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1、Chapter8-Friction8.1CharacteristicsofdryfrictionFriction-aresistiveforcethatpreventsorretardstheslippingofonebodywithrespecttoanother.2typesoffriction1).Dry(coulomb)-non-lubricated2).fluidCoefficientsoffrictionStaticfrictionforce-theforcethatkeepstheblock
2、frommoving.Asincreases,increasesuntilitreachesIfincreasesmore,thefrictioncannolongerbalanceitandtheblockbeginssliding.Whentheblockbeginsslidingthemagnitudeofdropstothekineticfrictionforce.ExperimentalevidenceshowsandAngleoffrictionAlso:Thus:Angleofrepose8
3、.2Problemsinvolvingdryfriction4typesoffrictionproblems1).Equilibrium(numberofunknowns=#ofequationsofequilibrium)Noevidencethathasreacheditsmaximumvalue,thusF=CANNOTbeused.2).Impendingmotionatallpoints(#ofunks=#ofEOE+#offrictionequations)Frictionforceisino
4、ppositedirectionofimpendingmotion.3).Tippingorimpendingmotionatsomepoints(#unks<#E.O.E.+#F.E.)·Sliding:mustdocomputationsforallthedifferentsituations-assumecondition1,solve-assumecondition2,solveetc.·Tipping:-blockwillslip-blockwilltip4).BodyisslidingExam
5、ples1).Given:Asupportblockisacteduponby2forcesasshown.and.Find:DeterminetheforcePrequiredto:a).Starttheblockmovinguptheinclineb).Keepitmovingupc).Preventitfromslidingdowna).FBDUsebecausemotionisimpending.b).Sameasa).onlyuse=0.25P=648.7Nc).FBD2).Given:betw
6、eenallsurfaces.ABC100kgD150kgPFind:DeterminethesmallestforcePrequiredtostartblockDmoving.FBDBlockCWc=981NyxTABFBDBlockDP3).Given:Three4kgpackagesA,B,andCareplacedonaconveyorbeltwhichisatrest.BetweenthebeltandbothpackagesAandC,.BetweenpackageBandthebelt,.T
7、hepackagesareplacedonthebeltsothattheyareincontactwitheachotherandatrest.Find:Determinewhich,ifany,ofthepackageswillmoveandthefrictionforceactingoneachpackage.ConsiderCbyitself-assumeCisinequilibrium.Thus,packageCdoesnotmove.ConsiderBbyitself-assumeBisine
8、quilibriumThus,packageBmovesConsiderAandBtogether-assumeAandBareinequilibriumPackageABothAandB,thus,packagesAandBmove
