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Finding Rotational Energy and Acceleration of Decending Mass

  1. Nov 3, 2009 #1
    1. A disk with radius of 0.3m and a mass of 4kg is free to rotate about an axle through its center. A cord wrapped around the circumference supports a weight with mass 5kg.

    a) What is the acceleration of descending mass if it is released?

    b) 5 seconds after release what is the rotational energy of the disk?



    2. I disk = (1/2)MR2


    3. I know that Idisk = (1/2)(4)(.3)2 is 0.18. After that I am lost.
     
  2. jcsd
  3. Nov 3, 2009 #2

    kuruman

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    Draw two free body diagrams, one for the disk and one for the hanging mass. Write Newton's 2nd Law pertinent to each FBD.
     
  4. Nov 3, 2009 #3
    Okay that sounds right I have that done but how do we get the rotational energy after 5 seconds??
     
  5. Nov 3, 2009 #4

    kuruman

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    If you know the angular acceleration, you can get the rotational speed at t = 5 s from the kinematic equations and from that the rotational energy.
     
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