The Law of Action-Reaction (Revisited)

Collisions are governed by Newton's laws. The law of action-reaction (Newton's third law) explains the nature of the forces between the two interacting objects.   ReadWatchInteract TPCandeLearning What'sNEWatTPC? PhysicsTutorial 1-DKinematicsNewton'sLawsVectors-MotionandForcesinTwoDimensionsMomentumandItsConservationWorkandEnergyCircularMotionandSatelliteMotionThermalPhysicsStaticElectricityElectricCircuitsVibrationsandWavesSoundWavesandMusicLightandColorReflectionandMirrorsRefractionandLenses PhysicsInteractives AboutthePhysicsInteractivesTaskTrackerUsagePolicyKinematicsNewtonsLawsVectorsandProjectilesForcesin2DMomentumandCollisionsWorkandEnergyCircularandSatelliteMotionBalanceandRotationStaticElectricityElectricCircuitsMagnetismWavesandSoundLightandColorReflectionandMirrorsRefractionandLensesChemistry VideoTutorial KinematicsNewton'sLawsVectorsandProjectilesForcesinTwoDimensionsMomentumandCollisionsWork,Energy,andPowerCircularMotionandGravitationStaticElectricityElectricCircuitsLightandColorReflectionandMirrorsRefractionandLenses MultimediaStudios 1-DimensionalKinematicsNewton'sLawsVectorsandProjectilesMomentumandCollisionsWorkandEnergyCircular,Satellite,andRotationalMotionEinstein'sTheoryofSpecialRelativityStaticElectricityWaves,SoundandLightRayOpticsQuickTimeMovies PracticeReviewTest ConceptBuilders AbouttheConceptBuildersVersion2PricingForSchoolsDirectionsforVersion2MeasurementandUnitsRelationshipsandGraphsKinematicsNewton'sLawsVectorsandProjectilesForcesin2-DimensionsMomentumandCollisionsWorkandEnergyCircularandSatelliteMotionRotationandBalanceStaticElectricityElectricCircuitsWavesandSoundLightandColorReflectionandRefractionChemistry MindsOnPhysics AboutMOPsFeaturesTeacherAccountsPricingForSchoolsTaskTrackerDirectionsKinematicConceptsKinematicGraphingNewton'sLawsVectorsandProjectilesForcesin2-DimensionsMomentumandCollisionsWorkandEnergyCircularandSatelliteMotionStaticElectricityElectricCircuitsWaveMotionSoundandMusicLightandColorReflectionandMirrorsRefractionandLenses ConceptCheckers AboutConceptCheckers ForSimulations TheReviewSession 1-DKinematicsNewton'sLawsofMotionVectorsandProjectilesForcesinTwoDimensionsMomentumandCollisionsWorkandEnergyPacketCircularMotionandGravitationStaticElectricityReviewElectricCircuitsWavesSoundandMusicLightandColorReflectionandMirrorsRefractionandLenses TheCalculatorPad ProblemSetsHabitsofanEffectiveProblemSolverANotetoStudentsNotetoInstructors PhysicsHelp GraphingPracticeRecognizingForcesVectorDirectionVectorAddition ACTTestCenter AbouttheACTACTPreparationACTTipsForTeachersOtherResources Teacher-Tools CurriculumCorner SolutionsGuideSolutionsGuideDigitalDownloadUsagePolicyMotioninOneDimensionNewton'sLawsVectorsandProjectilesForcesinTwoDimensionsMomentumandCollisionsWork,EnergyandPowerCircularMotionandGravitationStaticElectricityElectricCircuitsWaveBasicsSoundandMusicLightandColorReflectionandMirrorsRefractionandLenses QuestionBank ContentsPurchasingtheCDPurchasingtheDigitalDownload NGSSCorner AbouttheNGSSCornerNGSSSearchForceandMotionDCIs-HighSchoolEnergyDCIs-HighSchoolWaveApplicationsDCIs-HighSchoolForceandMotionPEs-HighSchoolEnergyPEs-HighSchoolWaveApplicationsPEs-HighSchoolCrosscuttingConceptsThePracticesPhysicsTopicsNGSSCorner:ActivityListNGSSCorner:Infographics TeacherToolkits