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Homework Help: Hula Hoop Rotational Motion Problem

  1. Apr 9, 2013 #1
    1. The problem statement, all variables and given/known data
    In throwing a hulahoop with back spin, you toss the hoop to the right, and it (1) moves right with speed v0, but rotates ccw with speed ω0. At some point, O, (2) it will change direction, and at that point it will start moving to the, left, but still be sliding because it is rotating too quickly. Finally, (3) it will overcome the rotation and continue to roll to the left without slipping. What direction does the friction point in each of these cases? How do you know?

    2. Relevant equations

    3. The attempt at a solution

    For (1):
    Friction force is left because motion is to the right and the hoop is slowing down

    I don't know how to approach 2 and 3. I tried drawing the free body diagrams, and my text suggests I consider directions of linear and angular acceleration, but I don't know how this will help me!
  2. jcsd
  3. Apr 9, 2013 #2


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    Gold Member

    To work out which way friction will act at a contact, think about which way the surfaces would move in relation to each other if there were no friction. Friction will always act to oppose that motion.
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