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sign up log in tour help Take the 2minute tour × Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required. What role does static friction force play for a rolling object? How can I know what direction it points? Fundamentally, what does a friction force do for a rolling object? I am very confused by the way my textbook explains it. In my textbook , it says: A wheel rolling on a horizontal flat surface at a constant velocity experiences no friction force. Why? On the same surface, there is an acceleration of the wheel pointing to the right (probably caused by a force), so the ball is angularly accelerating in the clockwise direction. In this case, a friction force appears, and it is also pointing to the right. How come? What does the friction do for this wheel? On an inclined plane, a ball freely rolls down the surface. The direction of friction is up the ramp, which confuses me because in the previous example the friction force was in the direction of the wheel's acceleration. And there is a difference if a wheel is freely rolling and if there is a torque acting on the ball's center of mass. WHY??? What does friction do in these cases? How does it cause an object to roll? forces friction asked Jan 11 at 18:37 Richard Song 6 1 – Static friction, as it acts in rolling without slipping. Richard Song Jan 11 at 18:41 – And static friction supposedly prevents the wheel from sliding, right? Richard Song Jan 11 at 18:46 – Then how do I know which way something would slide? How do I know which way a wheel WOULD slide? Richard Song Jan 11 at 18:58 – Related: physics.stackexchange.com/questions/7657/… dmckee ♦ Jan 11 at 22:25 2 Answers Hopefully this will answer at least some of your questions. If you consider a ball intially at rest on a frictionless surface, if a force is exerted through the centre of mass of the ball it will slide across the surface with no rotation, there will only be translational motion. If you consider the case where there is friction, if the force is again applied to a stationary ball the frictional force will act in the opposite direction to the force but that rests on the ground. This friction applies a torque to the ball which starts the rotation. So static friction is infact necessary for a ball to begin rolling. at the edge of the ball The condition for rolling at speed is that the angular velocity of the ball is given by from this the top of the ball will move at speed the centre of mass of the ball will move at and the bottom edge of the ball will instantaneously be at rest. So as the edge touching the ground is stationary it experiences no friction. So friction is necessary for a ball to start rolling but once the rolling condition has been met the ball experiences no friction. answered Jan 11 at 19:03 Chris2807 770 1 1 16 – Thank you for your explanations. I have concluded the following: please correct me if I am wrong. Richard Song Jan 11 at 19:59 – If a ball has a force acting through its center of mass (and therefore no torque is generated by that force), with no friction it will just slide forward. If the force acted towards the right, then there would be a friction force acting towards the left, thus generating a torque about the center of mass. Richard Song Jan 11 at 20:02 – In regard to Chris2807's answer, I have a question: If a ball is rolling down the hill on its own, there is a continual component of gravitational force that pulls it down the incline. Since this force produces no torque about the center of mass, I have concluded that a frictional force must be exerted on the contact point at all times when the ball slides down the incline, or else it would slide. What do you mean by the fact that once the rolling condition is met the ball experiences no friction? Richard Song Jan 11 at 20:17 1 Nice. You can also take the opposite approach: consider what happens if an external couple (pure torque) is

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What Role Does Static Friction Force Play for a Rolling Object

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    Takethe2minutetour PhysicsStackExchangeisaquestionandanswersiteforactiveresearchers,academicsandstudentsofphysics.It's100%free,noregistrationrequired.

    Whatroledoesstaticfrictionforceplayforarollingobject?HowcanIknowwhatdirectionitpoints?

    Fundamentally,whatdoesafrictionforcedoforarollingobject?Iamveryconfusedbythewaymytextbookexplainsit.Inmytextbook,itsays:

    Awheelrollingonahorizontalflatsurfaceataconstantvelocityexperiencesnofrictionforce.Why?

    Onthesamesurface,thereisanaccelerationofthewheelpointingtotheright(probablycausedbyaforce),sotheballisangularlyacceleratingintheclockwisedirection.Inthiscase,africtionforceappears,anditisalsopointingtotheright.Howcome?Whatdoesthefrictiondoforthiswheel?

    Onaninclinedplane,aballfreelyrollsdownthesurface.Thedirectionoffrictionisuptheramp,whichconfusesmebecauseinthepreviousexamplethefrictionforcewasinthedirectionofthewheel'sacceleration.

    Andthereisadifferenceifawheelisfreelyrollingandifthereisatorqueactingontheball'scenterofmass.WHY???

    Whatdoesfrictiondointhesecases?Howdoesitcauseanobjecttoroll?

    forces friction

    askedJan11at18:37RichardSong6 1

    Staticfriction,asitactsinrollingwithoutslipping. RichardSong Jan11at18:41

    Andstaticfrictionsupposedlypreventsthewheelfromsliding,right? RichardSong Jan11at18:46

    ThenhowdoIknowwhichwaysomethingwouldslide?HowdoIknowwhichwayawheelWOULDslide?RichardSong Jan11at18:58

    Related:physics.stackexchange.com/questions/7657/ dmckee Jan11at22:25

    2Answers

    Hopefullythiswillansweratleastsomeofyourquestions.Ifyouconsideraballintiallyatrestonafrictionlesssurface,ifaforceisexertedthroughthecentreofmassoftheballitwillslideacrossthesurfacewithnorotation,therewillonlybetranslationalmotion.

