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Homework Help: How do you find centripetal force without knowing velocity?

  1. Sep 20, 2012 #1
    1. The problem statement, all variables and given/known data

    A tetherball goes around the pole at a constant velocity. The rope has a 2m length, makes a 30 degree angle below the horizon and the ball's mass is .5 kg.

    2. Relevant equations



    3. The attempt at a solution
    I'm stuck at trying to find centripetal force.
  2. jcsd
  3. Sep 20, 2012 #2


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    Hello annamarie424. Welcome to PF !

    Find it from the centripetal force, which is the component of the force producing the circular motion.
  4. Sep 21, 2012 #3


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    Hi annamarie424 - Draw a force diagram showing the pole vertically with the ball and string making a 30 degree angle. Draw on the forces acting on the ball. In this view the ball isn't moving (eg it's not falling down, nor is it rising up) so some of the forces are in balance (eg they sum to zero).

    If you are still stuck try putting your diagram on an image hosting site and providing a link. Once you have made 10 (?) posts on this forum you can upload images directly.
  5. Sep 21, 2012 #4


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    Note it's not really a tetherball, the rope is not winding around the pole, instead the rope is attached to a frictionless pivot and the length (2 m) and angle (30 °) remain constant.
  6. Sep 22, 2012 #5
    centripetal force in this case is just the net radial force. Take some tension T in the rope and resolve the components of the weight in radial and tangential directions.
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