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Homework Help: Tension headache!

  1. Aug 29, 2013 #1
    1. The problem statement, all variables and given/known data
    a mass of 0.5kg is suspended from a flywheel. if the mass is released from rest and falls a distance of 0.5m in 1.5s, calculate : The tension in the rope. acceleration is 0.444ms-2.

    2. Relevant equations
    t-w = -ma

    3. The attempt at a solution

    t = m(ag)

    t = 0.5(0.4444=9.81)

    t = 5.127n

    w = mg = 0.5x9.81 4.9n

    so t-w 0.227n is rope tension.

    Check by inserting numbers into equation of t-w = -ma

    so: 5.127 - 4.905 = 0.5 x 0.222.

    The last equation doesn`t marry up?
  2. jcsd
  3. Aug 29, 2013 #2
    The tension in the rope is:

    T = m(g - a) = 0,5(9,81 - 0,44)

  4. Aug 29, 2013 #3
    If you had the mass just sitting there with no acceleration then it would be simply T=mg
    Since it's accelerating you have to subtract the force acting on it from the acceleration that it is going at which is F=ma so therefore:
    T=mg-ma=m(g-a)=0.5(9.81-0.444)=4.683N should be your answer?
  5. Aug 29, 2013 #4
    From the beginning

    t-w = -ma

    t=m(g-a) = 0.5(9.81-0.44)

    = 4.683n so t = 4.683n

    w=mg so w = 0.5x9.81 = 4.905n

    S0 t-w = -ma

    Checking...... 4.683-4.905 = -0.5 x 0.444

    Correct... i think.
  6. Aug 29, 2013 #5


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    Gold Member

    It is convenient to denote the positive direction in the direction of acceleration. In this case, the mass accelerates downwards so take down to be positive. This gives mg - T = ma. Solving gives T = m(g-a). It is difficult to see where you went wrong in your attempt.
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