AbouttheToolkitsPosition-Velocity-AccelerationPosition-TimeGraphsVelocity-TimeGraphsFreeFallNewton'sFirstLawNewton'sSecondLawNewton'sThirdLawTerminalVelocityVectorsProjectileMotionForcesin2DimensionsImpulseandMomentumChangeMomentumConservationWork-EnergyFundamentalsWork-EnergyRelationshipCircularMotionRollerCoasterPhysicsUniversalGravitationSatelliteMotionChargeandChargingCoulombsLawElectricFieldsCircuitConceptsSeriesCircuitsParallelCircuitsVibrationalMotionDescribing-WavesWaveBehaviorToolkitStandingWavePatternsSoundWavesResonatingAirColumnsWaveModelofLightColorPlaneMirrorsCurvedMirrorsSnellsLawTotalInternalReflectionLenses ReasoningCenter PhilosophyCRSUsageResourceCD TheLaboratory AboutTeacherGuideUsingLabNotebooksShare ThePhotoGallery 1-DKinematicsNewton'sLawsVectors-MotionandForcesinTwoDimensionsMomentumandItsConservationWork,Energy,andPowerCircularMotionandSatelliteMotionThermalPhysicsStaticElectricityCurrentElectricityWavesSoundWavesandMusicLightWavesandColorReflectionandRayModelofLightRefractionandRayModelofLight Store MyCartSubscriptionSelectionSeatCalculator TaskTracker ClassesEditProfileSettingsTasksandCoursesSubscriptionsSubscriptionLocatorTeacherResources   ReadWatchInteract TPCandeLearning What'sNEWatTPC? PhysicsTutorial 1-DKinematics Newton'sLaws Vectors-MotionandForcesinTwoDimensions MomentumandItsConservation WorkandEnergy CircularMotionandSatelliteMotion ThermalPhysics StaticElectricity ElectricCircuits VibrationsandWaves SoundWavesandMusic LightandColor ReflectionandMirrors RefractionandLenses PhysicsInteractives AboutthePhysicsInteractives TaskTracker UsagePolicy Kinematics NewtonsLaws VectorsandProjectiles Forcesin2D MomentumandCollisions WorkandEnergy CircularandSatelliteMotion BalanceandRotation StaticElectricity ElectricCircuits Magnetism WavesandSound LightandColor ReflectionandMirrors RefractionandLenses Chemistry VideoTutorial Kinematics Newton'sLaws VectorsandProjectiles ForcesinTwoDimensions MomentumandCollisions Work,Energy,andPower CircularMotionandGravitation StaticElectricity ElectricCircuits LightandColor ReflectionandMirrors RefractionandLenses MultimediaStudios 1-DimensionalKinematics Newton'sLaws VectorsandProjectiles MomentumandCollisions WorkandEnergy Circular,Satellite,andRotationalMotion Einstein'sTheoryofSpecialRelativity StaticElectricity Waves,SoundandLight RayOptics QuickTimeMovies PracticeReviewTest ConceptBuilders AbouttheConceptBuilders Version2 PricingForSchools DirectionsforVersion2 MeasurementandUnits RelationshipsandGraphs Kinematics Newton'sLaws VectorsandProjectiles Forcesin2-Dimensions MomentumandCollisions WorkandEnergy CircularandSatelliteMotion RotationandBalance StaticElectricity ElectricCircuits WavesandSound LightandColor ReflectionandRefraction Chemistry MindsOnPhysics AboutMOPs Features TeacherAccounts PricingForSchools TaskTrackerDirections KinematicConcepts KinematicGraphing Newton'sLaws VectorsandProjectiles Forcesin2-Dimensions MomentumandCollisions WorkandEnergy CircularandSatelliteMotion StaticElectricity ElectricCircuits WaveMotion SoundandMusic LightandColor ReflectionandMirrors RefractionandLenses ConceptCheckers AboutConceptCheckers ForSimulations TheReviewSession 1-DKinematics Newton'sLawsofMotion VectorsandProjectiles ForcesinTwoDimensions MomentumandCollisions WorkandEnergyPacket CircularMotionandGravitation StaticElectricityReview ElectricCircuits Waves SoundandMusic LightandColor ReflectionandMirrors RefractionandLenses TheCalculatorPad ProblemSets HabitsofanEffectiveProblemSolver ANotetoStudents NotetoInstructors PhysicsHelp GraphingPractice RecognizingForces VectorDirection VectorAddition ACTTestCenter AbouttheACT ACTPreparation ACTTips ForTeachers OtherResources Teacher-Tools CurriculumCorner SolutionsGuide