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Moment of inertia of a disc

  1. Dec 16, 2009 #1
    1. The problem statement, all variables and given/known data

    A) a horizontal disc of diameter 12.0cm is spinning freely about a vertical axis through its centre at an angular speed of 72 revolutions per minute. a piece of putty of mass 5.0g drops onto and sticks to the disc at a distance of 4.0cm from the centre. The angular speed reduces to 60 revolutions per minute. calculate the moment of inertia of the disc. No external torques are applied to the system during this process.

    B) A constant tangential force is now applied to the rim of the disc which brings the disc to rest in 6.0 s. Calculate the magnitude of this force.

    C) calculate the rotational energy of the system before and after the putty is added to the disc. comment on your answer.
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    2. Relevant equations



    3. The attempt at a solution

    HELP!!!!!! GOT TILL SATURDAY TO UNDERSTAND IT!!!!!
     
  2. jcsd
  3. Dec 16, 2009 #2

    rock.freak667

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    No attempt? No thoughts?

    Is anything conserved when the putty sticks to the disk?
     
  4. Dec 16, 2009 #3
    momentum?
     
  5. Dec 16, 2009 #4

    rock.freak667

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    specifically angular momentum. Can you apply the law of conservation of momentum to find the moment of inertia of the disk?
     
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