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About pedulum

  1. Dec 2, 2007 #1
    Hi, for this question
    http://tinyurl.com/2qgco5
    I have problem for question b

    My attempts for question (b)
    The potential energy of the pendulum when it is released from the horizontal position
    =The kinetic energy of pendulum at lowest point
    =mgL

    In order to make it swing in a complete circle, it needs to reach the height 2(L-d)
    hence mgL=mg(2(L-d)); and I get d=L/2

    This is wrong, what it wants is d=3/5L, may I know what is my mistake, and what is the correct approach?
     
  2. jcsd
  3. Dec 2, 2007 #2

    Doc Al

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    Staff: Mentor

    This assumes that the sphere can make a complete revolution about the peg with a speed of zero at the top. This is incorrect--figure out the minimum speed needed at the top. Hint: Consider Newton's 2nd law and circular motion.
     
  4. Dec 3, 2007 #3
    Thanks!
     
  5. Dec 3, 2007 #4
  6. Dec 3, 2007 #5

    Doc Al

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    Staff: Mentor

    See my response in the other thread.
     
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