Reaction (physics) - Wikipedia

Centripetal and centrifugal force Reaction(physics) FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch AsdescribedbythethirdofNewton'slawsofmotionofclassicalmechanics,allforcesoccurinpairssuchthatifoneobjectexertsaforceonanotherobject,thenthesecondobjectexertsanequalandoppositereactionforceonthefirst.[1][2]Thethirdlawisalsomoregenerallystatedas:"Toeveryactionthereisalwaysopposedanequalreaction:orthemutualactionsoftwobodiesuponeachotherarealwaysequal,anddirectedtocontraryparts."[3]Theattributionofwhichofthetwoforcesistheactionandwhichisthereactionisarbitrary.Eitherofthetwocanbeconsideredtheaction,whiletheotherisitsassociatedreaction. Contents 1Examples 1.1Interactionwithground 1.2Gravitationalforces 1.3Supportedmass 1.4Massonaspring 2Causalmisinterpretation 2.1'Equalandopposite' 2.2Centripetalandcentrifugalforce 3Seealso 4References 5Bibliography Examples[edit] Interactionwithground[edit] Whensomethingisexertingforceontheground,thegroundwillpushbackwithequalforceintheoppositedirection.Incertainfieldsofappliedphysics,suchasbiomechanics,thisforcebythegroundiscalled'groundreactionforce';theforcebytheobjectonthegroundisviewedasthe'action'. Whensomeonewantstojump,heorsheexertsadditionaldownwardforceontheground('action').Simultaneously,thegroundexertsupwardforceontheperson('reaction').Ifthisupwardforceisgreaterthantheperson'sweight,thiswillresultinupwardacceleration.Whentheseforcesareperpendiculartotheground,theyarealsocalledanormalforce. Likewise,thespinningwheelsofavehicleattempttoslidebackwardacrosstheground.Ifthegroundisnottooslippery,thisresultsinapairoffrictionforces:the'action'bythewheelonthegroundinbackwarddirection,andthe'reaction'bythegroundonthewheelinforwarddirection.Thisforwardforcepropelsthevehicle. Gravitationalforces[edit] TwobodiessimilartotheSunandEarth,i.e.withanextremedifferenceinmass–theredXmarksthebarycenter TheEarth,amongotherplanets,orbitstheSunbecausetheSunexertsagravitationalpullthatactsasacentripetalforce,holdingtheEarthtoit,whichwouldotherwisegoshootingoffintospace.IftheSun'spullisconsideredanaction,thenEarthsimultaneouslyexertsareactionasagravitationalpullontheSun.Earth'spullhasthesameamplitudeastheSunbutintheoppositedirection.SincetheSun'smassissomuchlargerthanEarth's,theSundoesnotgenerallyappeartoreacttothepullofEarth,butinfactitdoes,asdemonstratedintheanimation(nottoprecisescale).Acorrectwayofdescribingthecombinedmotionofbothobjects(ignoringallothercelestialbodiesforthemoment)istosaythattheybothorbitaroundthecenterofmass,referredtoinastronomyasthebarycenter,ofthecombinedsystem. Supportedmass[edit] Anymassonearthispulleddownbythegravitationalforceoftheearth;thisforceisalsocalleditsweight.Thecorresponding'reaction'isthegravitationalforcethatmassexertsontheplanet. Iftheobjectissupportedsothatitremainsatrest,forinstancebyacablefromwhichitishanging,orbyasurfaceunderneath,orbyaliquidonwhichitisfloating,thereisalsoasupportforceinupwarddirection(tensionforce,normalforce,buoyantforce,respectively).Thissupportforceisan'equalandopposite'force;weknowthisnotbecauseofNewton'sthirdlaw,butbecausetheobjectremainsatrest,sothattheforcesmustbebalanced. Tothissupportforcethereisalsoa'reaction':theobjectpullsdownonthesupportingcable,orpushesdownonthesupportingsurfaceorliquid.Inthiscase,therearethereforefourforcesofequalmagnitude: F1.gravitationalforcebyearthonobject(downward) F2.gravitationalforcebyobjectonearth(upward) F3.forcebysupportonobject(upward) F4.forcebyobjectonsupport(downward) ForcesF1andF2areequaldueofNewton'sthirdlaw;thesameistrueforforcesF3andF4. ForcesF1andF3areequalifandonlyiftheobjectisinequilibrium,andnootherforcesareapplied.