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Homework Help: Kinetics of Rigid bodies

  1. Jan 7, 2012 #1
    1. The problem statement, all variables and given/known data
    The rotor of an electric motor of mass 200 kg has a radius of gyration of 150 mm. Calculate the torque required
    to accelerate it from rest to 1,500 rev/min in 6 seconds. Friction resistance may be neglected.

    2. Relevant equations
    ƩMO = Iα
    I = 1/2 (mr2
    I am really really stuck here, I don't know what equations I need to use, if this were a straight line question I would use v = u + at, but it isn't so that is wrong

    3. The attempt at a solution
    All I have so far is this
    I = 1/2 (mr2
    I = 0.5 * 200 * 0.152
    I = 2.25 kgm2

    Really lost. I just need some pointers please.

    Kind regards

    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jan 7, 2012 #2


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    Science Advisor
    Homework Helper
    Gold Member

    In calculating I, the radius of gyration is not the radius of a disk. You can look up the formula for it, which is [itex]r = \sqrt{I/m} [/itex] (and note how it is derived).

    You can find the angular acceleration α using the kinematic equations of rotational motion (watch units).
  4. Jan 8, 2012 #3
    Brilliant, those are the words I didn't know to find the equation I couldn't find of variables I couldn't relate.

    Found them all.

    Thank you
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