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1、PhysicsProfessionalEnglish物理学专业英语PhysicsProfessionalEnglishB8CircularmotionNEWWORDSPage.34NEWWORDSPage.35GravitationalforceB9GravitationAllmassesattracteachotherwithagravitationalforce.Ifpointmassesm1andm2,areadistancerapart,andFistheforceoneach,thenaccordingtoNewton'slawofgravitation:Withasuitable
2、constant,theaboveproportioncanbeturnedintoanequation:B9GravitationGiscalledthegravitationalconstant.Itisfoundbyexperimentusinglargelaboratorymassesandanextremelysensitiveforce-measuringsystem.In51units,thevalueofGis6.67X10-11Nm2kg-2.Theequationontheleftisalsovalidforsphericalmassesofuniformdensity,
3、withcentresrapart,asabove.Note:Newton'slawofgravitationisanexampleofaninversesquarelaw.Ifthedistancerdoubles,theforceFdropstoonequarter,andsoon.Gravitationalforcesarealwaysforcesofattraction.Gravitationalforcesareextremelyweak,unlessatleastoneoftheobjectsisofplanetarymassormore.B9GravitationGravita
4、tionalfieldIfamassfeelsagravitationalforce,thenitisinagravitationalfield.Thegravitationalfieldstrength.gisdefinedlikethis:B9GravitationForexample,ifamassof2kgfeelsagravitationalforceof10N,thengis5Nkg-1.Note:Gravitationalfieldstrengthisavector.gisavariableandcanhavedifferentvalues.Thesymbolgabovedoe
5、snotimplytheparticularvalueof9.81Nkg-1neartheEarth'ssurface.Theforceactingonamassinagravitationalfieldcanbefoundbyrearrangingtheequationabove:F=mg.B9GravitationNote:Thegravitationalfieldaroundasphericalmassisshownabove.Itiscalledaradialfieldbecauseofitsshape.Insidethemasstheequationontheleftdoesnot
6、apply.gfallstozeroatthecentre.Equipotentiallinesareexplainedonthenextpage.B9GravitationTheEarth’sgravitationalfieldstrengthB9GravitationAtthesurfaceIfMistheEarth'smass,Risitsradius,andgoisthegravitationalfieldstrengthatitssurface,then:(1)B9GravitationNote:gois9.81Nkg-1•Itismorecommonlyknownasg(with
7、outthe0)'Herehowever,the0hasbeenaddedtodistinguishitfromotherpossiblevaluesofg.Usingmeasuredvaluesofgo'R,andGintheaboveequation,theEarth'smassMcanbecalculated.WithRknown,theEarth'saveragedensitycanalsobefou