(ThishasnothingtodowithNewton'sthirdlaw.) Massonaspring[edit] Ifamassishangingfromaspring,thesameconsiderationsapplyasbefore.However,ifthissystemisthenperturbed(e.g.,themassisgivenaslightkickupwardsordownwards,say),themassstartstooscillateupanddown.Becauseoftheseaccelerations(andsubsequentdecelerations),weconcludefromNewton'ssecondlawthatanetforceisresponsiblefortheobservedchangeinvelocity.Thegravitationalforcepullingdownonthemassisnolongerequaltotheupwardelasticforceofthespring.Intheterminologyoftheprevioussection,F1andF3arenolongerequal. However,itisstilltruethatF1=F2andF3=F4,asthisisrequiredbyNewton'sthirdlaw. Causalmisinterpretation[edit] Theterms'action'and'reaction'havethemisleadingsuggestionofcausality,asifthe'action'isthecauseand'reaction'istheeffect.Itisthereforeeasytothinkofthesecondforceasbeingtherebecauseofthefirst,andevenhappeningsometimeafterthefirst.Thisisincorrect;theforcesareperfectlysimultaneous,andarethereforthesamereason.[4] Whentheforcesarecausedbyaperson'svolition(e.g.asoccerplayerkicksaball),thisvolitionalcauseoftenleadstoanasymmetricinterpretation,wheretheforcebytheplayerontheballisconsideredthe'action'andtheforcebytheballontheplayer,the'reaction'.Butphysically,thesituationissymmetric.Theforcesonballandplayerarebothexplainedbytheirnearness,whichresultsinapairofcontactforces(ultimatelyduetoelectricrepulsion).Thatthisnearnessiscausedbyadecisionoftheplayerhasnobearingonthephysicalanalysis.Asfarasthephysicsisconcerned,thelabels'action'and'reaction'canbeflipped.[4] 'Equalandopposite'[edit] OneproblemfrequentlyobservedbyphysicseducatorsisthatstudentstendtoapplyNewton'sthirdlawtopairsof'equalandopposite'forcesactingonthesameobject.[5][6][7] Thisisincorrect;thethirdlawreferstoforcesontwodifferentobjects.Incontrast,abooklyingonatableissubjecttoadownwardgravitationalforce(exertedbytheearth)andtoanupwardnormalforcebythetable,bothforcesactingonthesamebook.Sincethebookisnotaccelerating,theseforcesmustbeexactlybalanced,accordingtoNewton'ssecondlaw.Theyaretherefore'equalandopposite',yettheyareactingonthesameobject,hencetheyarenotaction-reactionforcesinthesenseofNewton'sthirdlaw.Theactualaction-reactionforcesinthesenseofNewton'sthirdlawarethebookpushingdownonthetableandthebook'supwardgravitationalforceontheearth. Moreover,theforcesactingonthebookarenotalwaysequallystrong;theywillbedifferentifthebookispusheddownbyathirdforce,orifthetableisslanted,orifthetable-and-booksystemisinanacceleratingelevator.Thecaseofanynumberofforcesactingonthesameobjectiscoveredbyconsideringthesumofallforces. Apossiblecauseofthisproblemisthatthethirdlawisoftenstatedinanabbreviatedform:Foreveryactionthereisanequalandoppositereaction,[8]withoutthedetails,namelythattheseforcesactontwodifferentobjects.Moreover,thereisacausalconnectionbetweentheweightofsomethingandthenormalforce:ifanobjecthadnoweight,itwouldnotexperiencesupportforcefromthetable,andtheweightdictateshowstrongthesupportforcewillbe.Thiscausalrelationshipisnotduetothethirdlawbuttootherphysicalrelationsinthesystem. Centripetalandcentrifugalforce[edit] Anothercommonmistakeistostatethat"thecentrifugalforcethatanobjectexperiencesisthereactiontothecentripetalforceonthatobject."[9][10] Ifanobjectweresimultaneouslysubjecttobothacentripetalforceandanequalandoppositecentrifugalforce,theresultantforcewouldvanishandtheobjectcouldnotexperienceacircularmotion.Thecentrifugalforceissometimescalledafictitiousforceorpseudoforce,tounderscorethefactthatsuchaforceonlyappearswhencalculationsormeasurementsareconductedinnon-inertialreferenceframes.[11] Seealso[edit] Groundreactionforce Reactivecentrifugalforce IsaacNewton IbnBajjah Reactionengine/jetengine Shearforce References[edit] ^Taylor,JohnR.