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Circular Motion: Ball on String

  1. Apr 17, 2017 #1
    1. The problem statement, all variables and given/known data
    A 1.5 kg ball is tied to a string that is .75 m long and whirled in a verticle circle.
    A) Find the tension in the string at the top of the circle
    B) Find the tension in the string at the bottom of the circle

    2. Relevant equations
    Fc=mac
    ac= v2/r

    3. The attempt at a solution
    A)
    Fnet= Ft + Fg
    Fnet = Ft + (1.5)(10)

    B)
    Fnet = Ft - Fg
    Fnet = Ft - (1.5)(10)

    As you can see, I did not get very far in my calculations as I am very confused where to go from here. Both equations would require velocity which is not given and I don't know how to find it.
     
  2. jcsd
  3. Apr 17, 2017 #2

    kuruman

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    Do you know how fast the ball is moving?
     
  4. Apr 17, 2017 #3

    haruspex

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    .... or any other piece of information?
     
  5. Apr 17, 2017 #4

    scottdave

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    Perhaps the spin rate is 'just enough' to keep it going. That would happen when, at the top, it is just fast enough to overcome gravity acceleration. So what is Ft + Fg at the top of the circle?
     
  6. Apr 17, 2017 #5

    kuruman

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    Fg is mg and you know what that is. You are asked to find the tension and that depends on the speed. If the speed is just enough for the ball to go around the circle, what do you think the tension ought to be? I think you should revisit the problem and see if you forgot to post some needed information.
     
  7. Apr 17, 2017 #6

    haruspex

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    Scottdave is not the originator of the thread.
     
  8. Apr 17, 2017 #7

    kuruman

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    Oops, I missed that. Thanks and sorry Scottdave for directing my reply to you instead of the OP.
     
  9. Apr 17, 2017 #8

    haruspex

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    It's an easy mistake when posters don't use the reply or quote buttons, so it is not obvious whom they are addressing (hint @scottdave)
     
  10. Apr 18, 2017 #9
    Sorry for the late reply. Hate to say it but that was all the information that was given in the problem.
     
  11. Apr 18, 2017 #10

    kuruman

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    If you have ever swung a mass at the end of a string, you would know that the faster you swing the mass around, the harder the string pulls on your hand. Tension depends on speed. If the speed is not known, the tension cannot be known. You might wish to ask for clarifications from whoever asked you to do this problem.
     
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