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Friction in rotational movement

  1. Feb 3, 2005 #1
    First, sorry for my bad english, it is a hard job for me.

    I am an engineering student and I have found that something doesn´t match.

    Imagine a cylinder, a wheel for example. Case 1) If it if it descends by a plane inclined because of its weight, the friction direction goes in the same way that the angular acceleration, Case 2) But imagine that you give a torque to this wheel to make it raise by the slope. In this case the angular acceleration goes in opposite that in case 1 (and the turn) but the friction force will go in the same direction that before. Is this right? Because some professors claim that the friction force goes always in the same direction that the angular acceleration, but I think that it deppends it the object is pasive (when you let it go downhill) or active (it makes the torque, like in a car wheel). I hope someone confirm me this, is very important for my investigation


    Edited: you can try an applet in this page, it works pretty well:
    Webpage with java applet
    Last edited: Feb 3, 2005
  2. jcsd
  3. Feb 3, 2005 #2


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    Science Advisor

    I THINK you are referring to the wheel turning faster as it goes down the ramp and slower as it goes up. (Otherwise, there is not necessarily any rotational acceleration.)

    If there were no friction at all, the wheel could slide up or down the ramp with turning at all. The only force causing the wheel to turn faster (or slower) is the friction force (NOT gravity- that causes linear acceleration): F= ma so, yes, the friction force is in the same direction as the acceleration.

    Notice that as the wheel is rolling down the ramp, the angular acceleration is directed back up the ramp, the same direction as the friction.

    When the wheen is rolling up the ramp, the angular acceleration is directed down the ramp, again the same direction as the friction.

    You say "but I think that it depends it the object is passive (when you let it go downhill) or active (it makes the torque, like in a car wheel)." so you may be thinking of the wheel applying force to go up, rather than a wheel simply rolling up the slope (and slowing as it goes). I think your "professors" are talking about the "passive" case that I am since in your "active" case, there might be know angular acceleration at all.
    Last edited by a moderator: Feb 3, 2005
  4. Feb 3, 2005 #3
    Yes, my teachers are in the passive case, but they don´t understand it when i try to explain them the active case. :uhh:

    Yes, if it is passive, but if the object is active (like the wheel of a mountain bike) then, thinking of the wheel applying force to go up, in this case the angular acceleration will go in oppositte way to the friction force. The same case if you aply a torque to the wheel to force it to go up.

    My question is, am I right or not? I am sure that I am, but I need it to be confirmed by someone else besides me
  5. Feb 3, 2005 #4

    Doc Al

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    Staff: Mentor

    I'm not sure I understand the question, but I will offer these comments for situations involving rolling without slipping:

    (1) If friction is the only torque-producing force on the wheel, then of course the friction must act so as to create the angular acceleration. If other torque-producing forces act, then (obviously) the net torque is what determines the acceleration.

    (2) Static friction is a "passive" force, in that it will be the minimum it needs to be to prevent slipping, up to its limit ([itex]= \mu N[/itex]).

    If you imagine a mountain bike accelerating on a flat surface (for simplicity), then the friction points in the same direction as the linear acceleration. (Friction is the only external horizontal force on the bike.) The friction exerts a torque on the wheel, but so does the biker. The net torque causes the angular acceleration.
  6. Feb 3, 2005 #5
    That is what I tried to say, then I were not wrong. Thanks
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