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Chapter1FromoEinstein
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Gravityisbyfartheweakestofthefourfualforcesthatexistihersbeiromagicfordthestrongandweaknuclearforces.Yetedistaisgravitythatdomihisisbecausegravityisoiveaeverbesed.Sowhilemeobjectsareelectrieutral，theyeverbegravitatiohegravitatioweehmassalullthoseobjectstogether，andalwaysiheybeassive.
Itisthankstogeniuseslikeohatweuandgravityatall，yetitisgravity，morethananyotherforce，thatuestopreseswiththemostenigmaticofpuzzles.Toua'sstartatthebeginning，ahehistorientofgravity.
&ravity
Itseemssafetoassumethatmankindhasalhenwedropawillfalldownwards.Inthissense，we'vealwaysbeeheexistenceofgravity.Itwastheotionthatappearstohavebeenthefoahinkers.
Aristotle，who'sPhysicsplayedadominantroleinEuropeailaslateasthe17thtury，basedhisexplanationofgravityoobjeovetowardstheirnaturalplatheUhisplacewas，hereasoned，shouldbedetermiioiion.Morespecifically，itshoulddependonhowmutaihefourelemeer，Air，andFire.
AristuedthatobjeposedpredominantlyfromEarthandWatershouldmovetowardsthetreoftheUohim，thetreoftheUheplaeathhisfeet.ObjectsmadefromEarth，andthatarethrownintotheair，mustthereforemovedround.Water，hereasoned，islighterthah，asbeverifiedbydumpioahalf-fullglassofit.AllWatermustthereforeetorestontopoftheEarth.Similarly，AirislighterthanWater，asbubblesriseihenaturalplaceforAiristhereforeaboveWater，auralplaceforFireisabovetheAir.
Thisframeworkprovidesalogicalsortofordertotheworldthatwasobserved，intermsofwhatwerethoughttobethebasistituentsofmatteratthetime.Itevenallowsoatementsaboutthespeedatwhichobjectsshouldfall.Aristuedthatthevelocityofafallishouldbeinproportiontoitsweight，andiioyofthemediumthroughwhichittravels.Thatis，Aristotlethoughtthataweighs2kgshouldfalltwiceasfastasoweighs1kg.
Sadly，Aristotle'stheoryotbecorreowknowthatthereisheUniverseforobjeovetowards.Weonstrate，bydirectexperimentation，thattherateatwhiobjectacceleratesduetogravityisnotinproportiontoitsmass.Infabeshownthatallobjectsfallatthesamerate.Thisdiscoverywasoohemoderandingofgravity，andsoitbearssomefurtherexplanation.
&hatallobjectsaccelerateuyatthesamerateisnotanobviouso，ifIweretodropafeatherfrommylefthand，andalumpht，Ishouldohittheflooratthesametime.Thelumpettherefirst.Sowhatismeaement‘allobjectsaccelerateuyatthesamerate’？Touhisphrase，wehavetothinkaboutalloftheforcesthatatheseobjects.
&goofthefeatheritisabythefravity，butitisalsoabyotherforces.Asitstartstodrop，thereisresistasmotionfromtheairaroundit.Thisslowsthefeathermorethaheheavierlumpofiron.Anyslightgustofairwillalsohavealargeefethefeatherwhileperturbiionofthelumpofmetalbyonlyatinyamount.So，themeaningofthephrase‘allobjectsaccelerateuyatthesamerate’isnotastatementaboutthemotiosieeher，itisastatementaboutwhatshouldhappentoaweretofalluheinfluenceofgravityalois，ifallotheriionsaresuppressed，thesshouldfallatthesamerate.
Galileoiswidelycreditedwithhavingshowhofthisproposition.In1638，hesupposedlydroppedballswithdiferehetopoftheleaningtowerofPisa.Theballswerefoundtofallatthesamerate，iheiroreredperhapseveically，thesameresultwasdemoheApolloastronautDavidScott.Scottdroppedafeatherandahammerwhilestandingonthesurfaoon.Thereisnoairooslowthemotioher，andsobothobjededathisfeetatthesametime（seeFigure1）.TodaywerefertothispheheUyofFreeFall.Itisakeyihon'saein'stheravity，aswewillseelater.
1.（a）GalileballsoftheleaningtowerofPisa;and（b）apaintingofDavidScahammeraheronthesurfaoon.
&heravity
SirIsaa’stheravityandmotionublishedinhisPrincipiaMathemati1687，anditgedtheworldforever.Thiswasthefirstgeifictheoryofhowgravityworked.Ule，ondidn’ttryandseekairavity.Iifieditsefedintheprocessdeducedphysicallawsthataccuratelydescribedthemotionofnotosoalsothemotioself，aswellasallotherbodiesintheSolarSyste
&owastrulyaweednewbrahematidforthefirsttimedemohelawsofphysicsthatapplytous，oh，arethesameasthosethatapplytotheheaveheplexityofmotion，whichAristotlehadtriedtoexplain，wasboileddowntoahandfulofsimplelaws.helorious，aungedforover200years.Inonebook，ioheworldsofsdustry，androvidedtheframehiyofusstillthinktoday.
ThebasigredieheoryaretheexistenceofabsolutespadtimeasthestageonwhichallmotionodtheexisteniversalgravitatiosiweeneverypairofmassivebodiesintheUhatisall.
&onearstomostofusinourday-todayexperience，simplytheeternalandungarenawithinwhichallmatterexists.Ipositiooinspadworkoutthedistaoa，Y，bysimplyusiapeheldoutinastraightliohereisnoambiguityinthisprocess.ObjedYmaymoveihespaceitselfisfixedandfed.
Likewise，oionoftimefollowedtheeverydayintuitionthatmostofusgrowupwith.Instantsoftimeunfoldiheory，oher.Objegetheirpositiaime，buttimeitselfisuniversalandthesameforeveryoheoryalltrueeasuretimeinthesameway，justasalltruemeasuringtapesmeasurethesamedistawogives.
Ntoon，allbodiesmoveatatrate，uedoerhisarturefromAristotle’sphysics）.Ifaforceshouldaobjetheefectofthatforceistocausetheobjecttoaccelerate.Moreforsmoreac，and，ifahasmoremass，thenitoreachthesameac.Withinthisframewravityissimplyaeringonallmassivebodies，pulliher.hathisgravitatiobeproportionaltothemassofeachoftheobjectsitactsbetween，andinverselyproportionaltothesquareofthedistahatis，thefravitybetweentwomassivebodiesmustbegiveionoftheform:
whereMahemassesofthetwoobjequestion，andristhedistahissimpleequatiohon'slawsofmotion，issuittogetagoodapproximatioiomajorityofastrophysicalbodiesaswellasallbodiesthatexistoh.
Thatallbodiesfalluon'sgravityatthesamerate，andhenceobeyGalileo'sobservation，beseehation'smeiforcecausesaheavybodytoacceleratemoreslowlythanalightone.siderthistogetherwiththefactthat，agtohegravitatioihmass.Ion'stheorythesetwothiexactlytherequiredratetoceleachotherout.Abodybeingabyon'sgravity，aon'slawsofmotion，mustthereforeaccelerateatafixedrate，ismass.ThatthishappensisheUyofFreeFallisbuiltion'stheoryfromthestart.
&greatsuewton'stheorywasthatitcouldbeusedtoderivethelawsofmotios.TheselawshadbeendeducedempiricallybyJohannesKeplerearlierihtury，usingg-edgeastronomicaldata.Kepler'slawsstatedthat:
&ofaplaaheSunisatohefociofthisellipse.
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