功、功率和简单机械(Work-Power-Simple-Machines)

Work,Power,&MachinesWhatiswork?Theproductoftheforceappliedtoanobjectandthedistancethroughwhichthatforceisapplied.Isworkbeingdoneornot?MowingthelawnWeight-liftingMovingfurnitureupaflightofstairsPushingagainstalockeddoorSwingingagolfclubYESYESYESNOYESCalculatingWorkAllorpartoftheforcemustactinthedirectionofthemovement.work=forcexdistanceDoyoudomoreworkwhenyoufinishajobquickly?WorkdoesNOTinvolvetime,onlyforceanddistance.Noworkisdonewhenyoustandinplaceholdinganobject.Labelingwork:w=FxdNewtonXmeter(Nm)Whichalso=(kgxm2)s2TheJoule1newton-meterisaquantityknownasajoule(J).NamedafterBritishphysicistJamesPrescottJoule.Howquicklyworkisdone.Amountofworkdoneperunittime.Iftwopeoplemowtwolawnsofequalsizeandonedoesthejobinhalfthetime,whodidmorework?Samework.Differentpowerexerted.POWER=WORK/TIMEPowerThewattAunitnamedafterScottishinventorJamesWatt.Inventedthesteamengine.P=W/tJoules/second1watt=1J/swattsUsedtomeasurepoweroflightbulbsandsmallappliancesAnelectricbillismeasuredinkW/hrs.1kilowatt=1000WHorsepower(hp)=745.5wattsTraditionallyassociatedwithengines.(car,motorcycle,lawn-mower)Thetermhorsepowerwasdevelopedtoquantifypower.Astronghorsecouldmovea750Nobjectonemeterinonesecond.750NMachinesAdevicethatmakesworkeasier.Amachinecanchangethesize,thedirection,orthedistanceoverwhichaforceacts.Forcesinvolved:InputForceFIForceappliedtoamachineOutputForceFOForceappliedbyamachineTwoforces,thustwotypesofworkWorkInputworkdoneonamachine=Inputforcexthedistancethroughwhichthatforceacts(inputdistance)WorkOutputWorkdonebyamachine=Outputforcexthedistancethroughwhichtheresistancemoves(outputdistance)Canyougetmoreworkoutthanyouputin?Workoutputcanneverbegreaterthanworkinput.MechanicalAdvantage(MA)–expressedinaratioWITHNOUNITS!!Thenumberoftimesamachinemultipliestheinputforce.MA=outputforce/inputforce2typesofmechanicaladvantageIDEALInvolvesnofriction.IscalculateddifferentlyfordifferentmachinesUsuallyinputdistance/outputdistanceACTUALInvolvesfriction.CalculatedthesameforallmachinesDifferentmechanicaladvantages:MAequaltoone.(outputforce=inputforce)Changethedirectionoftheappliedforceonly.MechanicaladvantagelessthanoneAnincreaseinthedistanceanobjectismoved(do)EfficiencyEfficiencycanneverbegreaterthan100%.Why?Someworkisalwaysneededtoovercomefriction.Apercentagecomparisonofworkoutputtoworkinput.workoutput(WO)/workinput(WI)1.TheLeverAbarthatisfreetopivot,ormoveaboutafixedpointwhenaninputforceisapplied.Fulcrum=thepivotpointofalever.Therearethreeclassesofleversbasedonthepositioningoftheeffortforce,resistanceforce,andfulcrum.FirstClassLeversFulcrumislocatedbetweentheeffortandresistance.MakesworkeasierbymultiplyingtheeffortforceANDchangingdirection.Examples:SecondClassLeversResistanceisfoundbetweenthefulcrumandeffortforce.Makesworkeasierbymultiplyingtheeffortforce,butNOTchangingdirection.Examples:ThirdClassLeversEffortforceislocatedbetweentheresistanceforceandthefulcrum.DoesNOTmultiplytheeffortforce,onlymultipliesthedistance.Examples:Levers!!!!!!!!!!!Mechanicaladvantageoflevers.Ideal=inputarmlength/outputarmlengthinputarm=distancefrominputforcetothefulcrumoutputarm=distancefromoutputforcetothefulcrum2.TheWheelandAxleAleverthatrotatesinacircle.Acombinationoftwowheelsofdifferentsizes.Smallerwheelistermedtheaxle.IMA=radiusofwheel/radiusofaxle.3.TheInclinedPlaneAslantedsurfaceusedtoraiseanobject.Examples:ramps,stairs,laddersIMA=lengthoframp/heightoframpCanneverbelessthanone.4.TheWedgeAninclinedplanethatmoves.Examples:knife,axe,razorbladeMechanicaladvantageisincreasedbysharpeningit.5.TheScrewAninclinedplanewrappedaroundacylinder.Thecloserthethreads,thegreaterthemechanicaladvantageExamples:bolts,augers,drillbits6.ThePulleyAchain,belt,orropewrappedaroundawheel.CaneitherchangethedirectionortheamountofeffortforceEx.Flagpole,blinds,stagecurtainPulleytypesFIXEDCanonlychangethedirectionofaforce.MA=1MOVABLECanmultiplyaneffortforce,butcannotchangedirection.