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Pendulum problem Need help

  1. Oct 13, 2004 #1
    Pendulum problem...Need help!!!

    PROBLEM:
    A pendulum consists of a string of length L=4.50m and a bob of mass m=1.6kg. The string is brought to a horizontal position and the bob is given the minimum initial speed enabling the pendulum to make a full turn in the vertical plane with the string staying stretched to full length the whole way around. What is the maximum kinetic energy of the bob?

    I have tried what seems like a million times to solve this. I tried using the conservation of mechanical energy formula which is:
    Kf + Uf= Ki +Ui where K is kinetic energy and U is potential energy. I ended up with .5 *m *vsquared = m* g * h and then solving for velocity and then plugging it into .5 *m *vsquared and solving for kinetic energy.
    Somehow though that doesnt seem right but i am unsure of what else I can do.
     
  2. jcsd
  3. Oct 14, 2004 #2

    Tide

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    Energy conservation is a good start but you also need to apply the condition that the bob is able to make it all the way around to the top. This means that the minimum speed is such that the tension in the string is zero when the bob reaches the top so the centripetal force at that point is equal to mg. Once you figure out the intitial speed from that go back and find out what the maximum kinetic energy was during the trip around.
     
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