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Friction on a particle on rough horizontal rotating disc

  1. May 12, 2015 #1
    A particle is attached to an inextensible string. The other end of the string is attached to the centre of a rotating rough disc. The string is shorter than the radius of the disc so the particle remains on the disc and moves in uniform circular motion.
    I don't remember the quantities but the question was to find min and max values of tension in the string(for different speeds of rotation).
    Apparently min tension is when the particle is moving the most slowly and at his point friction is acting opposite the tension so Tmin - Fr = mv^2/r.
    Similarly the max tension is when speed is greatest and at this speed friction acts in the same direction as tension so Tmax + Fr = mv^2/r.
    I really don't understand why the direction of friction changes with different speeds and how can the friction act outward of the circle, wouldn't that be a non existent centrifugal force? I though in uniform circular motion friction acts tangentially and toward the circle and nowhere else.
    Can someone help me understand this? Any help is greatly appreciated. Cheers.

    Edit: I found the question. It's actually an elastic string. The string is extended and it's asking for min and max values of angular speed without changing the extension. So tension is constant and angular speed is changing. Sorry about that.
    Last edited: May 12, 2015
  2. jcsd
  3. May 12, 2015 #2

    Doc Al

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    Staff: Mentor

    What if the speed was zero (or at least very low)? Which way would the forces act on the particle? What if the speed was very high?

    In uniform circular motion, the net force must act radially (not tangentially) towards the center of the circle. If friction were the only force acting, then it would act toward the center. But that's not the case here. The string also exerts a force.
  4. May 12, 2015 #3


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    That explains a lot :-)
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