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1、EulerE842:AnIntroductiontoNaturalScience,EstablishingtheFundamentals....,fromhisOperaPostuma.TranslatedfromGermanbyE.Hirsch.50Chapter11.GeneralBasicRulesofNaturalScience.84.Ifabodyiseitheratrestormovesuniformlyinastraightline,wecanconcludethatitiseithernotpushedatallfromtheoutside,orthatth
2、eforcesthatactonitareinequi-librium.Thisruleimmediatelyfollowsfromtheconceptofpersistence;forsinceabodywillonitsowneitherstayatrestormoveuniformlyalongastraightline,noexternalforceisre-quiredtomaintainitinthatstate,butsuchaforcewouldinsteadchangeitsstate.Ifabodyremainsinthesamestate,thisis
3、acertainindicationthatnoexternalforceisactingonit.Inthatcaseitiseithernotpushedatallfromoutside,orwhenforcesarepresentthatexertapressureonit,thenitiscertainthatthesekeepeachotherinequilibrium,andtheactionofeachofthemiseliminatedbytheothers.Buthowseveralforcesthatactonabodykeepeachotherineq
4、uilibrium,istaughtbytheScienceofEquilibrium,whichisentirelybasedontheconsiderationthatwhentwoequalforcesactonabodyinoppositedirections,theyproducenochangeinthestateofthebody,andthesituationisequivalenttothatwherenoforcesarepresentatall.Thereforeifoneseesthatabodyremainsinitsstatealthoughit
5、isbeingpushedfromoneside,onecanconcludethatitispushedfromtheoppositesidewithequalstrength.Iseeforexample,thatabodylyingonthetableremainsatrest,althoughitwouldfalldownifthetablewerepenetrable,fromwhichIconclude,thattheimpenetrabilityofthetablepreventsitsfall,andconsequentlypushesthebodyupwa
6、rds.Butsince,inspiteofthispressure,thebodyremainsatrest,Iconcludethattheremustbeanotherforcethatpushesthebodyequallystronglydownwards,andthatiscalleditsweight;thatmeansthatinthiscasetheweightandtheforceduetotheimpenetrabilityofthetableareinequilibrium.85.Butifweseethatabodyatrestissetintom
7、otion,orthatamovingbodydoesnotcontinueatthesamespeed,orchangesitsdirection,wecanconcludewithcertaintythataforceisactingonit,andfromtheaboveonewillbeabletodetermineitsmagnitudeanddirection.Sinceeverybodyalwaysremainsinitsstateaslongasitisnotdisturbedbyanexterna