SolutionsGuideDigitalDownload UsagePolicy MotioninOneDimension Newton'sLaws VectorsandProjectiles ForcesinTwoDimensions MomentumandCollisions Work,EnergyandPower CircularMotionandGravitation StaticElectricity ElectricCircuits WaveBasics SoundandMusic LightandColor ReflectionandMirrors RefractionandLenses QuestionBank Contents PurchasingtheCD PurchasingtheDigitalDownload NGSSCorner AbouttheNGSSCorner NGSSSearch ForceandMotionDCIs-HighSchool EnergyDCIs-HighSchool WaveApplicationsDCIs-HighSchool ForceandMotionPEs-HighSchool EnergyPEs-HighSchool WaveApplicationsPEs-HighSchool CrosscuttingConcepts ThePractices PhysicsTopics NGSSCorner:ActivityList NGSSCorner:Infographics TeacherToolkits AbouttheToolkits Position-Velocity-Acceleration Position-TimeGraphs Velocity-TimeGraphs FreeFall Newton'sFirstLaw Newton'sSecondLaw Newton'sThirdLaw TerminalVelocity Vectors ProjectileMotion Forcesin2Dimensions ImpulseandMomentumChange MomentumConservation Work-EnergyFundamentals Work-EnergyRelationship CircularMotion RollerCoasterPhysics UniversalGravitation SatelliteMotion ChargeandCharging CoulombsLaw ElectricFields CircuitConcepts SeriesCircuits ParallelCircuits VibrationalMotion Describing-Waves WaveBehaviorToolkit StandingWavePatterns SoundWaves ResonatingAirColumns WaveModelofLight Color PlaneMirrors CurvedMirrors SnellsLaw TotalInternalReflection Lenses ReasoningCenter Philosophy CRS Usage ResourceCD TheLaboratory About TeacherGuide UsingLabNotebooks Share ThePhotoGallery 1-DKinematics Newton'sLaws Vectors-MotionandForcesinTwoDimensions MomentumandItsConservation Work,Energy,andPower CircularMotionandSatelliteMotion ThermalPhysics StaticElectricity CurrentElectricity Waves SoundWavesandMusic LightWavesandColor ReflectionandRayModelofLight RefractionandRayModelofLight Store MyCart SubscriptionSelection SeatCalculator TaskTracker Classes EditProfileSettings TasksandCourses Subscriptions SubscriptionLocator TeacherResources  StudentExtras  Teacher'sGuides ThePhysicsClassroom» PhysicsTutorial»MomentumandItsConservation»TheLawofAction-Reaction(Revisited) MomentumandItsConservation-Lesson2-TheLawofMomentumConservation TheLawofAction-Reaction(Revisited) TheLawofAction-Reaction(Revisited) MomentumConservationPrinciple IsolatedSystemsandCollisionAnalysis CollisionAnalysisandMomentumProblems UsingEquationsasGuidestoThinking MomentumConservationinExplosions Acollisionisaninteractionbetweentwoobjectsthathavemadecontact(usually)witheachother.Asinanyinteraction,acollisionresultsinaforcebeingappliedtothetwocollidingobjects.Newton'slawsofmotiongovernsuchcollisions.InthesecondunitofThePhysicsClassroom,Newton'sthirdlawofmotionwasintroducedanddiscussed.Itwassaidthat... ...ineveryinteraction,thereisapairofforcesactingonthetwointeractingobjects.Thesizeoftheforceonthefirstobjectequalsthesizeoftheforceonthesecondobject.Thedirectionoftheforceonthefirstobjectisoppositetothedirectionoftheforceonthesecondobject.Forcesalwayscomeinpairs-equalandoppositeaction-reactionforcepairs.   Newton'sLawsAppliedtoCollisions