物理课件动能定理(英文版)(可编辑).ppt

ENERGY KE PE Teacher swallowGao Review Work Force DisplacementorW F SMaker gaoyan KE1 definition thisisenergywhichsomethinghasbecauseitismoving 2 formula KE mv2 deriving ablockofmassmisacceleratedfromvelocityv1tov2byaresultantforceF whilegainingthisvelocity itsdisplacementissandaccelerationisa seefigbelow Fig 1 w FS masMaker gaoyanwhilev12 v22 2assow KE mv22 mv12 1 mv2 unit joule J 4 note w FS workdonebytheresultantforce notacomponent5 Exmple ahorizontalforceof50Nisappliedtoacart Thefrictionbetweenthecartandtheflooris20N ifthecarthastravelled3m seethediagrambelow calculate a TheworkdonebytheappliedforceF b Theworkdonebythefrictionalforcef c ThegaininKEofthecart ConsiderthebodydiagramTaketherightaspositiveGiven F 50Nf 20Ns 3mFind WF Wf KE Applyw F SWF 50N 3m 150JWf 20N 3m 60Jnegativeworkmeansthatenergyislostinovercomingfriction c Applyw KE WF Wf 150J 60J 90J PE1 definition thisisenergywhichsomethinghasbecauseofitsposition shapeorstate2 formula PE mgh3 unit joule J 4 note PE mg h mgh1 mgh2 PEisindependentofthepath justdependsontheh1 h2 conclusion 1 A5kgobjectisinitiallytravelingaloneafrictionlesssurfaceatavelocityof2m s 1 itispushedbya3nforceinthedirectionofmotionforadistanceof4m What stheKEoftheprojectafterthepush 2 A2Kgmassmovesacrossaroughsurfacewithafrictionalforceof2Nat4m s 1 a TheinitialKEofthemass Fig 4 Workdonebyfrictionontheblocktobringittorest Thefrictiondoesnegativework Itactsinthe same opposite directiontothedirectionofmotionandtendsto accelerate decelerate theblock b 3 Inaconstructionsite ablockwithamassof50kgisraisedslowlyupwardbyacranethoughaverticaldistanceof3m a Whatistheworkdonebythecrane b whatisthegaininPEoftheblock 4 A2kgmassispulledupbyastringthatpassesoverasmoothlightpulleyasshownopposite ifthehandmovest