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Angular Momentum of a rotating ball

  1. Nov 17, 2013 #1
    1. The problem statement, all variables and given/known data
    What is the angular momentum of a 0.210-kg ball rotating on the end of a thin string in a circle of radius 1.35m at an angular speed of 10.4 rad/s?


    2. Relevant equations
    I am using L = Iω


    3. The attempt at a solution
    I put I = (2/5)(0.210 kg)(1.35m)^2
    ω = 10.4 rad / sec
    When I try multiplying these out, I'm not getting the right answer. I come up with 1.592 kgm^2/sec and the answer is 3.98
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 17, 2013 #2
    You're using the moment of inertia of a sphere, But it isn't the sphere that's spinning. The ball is essentially a point mass at the end of the string. The moment of inertia for a point mass is [itex]I=mr^2[/itex]. Try this and you will get the answer.
     
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