高一物理课件：牛顿第二定律(英文版)课件

Newton'sfirstlawNewton'sfirstlaw1Oneday,StephenSuwasdrivingamotorcycleonthestreet.Meanwhile,DavidSunwasdrivingatrunkatafastspeed.StephenSudidnotstopintimeassoonasthetrafficlightturnedred.DavidsuncouldnotcontrolthevanandcollidedwithStephen.Theremainingquestionis,eventhoughStephenslammedonthebrake,theaccidenthappenedanyway.Whatcausedthistragedy??casestudyOneday,StephenSuwasdrivin2tendencyresistinertiamotionabsencewordlisttendencywordlist3AccordingtoNewton'sfirstlaw,anobjectinmotioncontinuesinmotionwiththesamespeedandinthesamedirectionunlessacteduponbyanunbalancedforce.inertiaAccordingtoNewton'sfirstla4DefinitionNewton’sfirstlawofmotion牛顿第一定律:EverybodycontinuesinitsstateofuniformmotionorrestunlessacteduponbyanexternalforceDefinitionNewton’sfirstlawo5ApplicationoftheNewton’sfirstlawSafetybeltsAnypassengersinthecarwillalsobedecceleratedtorestiftheyarestrappedtothecarbyseatbelts.Beingstrappedtightlytothecar,thepassengerssharethesamestateofmotionasthecar.Asthecaraccelerates,thepassengersacceleratewithit;asthecardecelerates,thepassengersdeceleratewithit;andasthecarmaintainsaconstantspeed,thepassengersmaintainaconstantspeedaswell.ApplicationoftheNewton’sfi6practiceIftwoobjects,auniformdiskandauniformsphere,havethesamemomentofinertiaabouttheiraxesofrotationandthesameangularvelocity,thenthediskhasthelargerrotationalkineticenergy.(T)

True

(F)

FalsepracticeIftwoobjects,aunif7Iftwoendsofaropearepulledwithforcesofequalmagnitudeandoppositedirection,thetensionatthecenteroftheropemustbezeroTrue

False

Iftwoendsofaropearepull8AdiskandahoopofthesamemassMandradiusRrollwithoutslippingacrossahorizontalfloor.Boththediskandthehooparemovingwithvelocityvwhenthefloorstartstoslopeupward.Whichstatementiscorrect?(B)(a)

Thediscmakesittoagreaterheightthanthehoop.

(b)

Thehoopmakesittoagreaterheightthanthedisk.

(c)

Boththediskandthehoopmakeittothesameheight.

Adiskandahoopofthesame9Newton'sSecondLaw

Newton'sSecondLaw10

Newton'sSecondLawofmotionpertainstothebehaviorofobjectsforwhichallexistingforcesarenotbalanced.Thesecondlawstatesthattheaccelerationofanobjectisdependentupontwovariables-thenetforceactingupontheobjectandthemassoftheobject.Theaccelerationofanobjectdependsdirectlyuponthenetforceactingupontheobject,andinverselyuponthemassoftheobject.Newton'sSecondLawofmot11Newton'sSecondLawtellsusthatrateofincrease(ordecrease)inthespeedofsomethingwhichismovingisproportionaltotheforceactingonit.

Imaginethatyouareridingabicyclealongaperfectlysmoothandlevelroadandyoudecidetostoppedaling.Ifthereisastrongwindpushingagainstyou,youwillstopcompletelyinashortertimethanyouwouldifthewindwaslight.Newton'sSecondLawtells12Applications

NowthatweknowNewton'sLawsofMotion,howdoweapplythem?Howcantheyletuspredictthemotionofanobjectifweknowalltheforcesactinguponit?Howcantheyletuspredicttheforcesonanobjectifweknowitsmotion?FORCE=MASStimesACCELERATIONorApplicationsNowthatwek13Kobe'scar,whichweighs1,000kg,isoutofgas.Kobeistryingtopushthecartoagasstation,andhemakesthecargo0.05m/s2.UsingNewton'sSecondLaw,youcancomputehowmuchforceKobeisapplyingtothecar.Answer=50newtonsKobe'scar,whichweighs1,00014Example1:Consideracratebeingpulledalongahorizontal,frictionlessfloor.AropeistiedarounditandamanpullsontheropewithaforceofT.Tisthetensionintherope.Whathappenstothecrate?

