高一物理课件：牛顿第二定律(英文版)PPT课件

Oneday,StephenSuwasdrivingamotorcycleonthestreet.Meanwhile,DavidSunwasdrivingatrunkatafastspeed.StephenSudidnotstopintimeassoonasthetrafficlightturnedred.DavidsuncouldnotcontrolthevanandcollidedwithStephen.Theremainingquestionis,eventhoughStephenslammedonthebrake,theaccidenthappenedanyway.Whatcausedthistragedy??tendencyresistinertiamotionabsenceAccordingtoNewton'sfirstlaw,anobjectinmotioncontinuesinmotionwiththesamespeedandinthesamedirectionunlessacteduponbyanunbalancedforce.Newton’sfirstlawofmotion牛顿第一定律:EverybodycontinuesinitsstateofuniformmotionorrestunlessacteduponbyanexternalforceApplicationoftheNewton’sfirstlawSafetybeltsAnypassengersinthecarwillalsobedecceleratedtorestiftheyarestrappedtothecarbyseatbelts.Beingstrappedtightlytothecar,thepassengerssharethesamestateofmotionasthecar.Asthecaraccelerates,thepassengersacceleratewithit;asthecardecelerates,thepassengersdeceleratewithit;andasthecarmaintainsaconstantspeed,thepassengersmaintainaconstantspeedaswell.Iftwoobjects,auniformdiskandauniformsphere,havethesamemomentofinertiaabouttheiraxesofrotationandthesameangularvelocity,thenthediskhasthelargerrotationalkineticenergy.(T)True(F)FalseIftwoendsofaropearepulledwithforcesofequalmagnitudeandoppositedirection,thetensionatthecenteroftheropemustbezeroTrueFalseAdiskandahoopofthesamemassMandradiusRrollwithoutslippingacrossahorizontalfloor.Boththediskandthehooparemovingwithvelocityvwhenthefloorstartstoslopeupward.Whichstatementiscorrect?(B)(a)Thediscmakesittoagreaterheightthanthehoop.(b)Thehoopmakesittoagreaterheightthanthedisk.(c)Boththediskandthehoopmakeittothesameheight.Newton'sSecondLawofmotionpertainstothebehaviorofobjectsforwhichallexistingforcesarenotbalanced.Thesecondlawstatesthattheaccelerationofanobjectisdependentupontwovariables-thenetforceactingupontheobjectandthemassoftheobject.Theaccelerationofanobjectdependsdirectlyuponthenetforceactingupontheobject,andinverselyuponthemassoftheobject.Newton'sSecondLawtellsusthatrateofincrease(ordecrease)inthespeedofsomethingwhichismovingisproportionaltotheforceactingonit.Imaginethatyouareridingabicyclealongaperfectlysmoothandlevelroadandyoudecidetostoppedaling.Ifthereisastrongwindpushingagainstyou,youwillstopcompletelyinashortertimethanyouwouldifthewindwaslight.NowthatweknowNewton'sLawsofMotion,howdoweapplythem?Howcantheyletuspredictthemotionofanobjectifweknowalltheforcesactinguponit?Howcantheyletuspredicttheforcesonanobjectifweknowitsmotion?FORCE=MASStimesACCELERATIONorKobe'scar,whichweighs1,000kg,isoutofgas.Kobeistryingtopushthecartoagasstation,andhemakesthecargo0.05m/s2.UsingNewton'sSecondLaw,youcancomputehowmuchforceKobeisapplyingtothecar.Answer=50newtonsExample1:Consideracratebeingpulledalongahorizontal,frictionlessfloor.AropeistiedarounditandamanpullsontheropewithaforceofT.Tisthetensionintherope.Whathappenstothecrate?Theseforcesareshownonthefreebodydiagramabove.Wehavedrawninalltheforcesactingontheobject.Thenetforceisthevectorsumoftheseforces.Fnet=F=T+n+wFnet,x=Fx=Tx+nx+wxFnet,y=Fy=Ty+ny+wyn=wExample2:Thisparticularexampleisstatedintermsofweighingafishinanacceleratingelevator.Itisalsofuntothinkofweighingyourselfinanacceleratingelevator.Whendoesanelevatoraccelerateupwards?Whendoesanelevatoracceleratedownwards?Theforcesactingonthefishareshowninthefree-bodydiagram.Tisthetensionsuppliedbythescale.Thisisthevaluethescalereads.Wemaycallittheapparentweightofthefish.ThenetforceonthefishisFnet=T-mgThenetforceis(always!)equaltothemasstimestheacceleration.Thisfishismovingalongwiththeelevator.Inthisdiagramwehavetakentheaccelerationtobeupsoitispositive.Fnet=T-mg=maT=mg+maT=m(g+a)Whiletheelevatoracceleratesupward,theapparentweightofthefishisgreaterthanitstrueweight,mg.Theforcesonthefishareagainshowninthefree-bodydiagramFnet=T-mgThenetforceis(always!)equaltothemasstimestheacceleration.Thisfishismovingalongwiththeelevator.Nowtheaccelerationtobeupsoitisnegative.Fnet=T-mg=m(-a)T=mg-maT=m(g-a)Whiletheelevatoracceleratesdownward,theapparentweightofthefishislessthanitstrueweight,mg.1.WhowasthescientistwhogaveustheLawsofMotion?Answer:SirIsaacNewton2.HowmanyLawsofMotionarethere?Answer:three3.Whatisanothernameforthefirstlawofmotion?Answer:LawofInertia4.Whichlawexplainswhyweneedtowearseatbelts?Answer:FirstLawofMotion5.Whichlawsaysthatforceisequaltomasstimesacceleration(F=MA)?Answer:SecondLawofMotion6.Whichlawsaysthatheavierobjectsrequiremoreforcethanlighterobjectstomoveoracceleratethem?Answer:SecondLawofMotion7.Whichlawexplainshowrocketsarelaunchedintospace?Answer:ThirdLawofMotion8.Whichlawsaysthatforeveryactionthereisanequalandoppositereaction?Answer:ThirdLawofMotionTNTisthebestgroup!Ilikeit!Yes,Iagree!