Newton'sthirdlawofmotionisnaturallyappliedtocollisionsbetweentwoobjects.Inacollisionbetweentwoobjects,bothobjectsexperienceforcesthatareequalinmagnitudeandoppositeindirection.Suchforcesoftencauseoneobjecttospeedup(gainmomentum)andtheotherobjecttoslowdown(losemomentum).AccordingtoNewton'sthirdlaw,theforcesonthetwoobjectsareequalinmagnitude.Whiletheforcesareequalinmagnitudeandoppositeindirection,theaccelerationsoftheobjectsarenotnecessarilyequalinmagnitude.InaccordwithNewton'ssecondlawofmotion,theaccelerationofanobjectisdependentuponbothforceandmass.Thus,ifthecollidingobjectshaveunequalmass,theywillhaveunequalaccelerationsasaresultofthecontactforcethatresultsduringthecollision. Considerthecollisionbetweentheclubheadandthegolfballinthesportofgolf.Whentheclubheadofamovinggolfclubcollideswithagolfballatrestuponatee,theforceexperiencedbytheclubheadisequaltotheforceexperiencedbythegolfball.Mostobserversofthiscollisionhavedifficultywiththisconceptbecausetheyperceivethehighspeedgiventotheballastheresultofthecollision.Theyarenotobservingunequalforcesupontheballandclubhead,butratherunequalaccelerations.Bothclubheadandballexperienceequalforces,yettheballexperiencesagreateraccelerationduetoitssmallermass.Inacollision,thereisaforceonbothobjectsthatcausesanaccelerationofbothobjects.Theforcesareequalinmagnitudeandoppositeindirection,yettheleastmassiveobjectreceivesthegreatestacceleration. Considerthecollisionbetweenamovingsevenballandaneightballthatisatrestinthesportoftablepool.Whenthesevenballcollideswiththeeightball,eachballexperiencesanequalforcedirectedinoppositedirections.Therightwardmovingsevenballexperiencesaleftwardforcethatcausesittoslowdown;theeightballexperiencesarightwardforcethatcausesittospeedup.Sincethetwoballshaveequalmasses,theywillalsoexperienceequalaccelerations.Inacollision,thereisaforceonbothobjectsthatcausesanaccelerationofbothobjects;theforcesareequalinmagnitudeandoppositeindirection.Forcollisionsbetweenequal-massobjects,eachobjectexperiencesthesameacceleration.     Considertheinteractionbetweenamaleandfemalefigureskaterinpairfigureskating.Awoman(m=45kg)iskneelingontheshouldersofaman(m=70kg);thepairismovingalongtheiceat1.5m/s.Themangracefullytossesthewomanforwardthroughtheairandontotheice.Thewomanreceivestheforwardforceandthemanreceivesabackwardforce.Theforceonthemanisequalinmagnitudeandoppositeindirectiontotheforceonthewoman.Yettheaccelerationofthewomanisgreaterthantheaccelerationofthemanduetothesmallermassofthewoman. Manyobserversofthisinteractionhavedifficultybelievingthatthemanexperiencedabackwardforce."Afterall,"theymightargue,"themandidnotmovebackward."Suchobserversarepresumingthatforcescausemotion.Intheirminds,abackwardforceonthemaleskaterwouldcauseabackwardmotion.ThisisacommonmisconceptionthathasbeenaddressedelsewhereinThePhysicsClassroom.Forcescauseacceleration,notmotion.Themalefigureskaterexperiencesabackwardsforcethatcauseshisbackwardsacceleration.Themaleskaterslowsdownwhilethewomanskaterspeedsup.Ineveryinteraction(withnoexception),thereareforcesactinguponthetwointeractingobjectsthatareequalinmagnitudeandoppositeindirection. CollisionsaregovernedbyNewton'slaws.Thelawofaction-reaction(Newton'sthirdlaw)explainsthenatureoftheforcesbetweenthetwointeractingobjects.Accordingtothelaw,theforceexertedbyobject1uponobject2isequalinmagnitudeandoppositeindirectiontotheforceexertedbyobject2uponobject1.       CheckYourUnderstanding ExpressyourunderstandingofNewton'sthirdlawbyansweringthefollowingquestions.Clickthebuttontocheckyouranswers. 1.Whiledrivingdowntheroad,afireflystrikesthewindshieldofabusandmakesaquiteobviousmessinfrontofthefaceofthedriver.ThisisaclearcaseofNewton'sthirdlawofmotion.Thefireflyhitthebusandthebushitsthefirefly.Whichofthetwoforcesisgreater:theforceonthefireflyortheforceonthebus? SeeAnswer   TrickQuestion!Eachforceisthesamesize.Foreveryaction,thereisanequal...(equal!).Thefactthatthefireflysplattersonlymeansthatwithitssmallermass,itislessabletowithstandthelargeraccelerationresultingfromtheinteraction.Besides,fireflieshavegutsandbuggutshaveatendencytobesplatterable.Windshieldsdon'thaveguts.Thereyouhaveit.       