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Homework Help: Angular speed/weight problem

  1. Jun 20, 2009 #1
    1. The problem statement, all variables and given/known data
    Suppose I tie a ball of mass M to a string of length L and I swing the ball on the string in a vertical fashion. If the speed at the top is V1 and the speed at the bottom is V2.
    Would I have:

    Let T = tension in string
    (top) -T - Mg = -MV1^2/R
    (bottom) T - Mg = MV2^2/R

    So to find Mg (the force of gravity), I would add the 2 equations to get:

    -2Mg = MV2^2/R - MV1^2/R.

    But somehow that is wrong?
  2. jcsd
  3. Jun 20, 2009 #2
    Last edited by a moderator: Apr 24, 2017
  4. Jun 20, 2009 #3


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    Homework Helper

    When the ball is moving through the bottom with velocity V2 its kinetic energy = 1/2*m*v2^2. As it moves towards the top, its kE decreases but its PE increases. At the top, KE = 1/2*m*v1^2 and rise in PE = 2mgR.Apply conservation of energy to find mg.
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