Example1:15Theseforcesareshownonthe"freebodydiagram"above.Wehavedrawninalltheforcesactingontheobject.Thenetforceisthevectorsumoftheseforces.Fnet=F=T+n+wFnet,x=F

x=Tx+nx+

wxFnet,y=Fy=Ty+

ny+

wyn=wTheseforcesareshownonthe16Example2:

Thisparticularexampleisstatedintermsofweighingafishinanacceleratingelevator.Itisalsofuntothinkofweighingyourselfinanacceleratingelevator.Whendoesanelevatoraccelerateupwards?Whendoesanelevatoracceleratedownwards?

Example2:17Theforcesactingonthefishareshowninthefree-bodydiagram.Tisthetensionsuppliedbythescale.Thisisthevaluethescalereads.Wemaycallittheapparentweightofthefish.ThenetforceonthefishisFnet=T-mgThenetforceis(always!)equaltothemasstimestheacceleration.Thisfishismovingalongwiththeelevator.Inthisdiagramwehavetakentheaccelerationtobeupsoitispositive.Fnet=T-mg=maT=mg+maT=m(g+a)Whiletheelevatoracceleratesupward,theapparentweightofthefishisgreaterthanitstrueweight,mg.

Theforcesactingonthefish18Theforcesonthefishareagainshowninthefree-bodydiagramFnet=T-mgThenetforceis(always!)equaltothemasstimestheacceleration.Thisfishismovingalongwiththeelevator.Nowtheaccelerationtobeupsoitisnegative.Fnet=T-mg=m(-a)T=mg-maT=m(g-a)Whiletheelevatoracceleratesdownward,theapparentweightofthefishislessthanitstrueweight,mg.Theforcesonthefishareaga191.WhowasthescientistwhogaveustheLawsofMotion?

Answer:SirIsaacNewton

2.HowmanyLawsofMotionarethere?

Answer:three

3.Whatisanothernameforthefirstlawofmotion?

Answer:LawofInertia

4.Whichlawexplainswhyweneedtowearseatbelts?Answer:FirstLawofMotion1.Whowasthescientistwhog205.Whichlawsaysthatforceisequaltomasstimes

acceleration(F=MA)?

Answer:SecondLawofMotion

6.Whichlawsaysthatheavierobjectsrequiremoreforcethanlighterobjectstomoveoracceleratethem?Answer:SecondLawofMotion

7.Whichlawexplainshowrocketsarelaunchedintospace?Answer:ThirdLawofMotion

8.Whichlawsaysthatforeveryactionthereisanequaland

oppositereaction?

Answer:ThirdLawofMotion

5.Whichlawsaysthatforcei21TNTisthebestgroup!Ilikeit!Yes,Iagree!ENDTNTisthebestgroup!Yes,Iag22Newton'sfirstlawNewton'sfirstlaw23Oneday,StephenSuwasdrivingamotorcycleonthestreet.Meanwhile,DavidSunwasdrivingatrunkatafastspeed.StephenSudidnotstopintimeassoonasthetrafficlightturnedred.DavidsuncouldnotcontrolthevanandcollidedwithStephen.Theremainingquestionis,eventhoughStephenslammedonthebrake,theaccidenthappenedanyway.Whatcausedthistragedy??casestudyOneday,StephenSuwasdrivin24tendencyresistinertiamotionabsencewordlisttendencywordlist25AccordingtoNewton'sfirstlaw,anobjectinmotioncontinuesinmotionwiththesamespeedandinthesamedirectionunlessacteduponbyanunbalancedforce.inertiaAccordingtoNewton'sfirstla26DefinitionNewton’sfirstlawofmotion牛顿第一定律:EverybodycontinuesinitsstateofuniformmotionorrestunlessacteduponbyanexternalforceDefinitionNewton’sfirstlawo27ApplicationoftheNewton’sfirstlawSafetybeltsAnypassengersinthecarwillalsobedecceleratedtorestiftheyarestrappedtothecarbyseatbelts.Beingstrappedtightlytothecar,thepassengerssharethesamestateofmotionasthecar.Asthecaraccelerates,thepassengersacceleratewithit;asthecardecelerates,thepassengersdeceleratewithit;andasthecarmaintainsaconstantspeed,thepassengersmaintainaconstantspeedaswell.ApplicationoftheNewton’sfi28practiceIftwoobjects,auniformdiskandauniformsphere,havethesamemomentofinertiaabouttheiraxesofrotationandthesameangularvelocity,thenthediskhasthelargerrotationalkineticenergy.(T)

