you let fall from the same height two weights, one many times heavier than the other you will see that the ratio of the times required for the motion does not depend [solely] on the weights, but that the difference in time is very small".

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  • $\begingroup$ Galileo was first to disprove that heavier object fall faster: with mental experiment: would bag of rocks fall faster than a single rock? $\endgroup$ – Peter M. Oct 27 '16 at 22:39
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How did Aristotle make these mistakes? Probably the resason is the same in most if not all cases: There was not a single experiment performed before Archimed. From atomism of Leukippos to Aristotle's laws of motion there was always only pure imagination or philosophy (except explanations based on observation, for instance the wrong life cycle of bees or the not reproducing eels). Aristoteles stated for instance acoustic dispersion: signals of different pitch should spread out with different velocities. Small wonder that many errors arose.

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