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1、过程装备与控制工程专业英语（ProcessequipmentandcontrolengineeringEnglish）InthispartoftheGeneralEquilibriumConditionsofASystem,wewillstudytheConditionsthatmustbemetinordertobalanceanobjectinitsforceandforce.AccordingtoNewton'sfirstlaw,theforceexertedonastationaryobjectmustbezero.Note,however,thatthislawdoesn
2、otmentiontherotationaleffectoftorqueorforce.Obviously,thenettorquemustbezero,otherwisetheobjectwillrotate.ThefundamentalproblemhereisthatNewton'sfirstlaw(andsecondlaw),whichwasoriginallydescribed,appliesonlytoverysmallobjects,ortonon-zero-massparticlesthatcanbeignoredinsize.However,itcanbeexte
3、ndedtoafinitesizeobjectasdescribedbelow.Considerasystemcomposedoftwomasspoints,assumingthattheforcesbetween1fand2faretheforcesbetweenthem(FIG.1.1).Theseforcesarecalledinternalforcesbecausetheyareproducedbytheinteractionofobjectswithinthesystem.Let'ssaythatinternalforceobeysNewton'sthirdlaw,and
4、wehave12ff=?Itis.Ifthereisalsotheinteractionbetweentheparticleandtheexternalobjectappliedtotheparticle,suchas1,2,FF,and3F,theseforcesarecalledexternalforces.Obviously,theforceactingonaparticularparticlemusthavethesameapplication,becausetheparticlesizecanbeignored.Ifeveryparticleinthesystemisin
5、equilibrium,wecansaythatthesystemisbalanced.Inthiscase,accordingtoNewton'sfirstlaw,theforceexertedoneachparticlemustbezero.FortheparticleAwehave:sigma=++=0121fFFFAandfortheparticleB,thesumoftheforcesonthesystemis123120abffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
6、ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff=+=Asshowninfigure1.1,wehave:12()()PABMrFrF=x+*sigmasigmasigmasigma,becausetheforce1fand2fhavethesameactionline,thecondi
7、tionsofthemomentcanberewrittenas1121223()0PMrFFffrF=x+++x=sigmabut12ff=?;Sotheconditionsofforceandmomentreducedto1210FFFF++==∑and111223()()()0PMRFRFRF=x+x+x=∑,inotherwords,ifthesystemisinbalance,thentheforceactingonitsmustbezero,butalso
