Aristotle

==== Motion ====
{{further | History of classical mechanics}}

Aristotle describes two kinds of motion: "violent" or "unnatural motion", such as that of a thrown stone, in the ''Physics'' (254b10), and "natural motion", such as of a falling object, in ''On the Heavens'' (300a20). In violent motion, as soon as the agent stops causing it, the motion stops also: in other words, the natural state of an object is to be at rest,{{sfn| Allain | 2016}}{{efn-ua | Rhett Allain notes that [[Newton's First Law]] is "essentially a direct reply to Aristotle, that the natural state is ''not to change'' motion.{{sfn| Allain | 2016}}}} since Aristotle does not address [[friction]].{{sfn| Drabkin | 1938 | pp=60–84}} With this understanding, it can be observed that, as Aristotle stated, heavy objects (on the ground, say) require more force to make them move; and objects pushed with greater force move faster.{{sfn| Susskind | 2011}}{{efn-ua | Leonard Susskind comments that Aristotle had clearly never gone [[ice skating]] or he would have seen that it takes force to stop an object.{{sfn| Susskind | 2011}}}} This would imply the equation{{sfn| Susskind | 2011}}

:: <math>F=mv</math>,

incorrect in modern physics.{{sfn| Susskind | 2011}}

Natural motion depends on the element concerned: the aether naturally moves in a circle around the heavens,{{efn-ua | For heavenly bodies like the Sun, Moon, and stars, the observed motions are "to a very good approximation" circular around the Earth's centre, (for example, the apparent rotation of the sky because of the rotation of the Earth, and the rotation of the moon around the Earth) as Aristotle stated.{{sfn| Rovelli | 2015 | pp=23–40}}}} while the 4 Empedoclean elements move vertically up (like fire, as is observed) or down (like earth) towards their natural resting places.{{sfn| Rovelli | 2015 | pp=23–40}}{{sfn| Drabkin | 1938 | pp=60–84}}{{efn-ua | Drabkin quotes numerous passages from ''Physics'' and ''On the Heavens'' (''De Caelo'') which state Aristotle's laws of motion.{{sfn| Drabkin | 1938 | pp=60–84}}}}

[[File:Aristotle's laws of motion.svg| thumb | upright=1.5 | Aristotle's laws of motion. In ''[[Physics (Aristotle)|Physics]]'' he states that objects fall at a speed proportional to their weight and inversely proportional to the density of the fluid they are immersed in.{{sfn| Drabkin | 1938 | pp=60–84}} This is a correct approximation for objects in Earth's gravitational field moving in air or water.{{sfn| Rovelli | 2015 | pp=23–40}}]]

In the ''Physics'' (215a25), Aristotle effectively states a quantitative law, that the speed, v, of a falling body is proportional (say, with constant c) to its weight, W, and inversely proportional to the density,{{efn-ua | Drabkin agrees that density is treated quantitatively in this passage, but without a sharp definition of density as weight per unit volume.{{sfn| Drabkin | 1938 | pp=60–84}}}} ρ, of the fluid in which it is falling:;{{sfn| Rovelli | 2015 | pp=23–40}}{{sfn| Drabkin | 1938 | pp=60–84}}

:: <math>v=c\frac{W}{\rho}</math>

Aristotle implies that in a [[vacuum]] the speed of fall would become infinite, and concludes from this apparent absurdity that a vacuum is not possible.{{sfn| Rovelli | 2015 | pp=23–40}}{{sfn| Drabkin | 1938 | pp=60–84}} Opinions have varied on whether Aristotle intended to state quantitative laws. Henri Carteron held the "extreme view"{{sfn| Drabkin | 1938 | pp=60–84}} that Aristotle's concept of force was basically qualitative,{{sfn| Carteron | 1923 | pages=1–32 and passim}} but other authors reject this.{{sfn| Drabkin | 1938 | pp=60–84}}

[[Archimedes]] corrected Aristotle's theory that bodies move towards their natural resting places; metal boats can float if they [[Archimedes' principle|displace enough water]]; floating depends in Archimedes' scheme on the mass and volume of the object, not, as Aristotle thought, its elementary composition.{{sfn| Rovelli | 2015 | pp=23–40}}

Aristotle's writings on motion remained influential until the [[Early Modern]] period. [[John Philoponus]] (in [[Late antiquity]]) and [[Galileo Galilei|Galileo]] (in [[Early modern period]]) are said to have shown by experiment that Aristotle's claim that a heavier object falls faster than a lighter object is incorrect.{{sfn| Wildberg | 2016}} A contrary opinion is given by [[Carlo Rovelli]], who argues that Aristotle's physics of motion is correct within its domain of validity, that of objects in the [[Earth]]'s gravitational field immersed in a fluid such as air. In this system, heavy bodies in steady fall indeed travel faster than light ones (whether friction is ignored, or not{{sfn| Rovelli | 2015 | pp=23–40}}), and they do fall more slowly in a denser medium.{{sfn| Susskind | 2011}}{{efn-ua | Philoponus and Galileo correctly objected that for the transient phase (still increasing in speed) with heavy objects falling a short distance, the law does not apply: Galileo used balls on a short incline to show this. Rovelli notes that "Two heavy balls with the same shape and different weight do fall at different speeds from an aeroplane, confirming Aristotle's theory, not Galileo's."{{sfn| Rovelli | 2015 | pp=23–40}}}}

Newton's "forced" motion corresponds to Aristotle's "violent" motion with its external agent, but Aristotle's assumption that the agent's effect stops immediately it stops acting (e.g., the ball leaves the thrower's hand) has awkward consequences: he has to suppose that surrounding fluid helps to push the ball along to make it continue to rise even though the hand is no longer acting on it, resulting in the Medieval [[theory of impetus]].{{sfn| Rovelli | 2015 | pp=23–40}}