AP_Physics_B_-_Hydrostaticsppt课件.ppt

FluidMechanics Hydrostatics APPhysicsB StatesofMatter BeforewebegintounderstandthenatureofaFluidwemustunderstandthenatureofallthestatesofmatter The3primarystatesofmatter solid Definiteshapeandvolume liquid Takestheshapeofitscontainer yethasadefinitevolume gas Takestheshapeandvolumeofitscontainer Special states Plasma Bose EinsteinCondensate Density The3primarystateshaveadistinctdensity whichisdefinedasmassperunitofvolume DensityisrepresentedbytheGreekletter RHO r WhatisaFluid Bydefinition afluidisanymaterialthatisunabletowithstandastaticshearstress Unlikeanelasticsolidwhichrespondstoashearstresswitharecoverabledeformation afluidrespondswithanirrecoverableflow Examplesoffluidsincludegasesandliquids Whyfluidsareusefulinphysics Typically liquidsareconsideredtobeincompressible ThatisonceyouplacealiquidinasealedcontaineryoucanDOWORKontheFLUIDasifitwereanobject ThePRESSUREyouapplyistransmittedthroughouttheliquidandovertheentirelengthofthefluiditself Pressure Oneofmostimportantapplicationsofafluidisit spressure definedasaForceperunitArea Example Awaterbedis2 0monasidean30 0cmdeep a Finditsweightifthedensityofwateris1000kg m3 b Findthepressurethethatthewaterbedexertsonthefloor Assumethattheentirelowersurfaceofthebedmakescontactwiththefloor 1 2m3 1200kg 11760N 2940N m2 HydrostaticPressure SupposeaFluid suchasaliquid isatREST wecallthisHYDROSTATICPRESSURETwoimportantpoints Afluidwillexertapressureinalldirections Afluidwillexertapressureperpendiculartoanysurfaceitcompacts NoticethatthearrowsonTOPoftheobjectsaresmallerthanattheBOTTOM ThisisbecausepressureisgreatlyaffectedbytheDEPTHoftheobject Sincethebottomofeachobjectisdeeperthanthetopthepressureisgreateratthebottom Pressurevs Depth Supposewehadanobjectsubmergedinwaterwiththetopparttouchingtheatmosphere IfweweretodrawanFBDforthisobjectwewouldhavethreeforcesTheweightoftheobjectTheforceoftheatmospherepressingdownTheforceofthewaterpressingup mg Fatm Fwater Fwater Fatm mg Pressurevs Depth Butrecall pressureisforceperunitarea Soifwesolveforforcewecaninsertournewequationin Note Theinitialpressureinthiscaseisatmosphericpressure whichisaCONSTANT Po 1x105N m2 AcloserlookatPressurevs Depth ABSOLUTEPRESSURE InitialPressure MayorMAYNOTbeatmosphericpressure Depthbelowsurface GaugePressure CHANGEinpressureortheDIFFERENCEintheinitialandabsolutepressure Example a Calculatetheabsolutepressureatanoceandepthof1000m Assumethatthedensityofwateris1000kg m3andthatPo 1 01x105Pa N m2 b Calculatethetotalforceexertedontheoutsideofa30 0cmdiametercircularsubmarinewindowatthisdepth 9 9x106N m2 2 80 x106N Aclosedsystem IfyoutakealiquidandplaceitinasystemthatisCLOSEDlikeplumbingforexampleoracar sbrakeline thePRESSUREisthesameeverywhere Sincethisistrue ifyouapplyaforceatonepartofthesystemthepressureisthesameattheotherendofthesystem Theforce ontheotherhandMAYorMAYNOTequaltheinitialforceapplied ItdependsontheAREA YoucantakeadvantageofthefactthatthepressureisthesameinaclosedsystemasithasMANYapplications TheideabehindthisiscalledPASCAL SPRINCIPLE Pascal sPrinciple AnotherExample Brakes Inthecaseofacar sbrakepads youhaveasmallinitialforceappliedbyyouonthebrakepedal Thistransfersviaabrakeline whichhadasmallcylindricalarea ThebrakefluidthenentersachamberwithmoreAREAallowingaLARGEFORCEtobeappliedonthebrakeshoes whichinturnslowthecardown Buoyancy Whenanobjectisimmersedinafluid suchasaliquid itisbuoyedUPWARDbyaforcecalledtheBUOYANTFORCE WhentheobjectisplacedinfluidisDISPLACESacertainamountoffluid Iftheobjectiscompletelysubmerged theVOLUMEoftheOBJECTisEQUALtotheVOLUMEofFLUIDitdisplaces Archimedes sPrinciple Anobjectisbuoyedupbyaforceequaltotheweightofthefluiddisplaced Inthefigure weseethatthedifferencebetweentheweightinAIRandtheweightinWATERis3lbs Thisisthebuoyantforcethatactsupwardtocanceloutpartoftheforce Ifyouweretoweightthewaterdisplaceditalsowouldweigh3lbs Archimedes sPrinciple Example Abargainhunterpurchasesa gold crownatafleamarket Aftershegetshome shehangsitfromascaleandfindsitsweightinairtobe7 84N Shethenweighsthecrownwhileitisimmersedinwater