牛顿运动定律(英文版)

Newton’sLawsofMotionReviewBackgroundSirIsaacNewton(1643-1727)anEnglishscientistandmathematicianfamousforhisdiscoveryofthelawofgravityalsodiscoveredthethreelawsofmotion.HepublishedtheminhisbookPhilosophiaeNaturalisPrincipiaMathematica(mathematicprinciplesofnaturalphilosophy)in1687.TodaytheselawsareknownasNewton’sLawsofMotionanddescribethemotionofallobjectsonthescaleweexperienceinoureverydaylives.“IfIhaveevermadeanyvaluablediscoveries,ithasbeenowingmoretopatientattention,thantoanyothertalent.”-SirIsaacNewtonNewton’sLawsofMotion1.Anobjectinmotiontendstostayinmotionandanobjectatresttendstostayatrestunlessacteduponbyanunbalancedforce.2.Forceequalsmasstimesacceleration(F=ma).3.Foreveryactionthereisanequalandoppositereaction.Newton’sFirstLawAnobjectatresttendstostayatrestandanobjectinmotiontendstostayinmotionunlessacteduponbyanunbalancedforce.Whatdoesthismean?Basically,anobjectwill“keepdoingwhatitwasdoing”unlessactedonbyanunbalancedforce.Iftheobjectwassittingstill,itwillremainstationary.Ifitwasmovingataconstantvelocity,itwillkeepmoving.Ittakesforcetochangethemotionofanobject.Whatismeantbyunbalancedforce?Iftheforcesonanobjectareequalandopposite,theyaresaidtobebalanced,andtheobjectexperiencesnochangeinmotion.Iftheyarenotequalandopposite,thentheforcesareunbalancedandthemotionoftheobjectchanges.SomeExamplesfromRealLifeTwoteamsareplayingtugofwar.Theyarebothexertingequalforceontheropeinoppositedirections.Thisbalancedforceresultsinnochangeofmotion.Asoccerballissittingatrest.Ittakesanunbalancedforceofakicktochangeitsmotion.Newton’sFirstLawisalsocalledtheLawofInertiaInertia:thetendencyofanobjecttoresistchangesinitsstateofmotionTheFirstLawstatesthatallobjectshaveinertia.Themoremassanobjecthas,themoreinertiaithas(andtheharderitistochangeitsmotion).MoreExamplesfromRealLifeApowerfullocomotivebeginstopullalonglineofboxcarsthatweresittingatrest.Sincetheboxcarsaresomassive,theyhaveagreatdealofinertiaandittakesalargeforcetochangetheirmotion.Oncetheyaremoving,ittakesalargeforcetostopthem.Onyourwaytoschool,abugfliesintoyourwindshield.Sincethebugissosmall,ithasverylittleinertiaandexertsaverysmallforceonyourcar(sosmallthatyoudon’tevenfeelit).Ifobjectsinmotiontendtostayinmotion,whydon’tmovingobjectskeepmovingforever?Thingsdon’tkeepmovingforeverbecausethere’salmostalwaysanunbalancedforceactinguponit.Abookslidingacrossatableslowsdownandstopsbecauseoftheforceoffriction.Ifyouthrowaballupwardsitwilleventuallyslowdownandfallbecauseoftheforceofgravity.Inouterspace,awayfromgravityandanysourcesoffriction,arocketshiplaunchedwithacertainspeedanddirectionwouldkeepgoinginthatsamedirectionandatthatsamespeedforever.Newton’sSecondLawForceequalsmasstimesacceleration.F=maAcceleration:ameasurementofhowquicklyanobjectischangingspeed.WhatdoesF=mamean?Forceisdirectlyproportionaltomassandacceleration.Imagineaballofacertainmassmovingatacertainacceleration.Thisballhasacertainforce.Nowimaginewemaketheballtwiceasbig(doublethemass)butkeeptheaccelerationconstant.F=masaysthatthisnewballhastwicetheforceoftheoldball.Nowimaginetheoriginalballmovingattwicetheoriginalacceleration.F=masaysthattheballwillagainhavetwicetheforceoftheballattheoriginalacceleration.MoreaboutF=maIfyoudoublethemass,youdoubletheforce.Ifyoudoubletheacceleration,youdoubletheforce.Whatifyoudoublethemassandtheacceleration?(2m)(2a)=4FDoublingthemassandtheaccelerationquadruplestheforce.So...whatifyoudecreasethemassbyhalf?Howmuchforcewouldtheobjecthavenow?WhatdoesF=masay?F=mabasicallymeansthattheforceofanobjectcomesfromitsmassanditsacceleration.Somethingverysmall(lowmass)that’schangingspeedveryquickly(highacceleration),likeabullet,canstillhaveagreatforce.Somethingverysmallchangingspeedveryslowlywillhaveaveryweakforce.Somethingverymassive(highmass)that’schangingspeedveryslowly(lowacceleration),likeaglacier,canstillhavegreatforce.Newton’sThirdLawForeveryactionthereisanequalandoppositereaction.Whatdoesthismean?Foreveryforceactingonanobject,thereisanequalforceactingintheoppositedirection.Rightnow,gravityispullingyoudowninyourseat,butNewton’sThirdLawsaysyourseatispushingupagainstyouwithequalforce.Thisiswhyyouarenotmoving.Thereisabalancedforceactingonyou–gravitypullingdown,yourseatpushingup.Thinkaboutit...Whathappensifyouarestandingonaskateboardoraslipperyfloorandpushagainstawall?Youslideintheoppositedirection(awayfromthewall),becauseyoupushedonthewallbutthewallpushedbackonyouwithequalandoppositeforce.Whydoesithurtsomuchwhenyoustubyourtoe?Whenyourtoeexertsaforceonarock,therockexertsanequalforcebackonyourtoe.Theharderyouhityourtoeagainstit,themoreforcetherockexertsbackonyourtoe(andthemoreyourtoehurts).ReviewNewton’sFirstLaw:Objectsinmotiontendtostayinmotionandobjectsatresttendtostayatrestunlessacteduponbyanunbalancedforce.Newton’sSecondLaw:Forceequalsmasstimesacceleration(F=ma).Newton’sThirdLaw:Foreveryactionthereisanequalandoppositereaction.VocabularyInertia:thetendencyofanobjecttoresistchangesinitsstateofmotionAcceleration:•achangeinvelocity•ameasurementofhowquicklyanobjectischangingspeed,directionorbothVelo