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Moment of Inertia HELP

  1. Nov 25, 2008 #1
    1. The problem statement, all variables and given/known data
    A wheel mounted on an axis that is not frictionless is initially at rest. A constant external torque of +49 N•m is applied to the wheel for 18 s, giving the wheel an angular velocity of +500 rev/min. The external torque is then removed, and the wheel comes to rest 120 s later.

    (a) the moment of intertia of the wheel
    (b) the frictional torque, which is assumed to be constant.

    2. Relevant equations
    angular acceleration *a* = angular velocity/ t = *w*/t
    *a* = *t*/I (torque / Moment of Inertia)
    I = (*t*t)/w
    *t* fric = *t*t2/t1

    3. The attempt at a solution

    (a) For part a, I used the I = equation and got 16.844 kgm^2
    (b) For part b, I used the equation *t* fric = and got 2.53 Nm

    Please help me with this problem. I believe that the answers are wrong...
  2. jcsd
  3. Nov 25, 2008 #2
    I think you solve this as 2 equations with 2 unknowns. Let T be frictional torque and I be moment of inertia.

    1) wheel speeds up: (49 - T) * 18 / I = 500

    2) wheel slows down: 500 / 120 = T/I
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