    Ifyouconsiderthecasewherethereisfriction,iftheforceisagainappliedtoastationaryballthefrictionalforcewillactintheoppositedirectiontotheforcebut thatrestsontheground.Thisfrictionappliesatorquetotheballwhichstartstherotation.Sostaticfrictionisinfactnecessaryforaballtobeginrolling.

    attheedgeoftheball

    Theconditionforrollingatspeed isthattheangularvelocityoftheballisgivenby

    fromthisthetopoftheballwillmoveatspeed thecentreofmassoftheballwillmoveat andthebottomedgeoftheballwillinstantaneouslybeatrest.Soastheedgetouchingthegroundisstationaryitexperiencesnofriction.

    2

    2 [.

    2 2

    Sofrictionisnecessaryforaballtostartrollingbutoncetherollingconditionhasbeenmettheballexperiencesnofriction.

    answeredJan11at19:03Chris2807770 1 1 16

    Thankyouforyourexplanations.Ihaveconcludedthefollowing:pleasecorrectmeifIamwrong.RichardSong Jan11at19:59

    Ifaballhasaforceactingthroughitscenterofmass(andthereforenotorqueisgeneratedbythatforce),withnofrictionitwilljustslideforward.Iftheforceactedtowardstheright,thentherewouldbeafrictionforceactingtowardstheleft,thusgeneratingatorqueaboutthecenterofmass. RichardSong Jan11at20:02

    InregardtoChris2807'sanswer,Ihaveaquestion:Ifaballisrollingdownthehillonitsown,thereisacontinualcomponentofgravitationalforcethatpullsitdowntheincline.Sincethisforceproducesnotorqueaboutthecenterofmass,Ihaveconcludedthatafrictionalforcemustbeexertedonthecontactpointatalltimeswhentheballslidesdowntheincline,orelseitwouldslide.Whatdoyoumeanbythefactthatoncetherollingconditionismettheballexperiencesnofriction? RichardSong Jan11at20:17

    1 Nice.Youcanalsotaketheoppositeapproach:considerwhathappensifanexternalcouple(puretorque)is

  • suppliedaroundahorizontalaxis.Withoutfrictiontheballspinsinplace,butwithfrictiontheballappliesaforcetothegroundandthereactionforcebothcancelsmuchoftheappliedcoupleandcausestheballtobegintranslationalmotion. dmckee Jan11at22:23

    @dmckee+1forthatcomment,prettycooleffect.Acoolexampleofthisisprofessionaltenpinbowlers,theyproducelotsofsidespinontheballwhichhasnoeffect(astheboardsareverypolishedandhavelittlefrictionwiththeball)untilthelastfewfeetwheretheyarelesswellpolishedandtheballmoveslaterally.Havealookatthisyoutube.com/watch?v=NiBXo2KjdBA Chris2807 Jan11at23:06

    1. Ifthewheelisrollingatconstantvelocitywithoutanyotherforcesacting,thenthereisnotendencyforslippagebetweenthewheelandthesurface,sothereisnofriction.Frictionwillonlyacttotrytopreventslippagebetween2surfaces.

    2. Presumablyinthiscasethereisatorqueactingontheaxisoftheball/wheel?Thistorquewillacttogiveangularaccelerationtothewheel(clockwise).Iftherewasnofrictionthentheangularvelocitywouldincreaseandtherewouldbeslippagebetweenthewheelandthesurface.However,frictionexistsandwilltrytopreventtheslippage,whichiswhyitactstotheright.Intryingtopreventthisslippage,thefrictionalsoexertsahorizontallinearforceonthewheel,givingitaccelerationtotheright,soitspeedsup.Thisishowacaraccelerateshorizontally,usingfrictionbetweenthewheelsandtheroad.

    3. Ontheinclinedplane,theballwilltendtoslidedownwardsandiftherewasnofrictionthenagaintherewouldbeslippagebetweentheballandtheinclinedsurface.Perhapsthebestwaytothinkaboutitis:"Whichwayisslippagegoingtooccuriftherewasnofriction?"Frictionwillalwaystrytocounteractslippage,sohereitactsuptheslopetotrytopreventtheballslippingdowntheslope.Thishastheeffectofapplyingamomenttotheball,whichwillcauseittostartrolling.

    answeredJan11at19:09Time4Tea903 1 16

    Sowhatifthereisaforcethatcausesnotorqueandaforcethatcausestorquethatactonawheel.Soforexample,oneforceactsonthecenterofmasswhileanotheractsonthetopofthewheel.Thewheelisonaflatsurface.Doesthewheelneedfrictiontoroll? RichardSong Jan11at21:29

    Fortheforcethroughthecenter,thewheelwillneedfrictiontostartrolling,otherwiseitwilljustslidehorizontally.Fortheforceonthetop,ifthereisnofrictionthenthewheelwillstartbothmovinglaterallyandrotating.Thesubsequentmotionwilldependonhowtheforcebehavesasthewheelturnswhetheritturnswiththewheelorstayspointinghorizontally(Iassumeitactsatthesamepoint). Time4Tea Jan11at21:56


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