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Homework Help: Circular motion problem

  1. Jan 26, 2015 #1
    1. The problem statement, all variables and given/known data
    object of mass 0.20 kg tied to a string is made to move in a vertical circle. When the object is at the highest point, the tension in the string is zero. Determine the tension in the string when the object is at the lowest point. [11.8 N]

    2. Relevant equations

    3. The attempt at a solution
    i tried it and could not arrive at the state answer in bracket. at highest point, weight + tension gives the centripetal force. since tension is said to be zero, at the hioghest point, weight = centripetal force.

    then at lowest point, tension - weight gives centripetal. tension is the sum of centripetal and weight = 2 x weight since centripetal was calculated to be equal to weight. but this is not the answer given.
    am i missing something?
  2. jcsd
  3. Jan 26, 2015 #2

    Suraj M

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    Your right about the first part. But the centripetal force acting on the body is not a constant.

    At the highest position
    mg = v²/r
    v =√gr
    this velocity is not the same velocity which it has at the lowest position
    To calculate that use the law of conservation of energy :
    ½mv²highest position = ½mv²lowest position + mg(2r)
    and then find your tension.
    Last edited: Jan 26, 2015
  4. Jan 26, 2015 #3


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    You did pretty much the entire problem. You did it well but you did too much. Let the OP do his own homework and just give hints.

    I know there is an urge to leap out and solve the problem. I have the same urge. But once you solve it, don't post the entire thing. If you do the entire thing for them they won't get the benefit of the homework.
  5. Jan 26, 2015 #4

    Suraj M

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    Gold Member

    ok DEvens .. i realised that and edited it ..and sorry.
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