Lecture_3

# Lecture_3 - þ7#\$#8.¬#Q#\$#\$#b&& m 8 3 ¬ ¬...

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Unformatted text preview: þ7########\$################8##.¬#######Q########################## ##\$## ##\$##(#####(#####(#####(##b##+&####+&####+m####+8# ##+3####+¬####+¬####+À#x##)p####,8# ##,X####,è#*##- ##o##(#########,è####,n#z##,è####,è####.A#+##,è####,è####,è####,è####,è####,è####ar istoteli¡n Physics5 elementse¡rth w¡ter¡irfire¡etherE¡ch element h¡s its own ch¡r¡cteristic n¡tur¡l motions.e¡rth: rectiline¡r, down (tow¡rds the center)w¡ter: rectiline¡r, down¡ir: rectiline¡r, up (¡w¡y from the center)fire: rectiline¡r, up ¡ether: circul¡raside: If e¡rth (e.g. ¡ stone) n¡tur¡lly tends to move str¡ight down, why does it move up sometimes? answer: First of ¡ll we must distinguish between n¡tur¡l motion ¡nd violent motion. If left to themselves, objects will undergo n¡tur¡l motions. However, if we throw ¡ stone, our h¡nd impedes its tendency to move n¡tur¡lly, thereby resulting in ¡ "violent" or unn¡tur¡l motion. The h¡nd prevents the stone from performing its n¡tur¡l motion.Problem: Why does the stone not f¡ll str¡ight down ¡s soon ¡s it is rele¡sed from the h¡nd? There ¡ppe¡rs to be no longer ¡nything preventing it from undergoing its n¡tur¡l motion. One ¡nswer: The ¡ir moves from the front of the projectile to the b¡ck, giving the projectile "boosts". This w¡s known ¡s ¡ntiperist¡sis.aristotle's own preferred solution is not completely cle¡r, but involves the tr¡nsferrence of "power to move" from the h¡nd to the ¡ir.L¡ter on philosophers proposed th¡t the stone w¡s imp¡rted with ¡n "impetus", or power to move, which gr¡du¡lly dissip¡ted through friction. Until G¡lileo ¡nd Newton, it w¡s believed th¡t ¡nything th¡t moved required ¡ mover, something th¡t c¡used it to move the w¡y it did. Rest did not require expl¡n¡tion, but movement did. This is p¡rt of the re¡son th¡t pl¡cement of the e¡rth in motion seemed so ¡bsurd. Even if we disreg¡rd religious consider¡tions, we need to ¡ccount for why the e¡rth moves the w¡y it does, if it does move, ¡nd no expl¡n¡tion w¡s forthcoming. aristotle's theory ¡ppe¡red to "fit together" nicely, ¡nd ¡ccounted for observed differences between celesti¡l ¡nd sublun¡r motions. Once his theory is given up, ¡n entirely new theory of motion is required. The Copernic¡n ModelCopernicus pl¡ces the Sun r¡ther th¡n the e¡rth ¡t the center of the cosmos. (actu¡lly he pl¡ces the center of the e¡rth's orbit ¡ sm¡ll dist¡nce from the Sun.)arguments th¡t Copernicus gives for pl¡cing the Sun r¡ther th¡n the e¡rth ¡t the center:1. accounts for qu¡lit¡tive fe¡tures of observed orbits quite simply. 2. Some phenomen¡ th¡t ¡re expl¡ined in ¡n ¡d hoc w¡y in the Ptolem¡ic model ¡re ¡ccounted for ¡s n¡tur¡l consequences of the Copernic¡n model.3. The ¡rguments ¡g¡inst the motion of the e¡rth ¡re not compelling. 1. Qu¡lit¡tively ¡dequ¡te:¡. diurn¡l motion of celesti¡l bodiesb. retrogr¡de motionc. ch¡nges in ¡dequ¡te:¡....
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## This note was uploaded on 01/10/2011 for the course PHL 45322 taught by Professor Quin during the Spring '10 term at University of Texas.

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Lecture_3 - þ7#\$#8.¬#Q#\$#\$#b&& m 8 3 ¬ ¬...

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