(2005).ClassicalMechanics.UniversityScienceBooks.pp. 17–18.ISBN 9781891389221. ^Shapiro,IlyaL.;deBerredo-Peixoto,Guilherme(2013).LectureNotesonNewtonianMechanics:LessonsfromModernConcepts.SpringerScience&BusinessMedia.p. 116.ISBN 978-1461478256.Retrieved28September2016. ^Thistranslationofthethirdlawandthecommentaryfollowingitcanbefoundinthe"Principia"onpage20ofvolume1ofthe1729translation. ^abBrown,David(1989)."Students'conceptofforce:theimportanceofunderstandingNewton'sthirdlaw".Phys.Educ.24(6):353–358.doi:10.1088/0031-9120/24/6/007.Eventhoughonebodymightbemore‘active’thantheotherbodyandthusmightseemtoinitiatetheinteraction(e.g.abowlingballstrikingapin),theforcebodyAexertsonbodyBisalwayssimultaneouswiththeforceBexertsonA. ^ColinTerryandGeorgeJones(1986)."Alternativeframeworks:Newton'sthirdlawandconceptualchange".EuropeanJournalofScienceEducation.8(3):291–298.Bibcode:1986IJSEd...8..291T.doi:10.1080/0140528860080305.ThisreporthighlightssomeofthedifficultiesthatchildrenexperiencewithNewton'sthirdlaw. ^CornelisHellingman(1992)."Newton'sThirdLawRevisited".PhysicsEducation.27(2):112–115.Bibcode:1992PhyEd..27..112H.doi:10.1088/0031-9120/27/2/011....followingquestioninwriting:Newton’sthirdlawspeaksabout‘action’and‘reaction’.Imagineabottleofwinestandingonatable.Ifthegravitationalforcethatattractsthebottleiscalledtheaction,whatforceisthereactiontothisforceaccordingtoNewton’sthirdlaw?Theanswermostfrequentlygivenwas:‘Thenormalforcethetableexertsonthebottle’. ^French,Anthony(1971),NewtonianMechanics,p. 314,…Newton’sthirdlaw,that‘‘actionandreactionareequalandopposite’’ ^Hall,Nancy."Newton'sThirdLawAppliedtoAerodynamics".NASA.Archivedfromtheoriginalon2018-10-03.foreveryaction(force)innaturethereisanequalandoppositereaction ^Adair,Aaron(2013),StudentMisconceptionsaboutNewtonianMechanics:OriginsandSolutionsthroughChangestoInstruction,ThiswasattackedbyNewtonwhotriedtohavethecentripetalforceontheplanets(fromgravitationalinteractions)bematchedbythecentrifugalforcesotherewouldbeabalanceofforcesbasedonhisthirdlawofmotion ^Aiton,Eric(1995),Swetz,Frank;et al.(eds.),AnEpisodeintheHistoryofCelestialMechanicsanditsUtilityintheTeachingofAppliedMathematics,LearnfromtheMasters,TheMathematicalAssociationofAmerica,ISBN 978-0883857038,...inoneofhisattacksonLeibnizwrittenin1711,Newtonsaysthatcentrifugalforceisalwaysequalandoppositetotheforceofgravitybythethirdlawofmotion. ^Singh,Chandralekha(2009),"CentripetalAcceleration:OftenForgottenorMisinterpreted",PhysicsEducation,44(5):464–468,arXiv:1602.06361,doi:10.1088/0031-9120/44/5/001,S2CID 118701050,Anotherdifficultyisthatstudentsoftenconsiderthepseudoforces,e.g.,thecentrifugalforce,asthoughtheywererealforcesactinginaninertialreferenceframe. Bibliography[edit] Feynman,R.P.,LeightonandSands(1970)TheFeynmanLecturesonPhysics,Volume1,AddisonWesleyLongman,ISBN 0-201-02115-3. Resnick,R.andD.Halliday(1966)Physics,Part1,JohnWiley&Sons,NewYork,646pp+Appendices. Warren,J.W.(1965)TheTeachingofPhysics,Butterworths,London,130pp. Retrievedfrom"https://en.wikipedia.org/w/index.php?title=Reaction_(physics)&oldid=1058345302" Categories:ClassicalmechanicsForce Navigationmenu Personaltools NotloggedinTalkContributionsCreateaccountLogin Namespaces ArticleTalk English Views ReadEditViewhistory More Search Navigation MainpageContentsCurrenteventsRandomarticleAboutWikipediaContactusDonate Contribute HelpLearntoeditCommunityportalRecentchangesUploadfile Tools WhatlinkshereRelatedchangesUploadfileSpecialpagesPermanentlinkPageinformationCitethispageWikidataitem Print/export DownloadasPDFPrintableversion Languages العربيةБеларускаяCatalàEspañolفارسی한국어HrvatskiМакедонски日本語NorsknynorskسنڌيSrpskohrvatski/српскохрватскиไทยУкраїнська粵語中文 Editlinks