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Mechanics Problem: Finding the speed at the center of a ring

  1. May 3, 2013 #1
    1. The problem statement, all variables and given/known data
    A homogeneous ring lays horizontally on two identical parallel rails. The first rail moves parallel to itself, with a constant speed v; the second rail is at rest. The angular distance between the ring-rail contact points, as seen from the center of the ring, is 2α for the first rail, and 2β for the second rail, see figure. Assuming that α << 1 and β = π/3, find the speed of the center of the ring.
    (and also, the first rail intersects the ring at two points)

    2. Relevant equations
    τ (torque) = F x r

    3. The attempt at a solution
    I wasn't quite sure where to start but I thought I might be able to use a torque balance to find the frictional forces, though I'm not sure where to go from there, or whether I am actually taking the correct approach.
  2. jcsd
  3. May 4, 2013 #2


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    2017 Award

    Staff: Mentor

    That looks like a good idea.
    Afterwards, you can consider rotation around a different axis to get the speed (and angular velocity) of the ring.
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