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Velocity of Pendulum at a certain point

  1. Oct 28, 2008 #1
    1. The problem statement, all variables and given/known data

    Tarzan, who weighs 668 N, swings from a cliff at the end of a convenient vine that is 23.0 m long (see the figure). From the top of the cliff to the bottom of the swing, he descends by 3.2 m.
    If the vine doesn't break, what is the maximum magnitude of the tension in the vine?

    http://capa-new.colorado.edu/hrw-lib/hrwpictures/8-38.jpg

    2. Relevant equations

    F=ma

    a_radial=v^2/r

    3. The attempt at a solution

    i know that the max tension is at the bottom, so i made a force diagram at the bottom of the path, and summed the forces to get

    F=T-mg
    mv^2/r + mg= T

    i found the mass, but my only missing variable is the velocity of the "pendulum."

    i am completely stuck on this part. Thanks for any help!
     
  2. jcsd
  3. Oct 28, 2008 #2
    Welcome to PF!

    Can you think of anyways you can get V from this information "From the top of the cliff to the bottom of the swing, he descends by 3.2 m."
     
  4. Oct 28, 2008 #3
    As energy is not lost nor created, Tarzan speed (kynetic energy) must come from another form of energy. In this case, he has descended 3.2m...
     
  5. Oct 28, 2008 #4
    thanks for the welcome!

    i have been thinking about it on the next homework problem actually, and i think it has something to do with conservation of momentum. something like:

    1/2(mv^2)+mgy= 1/2(mv^2) +No potential at the bottom.

    the initial velocity is 0, so the first initial kinetic energy is 0. is this reasoning right? or does he have some kinetic at the top?
     
  6. Oct 28, 2008 #5
    i guess that was right. i got the problem right, so that should help me on a few other problems. i will probably be back here asking more questions later though!
     
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