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Centripetal force problem with tension

  1. Jun 28, 2014 #1
    1. The problem statement, all variables and given/known data
    In a test of machine-part reliability, a specimen of mass m is swung in a vertical circle of constant radius .75 m. When the object is at the bottom the circular path, the tension in the supporting wire is found to be six times the weight of the object. Determine the rotation rate in revolutions per minute


    2. Relevant equations
    Fc = mv^2/r
    ω = v/r



    3. The attempt at a solution
    I drew a free body diagram and realized that the only forces acting on the system is the centripetal force, which is composed of the weight of the object and the tension so
    Fc = T - W
    mv^2 = 6(9.8)(M) - (9.8)(M)
    v^2 = 5(9.8)
    v^2 = 49
    v = 7 m/s

    ω = v/r
    ω = 7/.75
    ω = 9 and 1/3 radians per second

    So in 1 min there will be 560 radians
    560/ 2 pi
    So there will be 89.126 rotations per minute.
    This answer is wrong the correct answer is 77.2 rev/min
     
  2. jcsd
  3. Jun 28, 2014 #2

    AlephZero

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    Science Advisor
    Homework Helper

    Correct.

    Oops!

    The method is OK, except for that one mistake.
     
  4. Jun 28, 2014 #3
    Thanks!
     
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