《大學(xué)物理簡(jiǎn)明教程》 課件 3 Work and Energy

workWorkandEnergyenergyWork-EnergyTheoremconservationofmechanicalenergyconservativeforcepowerAforceonanobjectTheobjectmovinginthedirectionoftheforceFThereisworkdonebytheforceWorkWorkistheproductofthecomponentoftheFORCEinthedirectionofmotiontimestheDISTANCEthroughwhichtheforceactsUnit:Nm=J(Joules)JamesPrescottJoule(1818-1889)Workisascalarquantity.

dThefrictionforcedoesworktooNegativework!mθFfrα=180oWorkofFT

TransferringenergytoobjectPositiveworkRemovingenergyfromobjectNegativeworkWorkandenergyEnergy-theability(capacity)todoworkTheSIunitofenergyisalsoJoule(N·m)Applyingaconstant

netforceoveradistancedWhatisthefinalspeedoftheobject?(acceleration=a)

KineticEnergyofanobject(initialspeed=0)

ΔlF(l)θ

parallelcomponent,e.g.speed

WorkdonebygravityWorkdonebynormalforceNoworkbecauseNisperpendicularWorkdonebyfrictionexample

Whatisthespeedoftheboxatthebottom?

vi=0Aballthrownupwardwithinitialspeedof50m/sHowhighwillitgo?Gravitydoesnegativeworkmg

-mgWorkdonebygravityhGravity‘removes’kineticenergytransformsitintopotentialenergyEpIftheballfallsdown,itgainskineticenergyConservationofmechanicalenergymg

IneithercaseConservationofmechanicalenergy

Underwhatcondition??conservativeforcethetotal

work

donebythisforceinmovingaparticlebetweentwopointsisindependentofthepath.ifaparticletravelsinanyclosedloop,thenetworkdonebyaconservativeforceiszeromechanicalenergyTheworkdonebygravity/changeinpotentialenergydoesnotdependonthepathtakenThegravitationalforceisa‘conservativeforce’Frictionisa‘nonconservativeforce’

ItsworkdependsonthepathConservationofmechanicalenergyNononconservativeforcedoesworkPotentialenergyofaspringHooke'slawForceexertedbyaspring:Workifcompressedbyadistance

d-theshadowareaIfthespringisreleased,theobjectconnectedonthespringgainskineticenergy:

Potentialenergyofaspring(springenergy)v1？hv0x？Ek0Ep0Es00Totalenergy

energyisconservedNofrictionorairresistance2equationsSolvingfor2unknowns:v1&x

KineticEnergy

GravitationalPotentialEnergy

SpringPotentialEnergyMechanicalEnergy

OtherformsofenergyMechanicalenergyElectricalenergyChemicalenergyRadiantenergyNuclearenergyThermalenergyAclimberof55kgclimbsamountain,goingfromanaltitudeof1600mtothesummitat3300m.Whatisthepotentialenergygainduringthisclimb?If100gofricehas116Calories(1Calorie=4200J),howmuchricecanprovidethatenergy?9.16×105J188g(a)Calculatetheforceneededtobringa950-kgcartorestfromaspeedof90.0km/hinadistanceof120m(afairlytypicaldistanceforanon-panicstop).(b)Supposeinsteadthecarhitsaconcreteabutmentatfullspeedandisbroughttoastopin2.00m.Calculatetheforceexertedonthecarandcompareitwiththeforcefoundinpart(a).(negative)Work=kineticenergylost(a)(b)A60.0-kgskierwithaninitialspeedof12.0m/scoastsupa2.50-m-highriseasshowninthefigure.Findherfinalspeedatthetop,giventhatthecoefficientoffrictionbetweenherskisandthesnowis0.0800.(Hint:Findthedistancetraveleduptheinclineassumingastraight-linepathasshowninthefigure.)workfromgravity:workfromfriction:Kineticenergyatthetop:Frictionforce:Distanceuptheslope:POWERPoweristherateatwhichworkisdone:Whatisthepoweroutputfora60.0kgwomanwhorunsupa3.0mhighflightofstairsin3.50s,startingfromrestbuthavingafinalspeedof2.0m/s?Work(byher“l(fā)egforce”):SIunit:J/sorWatts

1horsepower=735.5W

(a)Howlongwouldittakea1.50×105-kgairplanewithenginesthatproduce100MWofpowertoreachaspeedof250m/sandanaltitudeof12.0kmifairresistancewerenegligible?Totalenergygained:Kineticenergy+Potentialenergy=Powerxtime(b)Ifitactuallytakes900s,whatisthepower?(c)Giventhispower,estimatethea