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Rotational Kinetic Energy and Work

  1. Nov 16, 2006 #1
    [​IMG]

    The puck in Figure P8.51 has a mass of 0.120 kg. Its original distance from the center of rotation is 40.0 cm, and it moves with a speed of 85.0 cm/s. The string is pulled downward 15.0 cm through the hole in the frictionless table. Determine the work done on the puck. (Hint: Consider the change of kinetic energy of the puck.)

    I tried doing it by finding the Kinetic Rotational Energy before and after but its not coming out right. For the KE after do I use the radius as 15, or is it 40-15?
     
  2. jcsd
  3. Nov 16, 2006 #2

    Doc Al

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    Staff: Mentor

    The puck ends up 15 cm closer to the center, so its new radius will be 40-15 cm.
     
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