高一物理课件：牛顿第二定律(英文版).ppt

Oneday,StephenSuwasdrivingamotorcycleonthestreet.Meanwhile,DavidSunwasdrivingatrunkatafastspeed.StephenSudidnotstopintimeassoonasthetrafficlightturnedred.DavidsuncouldnotcontrolthevanandcollidedwithStephen.Theremainingquestionis,eventhoughStephenslammedonthebrake,theaccidenthappenedanyway.Whatcausedthistragedy?tendencyresistinertiamotionabsenceAccordingtoNewtonsfirstlaw,anobjectinmotioncontinuesinmotionwiththesamespeedandinthesamedirectionunlessacteduponbyanunbalancedforce.Newtons first law of motion牛顿第一定律:Every body continues in its state of uniform motion or rest unless acted up on by an external forceApplicationoftheNewtonsfirstlawSafetybeltsAnypassengersinthecarwillalsobedecceleratedtorestiftheyarestrappedtothecarbyseatbelts.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.NewtonsSecondLawofmotionpertainstotheNewtonsSecondLawofmotionpertainstothebehaviorofobjectsforwhichallexistingforcesbehaviorofobjectsforwhichallexistingforcesarenotbalanced.Thesecondlawstatesthatthearenotbalanced.Thesecondlawstatesthattheaccelerationofanobjectisdependentupontwoaccelerationofanobjectisdependentupontwovariables-thenetforceactingupontheobjectvariables-thenetforceactingupontheobjectandthemassoftheobject.Theaccelerationofanandthemassoftheobject.Theaccelerationofanobjectdependsdirectlyuponthenetforceactingobjectdependsdirectlyuponthenetforceactingupontheobject,andinverselyuponthemassofupontheobject,andinverselyuponthemassoftheobject.theobject.Newtons Second Law tells us that rate of increase(or decrease)in the speed of something which is moving is proportional to the force acting on it.Imagine that you are riding a bicycle along a perfectly smooth and level road and you decide to stop pedaling.If there is a strong wind pushing against you,you will stop completely in a shorter time than you would if the wind was light.Now that we know Newtons Laws of Motion,how do we apply them?How can they let us predict the motion of an object if we know all the forces acting upon it?How can they let us predict the forces on an object if we know its motion?FORCE=MASStimesACCELERATIONorKobescar,whichweighs1,000kg,isoutofgas.Kobeistryingtopushthecartoagasstation,andhemakesthecargo0.05m/s2.UsingNewtonsSecondLaw,youcancomputehowmuchforceKobeisapplyingtothecar.Answer=50newtonsExample1:Consideracratebeingpulledalongahorizontal,frictionlessfloor.AropeistiedarounditandamanpullsontheropewithaforceofT.Tisthetensionintherope.Whathappenstothecrate?Theseforcesareshownonthefreebodydiagramabove.Wehavedrawninalltheforcesactingontheobject.Thenetforceisthevectorsumoftheseforces.Fnet=F=T+n+wFnet,x=F x=Tx+nx+wxFnet,y=Fy=Ty+ny+wyn=wExample2:Thisparticularexampleisstatedintermsofweighingafishinanacceleratingelevator.Itisalsofuntothinkofweighingyourselfinanacceleratingelevator.Whendoesanelevatoraccelerateupwards?Whendoesanelevatoracceleratedownwards?Theforcesactingonthefishareshowninthefree-bodydiagram.Tisthetensionsuppliedbythescale.Thisisthevaluethescalereads.Wemaycallittheapparentweightofthefish.ThenetforceonthefishisFnet=T-mgThe net force is(always!)equal to the mass times the acceleration.This fish is moving along with the elevator.In this diagram we have taken the acceleration to be up so it is positive.Fnet=T-m g=m a T=m g+m aT=m(g+a)While the elevator accelerates upward,the apparentweight of the fish is greater than its true weight,mg.The forces on the fish are again shown in the free-body diagram Fnet=T-mgThe net force is(always!)equal to the mass times the acceleration.This fish is moving along with the elevator.Now the acceleration to be up so it is negative.Fnet=T-m g=m(-a)T=m g-m aT=m(g-a)While the elevator accelerates downward,the apparentweight of the fish is less than its true weight,mg.1.WhowasthescientistwhogaveustheLawsofMotion?Answer:SirIsaacNewton 2.HowmanyLawsofMotionarethere?Answer:three3.Whatisanothernameforthefirstlawofmotion?Answer:LawofInertia4.Whichlawexplainswhyweneedtowearseatbelts?Answer:FirstLawofMotion5.Whichlawsaysthatforceisequaltomasstimesacceleration(F=MA)?Answer:SecondLawofMotion6.Whichlawsaysthatheavierobjectsrequiremoreforcethanlighterobjectstomoveoracceleratethem?Answer:SecondLawofMotion7.Whichlawexplainshowrocketsarelaunchedintospace?Answer:ThirdLawofMotion8.Whichlawsaysthatforeveryactionthereisanequalandoppositereaction?Answer:ThirdLawofMotionTNT is the best group!I like it!Yes,I agree!