True

(F)

FalsepracticeIftwoobjects,aunif29Iftwoendsofaropearepulledwithforcesofequalmagnitudeandoppositedirection,thetensionatthecenteroftheropemustbezeroTrue

False

Iftwoendsofaropearepull30AdiskandahoopofthesamemassMandradiusRrollwithoutslippingacrossahorizontalfloor.Boththediskandthehooparemovingwithvelocityvwhenthefloorstartstoslopeupward.Whichstatementiscorrect?(B)(a)

Thediscmakesittoagreaterheightthanthehoop.

(b)

Thehoopmakesittoagreaterheightthanthedisk.

(c)

Boththediskandthehoopmakeittothesameheight.

Adiskandahoopofthesame31Newton'sSecondLaw

Newton'sSecondLaw32

Newton'sSecondLawofmotionpertainstothebehaviorofobjectsforwhichallexistingforcesarenotbalanced.Thesecondlawstatesthattheaccelerationofanobjectisdependentupontwovariables-thenetforceactingupontheobjectandthemassoftheobject.Theaccelerationofanobjectdependsdirectlyuponthenetforceactingupontheobject,andinverselyuponthemassoftheobject.Newton'sSecondLawofmot33Newton'sSecondLawtellsusthatrateofincrease(ordecrease)inthespeedofsomethingwhichismovingisproportionaltotheforceactingonit.

Imaginethatyouareridingabicyclealongaperfectlysmoothandlevelroadandyoudecidetostoppedaling.Ifthereisastrongwindpushingagainstyou,youwillstopcompletelyinashortertimethanyouwouldifthewindwaslight.Newton'sSecondLawtells34Applications

NowthatweknowNewton'sLawsofMotion,howdoweapplythem?Howcantheyletuspredictthemotionofanobjectifweknowalltheforcesactinguponit?Howcantheyletuspredicttheforcesonanobjectifweknowitsmotion?FORCE=MASStimesACCELERATIONorApplicationsNowthatwek35Kobe'scar,whichweighs1,000kg,isoutofgas.Kobeistryingtopushthecartoagasstation,andhemakesthecargo0.05m/s2.UsingNewton'sSecondLaw,youcancomputehowmuchforceKobeisapplyingtothecar.Answer=50newtonsKobe'scar,whichweighs1,00036Example1:Consideracratebeingpulledalongahorizontal,frictionlessfloor.AropeistiedarounditandamanpullsontheropewithaforceofT.Tisthetensionintherope.Whathappenstothecrate?

Example1:37Theseforcesareshownonthe"freebodydiagram"above.Wehavedrawninalltheforcesactingontheobject.Thenetforceisthevectorsumoftheseforces.Fnet=F=T+n+wFnet,x=F

x=Tx+nx+

wxFnet,y=Fy=Ty+

ny+

wyn=wTheseforcesareshownonthe38Example2:

Thisparticularexampleisstatedintermsofweighingafishinanacceleratingelevator.Itisalsofuntothinkofweighingyourselfinanacceleratingelevator.Whendoesanelevatoraccelerateupwards?Whendoesanelevatoracceleratedownwards?

Example2:39Theforcesactingonthefishareshowninthefree-bodydiagram.Tisthetensionsuppliedbythescale.Thisisthevaluethescalereads.Wemaycallittheapparentweightofthefish.ThenetforceonthefishisFnet=T-mgThenetforceis(always!)equaltothemasstimestheacceleration.Thisfishismovingalongwiththeelevator.Inthisdiagramwehavetakentheaccelerationtobeupsoitispositive.Fnet=T-mg=maT=mg+maT=m(g+a)Whiletheelevatoracceleratesupward,theapparentweightofthefishisgreaterthanitstrueweight,mg.

Theforcesactingonthefish40Theforcesonthefishareagainsh