2.Foryears,spacetravelwasbelievedtobeimpossiblebecausetherewasnothingthatrocketscouldpushoffofinspaceinordertoprovidethepropulsionnecessarytoaccelerate.Thisinabilityofarockettoprovidepropulsioninspaceisbecause... a.spaceisvoidofairsotherocketshavenothingtopushoffof. b.gravityisabsentinspace. c.spaceisvoidofairandsothereisnoairresistanceinspace. d....nonsense!Rocketsdoaccelerateinspaceandhavebeenabletodosoforalongtime. SeeAnswer   Answer:D Itisacommonmisconceptionthatrocketsareunabletoaccelerateinspace.Thefactisthatrocketsdoaccelerate.Rocketsareabletoaccelerateduetothefactthattheyburnfuelandthrusttheexhaustgasesinadirectionoppositethedirectionwhichtheywishtoaccelerate.       3.Manypeoplearefamiliarwiththefactthatariflerecoilswhenfired.Thisrecoilistheresultofaction-reactionforcepairs.Agunpowderexplosioncreateshotgasesthatexpandoutwardallowingtherifletopushforwardonthebullet.ConsistentwithNewton'sthirdlawofmotion,thebulletpushesbackwardsupontherifle.Theaccelerationoftherecoilingrifleis... a.greaterthantheaccelerationofthebullet. b.smallerthantheaccelerationofthebullet. c.thesamesizeastheaccelerationofthebullet. SeeAnswer   Answer:B Theforceontherifleequalstheforceonthebullet.Yet,accelerationdependsonbothforceandmass.Thebullethasagreateraccelerationduetothefactthatithasasmallermass.Remember:accelerationandmassareinverselyproportional.         4.KentSwimm,whoistakingPhysicsforthethirdyearinarow(andnotbecausehelikesit),hasrowedhisboatwithinthreefeetofthedock.Kentdecidestojumpontothedockandturnaroundanddockhisboat.ExplaintoKentwhythisdockingstrategyisnotagoodstrategy. SeeAnswer   Don'tdothisathome(atleast,notifyouwishtodocktheboat)!AsKentjumpstoreachthedock,therowboatpushesKentforwardandthusKentpushestherowboatbackwards.Kentwillindeedreachthedock;butKent'srowboatwillbeseveralfeetawaywhenheturnsaroundtodockit.ThatmakesitverydifficultforKenttodocktheboat.       5.Aclownisontheicerinkwithalargemedicineball.Iftheclownthrowstheballforward,thenheissetintobackwardsmotionwiththesamemomentumastheball'sforwardmomentum.Whatwouldhappentotheclownifhegoesthroughthemotionofthrowingtheballwithoutactuallylettinggoofit?Explain.     SeeAnswer   Withoutactuallylettinggooftheball,theclownwillnotbedisplacedfromhisoriginalposition.Hemayslidebackwardsalittleandthenforwardsalittleashegoesthroughthemotionofthethrowingtheball.Butwhenheisdonedoingthemotion,hewillbeatrestinhisoriginalposition.       6.Dale,Mel,andShanelle areastronautsonaspaceship.Theyeachhavethesamemassandthesamestrength.DaleandMel decidetoplaycatchwithShanelle,intendingtothrowherbackandforthbetweenthem.DalethrowsShanelle toMelandthegamebegins.DescribethemotionofDale,MelandShanelle asthegamecontinues.Ifweassumethateachthrowinvolvesthesameamountofpush,thenhowmanythrowswillthegamelast?   SeeAnswer    Thegamewilllasttwothrowsandonecatch.WhenDalethrowsShanelle,thetwowilltravelinoppositedirectionsatthesamespeed.WhenMelcatchesShanelle,ShanellewillslowdowntohalfheroriginalspeedandmovetogetherwithMelatthatsamespeed.WhenMelthrowsShanelletowardsDale,thegreatestspeedthatShanellecanhaveisone-halftheoriginalspeed.ThegameisnowoversinceShanellewillnevercatchuptoDale.       NextSection: MomentumConservationPrinciple IsolatedSystemsandCollisionAnalysis CollisionAnalysisandMomentumProblems UsingEquationsasGuidestoThinking MomentumConservationinExplosions JumpToNextLesson: Page1 FollowUs PhysicsTutorial»MomentumandItsConservation»Lesson2-TheLawofMomentumConservation»TheLawofAction-Reaction(Revisited)