人教版英语八年级下Unit9课件5VIP免费

CircularmotionHaveyoueverbeentoanamusementpark?Fig.05.19WhysomebodyneedstoknowaboutcircularmotionOtherwisewewouldn’tgoonthoserides?Spacestation,satellites,livefromoverseasPlanets,CometsandMoonsUniformCircularMotionVelocityisavector,ithasbothdirectionandmagnitudeSimplestcase:directionconstant,magnitudechanges.LinearMotionWewillnowstudyanother“simple”case:magnitudeisconstantanddirectioncontinuallychanges.UniformCircularMotion8CircularMotionXθ=0yifHowdowelocatesomethingonacircle?GiveitsangularpositionθWhatisthelocationof900orπ/29CircularMotionistheangularposition.Angulardisplacement:ifNote:anglesmeasuredClockwise(CW)arenegativeandanglesmeasured(CCW)arepositive.ismeasuredinradians.2radians=360=1revolutionxyifWhatisaradian?C=2πr=(totalangleofonerotation)xrRadianmeasurerelatesdisplacementalongcircumferencetoangulardisplacement11CircularMotionXθ=0yifHowdowelocatesomethingonacircle?GiveitsangularpositionθWhatisthelocationof900orπ/2Fromθ=0toθ=900wehavegone¼ofwayaround¼of2π=π/212xyifrarclength=s=rrsisaratiooftwolengths;itisadimensionlessratio!ThisisaradianmeasureofangleIfwegoallthewayrounds=2πrandΔθ=2πQuestionAchildrunsaroundthecircularledgeofafountainwhichhasadiameterof6.5m.Afterthechildhasrun9.8m,herangulardisplacementisA)0.48rad.B)6.0rad.C)1.5rad.D)3.0rad.14Ifwemovearoundacirclewechangeθ.Howfastdoesθchange?Theaverageandinstantaneousangularvelocitiesωare:ttt0avlimandismeasuredinrads/sec.vandωForcircularmotion—HowfardoyoutravelduringΔtifyouarespinningatanangularvelocityω?Howfar?s=Δθr=ωΔtrHowfast?(speed)v=s/Δt=ωrv=ωrQuestionAchildrunsaroundthecircularledgeofafountainwhichhasadiameterof6.5m.Ifittakesher72secondstorunallthewayaround,herangularvelocityisA)0.087rad/sB)0.57m/sC)0.17rad/sD)0.044rad/sQuestionACDmakesonecompleterevolutioneverytenthofasecond.Theangularvelocityofpoint4is:A)thesameasforpt1.B)twicethatofpt2.C)halfthatofpt2.D)1/4thatofpt1.E)fourtimesthatofpt1.QuestionACDmakesonecompleterevolutioneverytenthofasecond.Whichhasthelargestlinear(tangential)velocity?A)point1B)point2C)point3D)point419Thetimeittakestogoonetimearoundaclosedpathiscalledtheperiod(T).Trv2timetotaldistancetotalavComparingtov=r:ω=v/rfT22fiscalledthefrequency,thenumberofrevolutions(orcycles)persecond.f=1/T20CentripetalAccelerationHere,v0.Thedirectionofvischanging.Ifv0,thena0.Thenetforcecannotbezero.Consideranobjectmovinginacircularpathofradiusratconstantspeed.xyvvvv21Conclusion:tomoveinacircularpath,anobjectmusthaveanonzeronetforceactingonit.ItisalwaystruethatF=ma.Whataccelerationdoweuse?22Thevelocityofaparticleistangenttoitspath.Foranobjectmovinginuniformcircularmotion,theaccelerationisradiallyinward.23Allthreeangles=Δθ=Δr/r=vΔt/rΔv=vΔθ=v2Δt/r24Themagnitudeoftheradialaccelerationis:vrrvar22Δv=vΔθ=v2Δt/rar=Δv/Δt=v2Δt/rΔtWhatmakesitgoround?AparticleincircularmotionisacceleratingsotheremustbeaforceprovidingthisaccelerationF=ma=mv2/rQuestionAspiderissittingonaturntablethatisrotatingat33rpm.TheaccelerationaofthespiderisA)greaterthecloserthespideristothecentralaxis.B)greaterthefartherthespiderisfromthecentralaxis.C)nonzeroandindependentofhislocation.D)zero.28Exam...