?大學(xué)物理?英文課件8PotentialEnergyandConservationofEnergy8-1PotentialEnergyPotentialenergyisassociatedwiththeconfigurationofasystemofobjectsthatexertforcesononeanothergravitationalpotentialenergyelasticpotentialenergybelongtosystem8-2PathIndependenceofConservativeForces

Thenetworkdonebyaconservativeforceonaparticlemovingaroundeveryclosedpathiszero.Theworkdonebyaconservativeforceonaparticlemovingbetweentwopointsdoesnotdependonthepathtakenbytheparticle8-2PathIndependenceofConservativeForces

SampleProblem:a2.0kgofslipperycheesethatslidesalongafrictionlesstrackfrompointatob.Thecheesetravelsthroughatotaldistanceof2.0malongthetrack,andanetverticaldistanceof0.80m.Howworkisdoneonthecheesebythegravitationalforceduringtheslide?8-2PathIndependenceofConservativeForces

8-3DeterminingPotentialEnergyValues

OnlychangesinUarephysicallymeaningful

GravitationalPotentialEnergy-onlydependsontheverticalpositionElasticPotentialEnergySampleProblemProblem:a2.0kgslothhangs5.0mabovetheground.(a)whatisthegravitationalpotentialenergyUofthesloth-Earthsystemifwetakethereferencepointy=0tobe(1)attheground,(2)atabalconyfloorthatis3.0mabovetheground,(3)atthelimb,(4)1.0mabovethelimb?Takethegravitationalpotentialenergytobezeroaty=0.Ifthesystemisisolatedandonlyconservativeforcesdowork,thenwegotprincipleofconservationofmechanicalenergy:

Mechanicalenergy8-4ConservationofMechanicalEnergyPrincipleofconservationofmechanicalenergyExample:pendulumswing,andthesystemiscomposedbypendulumandtheEarthSampleProblemProblem:achildofmassmisreleasedfromrestatthetopofawaterslide,atheighth=8.5mabovethebottomoftheslide.Assumingthatslideisfrictionless,findthechild’sspeedatthebottomoftheslide.Solution:Findingtheforceanalytically

8-5ReadingaPotentialEnergyCurveNeutralequilibriumpointEquilibriumpointsUnstableequilibriumpointStableequilibriumpointWorkisenergytransferredtoorfromasystembyexternalforceSingleparticle:work-kineticenergytheoremSystem(nofriction)energystatement

8-6WorkDoneonaSystembyanExternalForceSystem(withfriction)energystatementEth:thermalenergySampleProblemThesystem:crate-floor

Problem:afoodshipperpushesawoodcrateofcabbageheads(totalmassm=14kg)acrossaconcretefloorwithconstanthorizontalforceFofmagnitude40N.Inastraight-linedisplacementofmagnituded=0.50m,thespeedofthecratedecreasesfromv0v=0.20m/s.(a)HowmuchworkisdonebyforceF,andonwhatsystemdoesitdothework?(b)WhatistheincreaseΔEthinthethermalenergyofthecrateandfloor?Solution:(a)(b)8-7ConservationofEnergy

Lawofconservationofenergy:Thetotalenergyofasystemcanchangeonlybyamountsofenergythataretransferredtoorfromthesystem

Totalenergyofanisolatedsystemcannotchange

Powerisrateatwhichenergyistransferredbyaforcefromoneformtoanother

ΔEint:changeofinternalenergyIsolatedsystem:package-spring-floor-wall

SampleProblemProblem:a2.0kgpackageoftamalesslidesalongafloorwithspeed4.0m/s.Itthenrunsintoandcompressesaspring,untilthepackagemomentarilystops.Itspathtotheinitiallyrelaxedspringisfrictionless,butasitcompressesthespring,akineticf