普通物理英语课件13_KineticEnergy.pdfVIP

KineticEnergyandEnergyConservation

Physics1425Lecture13

MichaelFowler,UVa

MovingThingsHaveEnergy

•Energyistheabilitytodowork:to

deliveraforcethatactsthrougha•A

distance.

•Placingaweightgentlyonanaildoes5kg

nothing.

•Droppingtheweightonthenailcan

drivethenailintothewood.

•Iftheweightismovingwhenithits

thenail,ithastheabilitytodowork

drivingthenailin.Thisisitskinetic

energy.

HowMuchWorkDoestheMovingWeightDo?

•Aftercontactwiththenail,theforces•A

betweentheweightandthenailareequal

andopposite.Supposethenailisdrivenin

-F

atotaldistancex.

05kg

•IngoingthroughasmalldistanceΔx,the

workdoneonthenailΔW=FΔx.x0Δx

•Meanwhile,fortheweight-F=ma,theF

weighthassloweddown:-F=mΔv/Δt.

•ThereforeΔW=FΔx=-mΔvΔx/Δt.

•NowforsmallΔx,wecantakeΔx/Δt=v,Weighthitsnail

withspeedv0

soΔW=-mvΔv,andx00

WdW−mvdv1mv2

∫∫20

0v

0

WhereDidtheWeight’sEnergyComeFrom?

•We’veseenthatiftheweighthitsthenailand

comesrapidlytorest,itlosesenergy½mv02.

•ThisisitskineticenergyKatspeedv0.

•Let’ssupposeitgainedthatenergybybeing