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44
ChapterII
purposes-PhiloponusandhisfollowersconstructedanAristote-
lianulaw”ofmotion.Itexpressesmeanspeed(S)asafunctionof
force(F,whichisweightinthecaseoffreefallandtheresistance
ofthemedium(R):
S=F/R
Forontologicalandempiricalreasons,manyphysicistsofan-
tiquityandtheMiddleAgesreformulatedtherelationofthe
forceandtheresistance.Philoponus,inparticular,replacesthis
ulaw”heattributestoAristotlewithanotherone,whichdoes
notdividetheforcebytheresistancebutpostulatesthatthe
timetrequiredforanobjecttofallacertaindistancethrough
themediumwillbeinverselyproportionaltoitsweight(W),
plusacertaintime(x):
t=I/W+x
Itisonlythedeterminationofxthatthedensityofthemedium
playsarole.Thus,themeanspeedofafreefallisdirectlypropor-
tionaltotheweightofthebody,butisalsopartiallyinfluencedby
thedensityofthemedium.16
PhiloponuswasnottheonlyonewhotriedtoreformulatetheAris-
totelianulaw”.Theothertheories,knowntoAvicenna,suggestthattwo
bodies,suchasafeatherandastonewouldfallinavoidwiththesame
speed.Thisconclusionfollowsfromthebeliefthatabodycomposesof
atoms,whoseweightisthesame,andtheweightofaseparateatom-the
primarycauseofitsmotioninfreefall-inavoidwouldcausethesame
speed.Thedifferenceinthespeedsofdifferentbodiesresultsfrom
alesserorgreaterdensityofatoms.Lessdenselypackedatomshaveless
intenseaforcetocutthroughthemedium,and,thusthebodymoves
slower.Thefollowingulaw”isobtained:
16SeeM.Rashed,Naturalphilosohy,[in:]uTheCambridgeCompaniontoArabic
Philosophy”,P.Adamson,R.C.Taylor(eds),Cambridge2006,pp.295-296.See
alsoM.Wolff,uFallgesetzundMassbegriff”,Berlin1971;Idem,Philoponusand
theRiseofPreclassicalDynamics,[in:]uPhiloponusandtheRejectionofAristotelian
Science”,R.Sorabji(ed.),London1987,pp.125-160.