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Homework Help: Centripetal Force Question

  1. Mar 13, 2007 #1
    The problem statement, all variables and given/known data
    A 1.0kg ball is attached to a rigid vertical rod by the means of two massless strongs each 1.0m long. The strings are attached to the rod at points 1.0m apart. The system is rotating about the axis of the rod, both strings being taut and forming an equilateral triangle with the rod. The tension in the supper string is What is the tension in the lower string? What is the net force on the ball? What is the speed of the ball?

    Relevant equations
    The only equation I can think of is F = mv^2 / r
    where m = mass, r = radius, v = velocity, F = Centripetal Force

    The attempt at a solution
    I'm not sure how to do the entire question... does finding the tension need an angle? Because the string is not at the horizontal...
  2. jcsd
  3. Mar 13, 2007 #2
    At first glance, (and maybe I'm missing something) but it seems that something is missing. If you spin the rod faster and faster (once you've reached the point at which both strings are taut), the tension in the strings will increase until they finally break, and will continue to form an equilateral triangle.

    Is it implied in the problem that the system is rotating "just fast enough" for the lower string to be taut?

    However, on problems where forces are important, it's a good idea to draw a free-body diagram. What forces are acting on the ball? (You're missing one; usually one which is obvious)
  4. Mar 13, 2007 #3
    No other forces have been given to me in the question. I am also not sure of the forces that will be acting on the free body diagram of the ball? Would it only be the force of gravity, the normal force, and the forces of the strings.....?
  5. Mar 13, 2007 #4
    Yes, the force of gravity = you just had a mass there; I was making sure you were including that force.
  6. Mar 13, 2007 #5


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    I believe you left out a given..the tension in the upper string is ??????
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