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1、5TimeandDistance5-1MotionInthischapterweshallconsidersomeaspectsoftheconceptsoftimeand5-1Motiondistance.Ithasbeenemphasizedearlierthatphysics,asdoallthesciences,de-5-2Timependsonobservation.Onemightalsosaythatthedevelopmentofthephysicalsciencestotheirpresentformha
2、sdependedtoalargeextentontheemphasis5-3Shorttimeswhichhasbeenplacedonthemakingofquantitativeobservations.Onlywith5-4Longtimesquantitativeobservationscanonearriveatquantitativerelationships,whicharetheheartofphysics.5-5UnitsandstandardsoftimeManypeoplewouldliketopl
3、acethebeginningsofphysicswiththework5-6Largedistancesdone350yearsagobyGalileo,andtocallhimthefirstphysicist.Untilthattime,thestudyofmotionhadbeenaphilosophicalonebasedonargumentsthatcould5-7Shortdistancesbethoughtupinone'shead.MostoftheargumentshadbeenpresentedbyA
4、ristotleandotherGreekphilosophers,andweretakenas"proven."Galileowasskeptical,anddidanexperimentonmotionwhichwasessentiallythis:Heallowedaballtorolldownaninclinedtroughandobservedthemotion.Hedidnot,however,justlook;hemeasuredhowfartheballwentinhowlongatime.Thewayto
5、measureadistancewaswellknownlongbeforeGalileo,buttherewerenoaccuratewaysofmeasuringtime,particularlyshorttimes.Althoughhelaterdevisedmoresatisfactoryclocks(thoughnotliketheonesweknow),Galileo'sfirstexperimentsonmotionweredonebyusinghispulsetocountoffequalin-terval
6、softime.Letusdothesame.Wemaycountoffbeatsofapulseastheballrollsdownthetrack:"one...two...three...four...five...six...seven...eight..."Weaskafriendtomakeasmallmarkatthelocationoftheballateachcount;wecanthenmeasurethedistancetheballtravelledfromthepointofreleaseinon
7、e,ortwo,orthree,etc.,equalintervalsoftime.GalileoexpressedtheresultofhisobservationsinFig.5-1.Aballrollsdownanthisway:ifthelocationoftheballismarkedat1,2,3,4,...unitsoftimefromdinedtrack.theinstantofItsrelease,thosemarksaredistantfromthestartingpointinpropor-tiont
8、othenumbers1,4,9,16,...Todaywewouldsaythedistanceispropor-tionaltothesquareofthetime:Thestudyofmotion,whichisbasictoallofphysics,treatswiththequestions:
