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Homework Help: Calculating angular momentum of a sphere

  1. Oct 13, 2014 #1
    I've tried this problem at least 10 times using the equations in my book and I keep getting the same answer. What am I doing wrong?
    1. The problem statement, all variables and given/known data
    A solid steel sphere of density 7.6 g/cm3
    mass 0.2 kg spin on an axis through its center
    with a period of 1.5 s.
    Given Vsphere =4/3π^3, what is its angular momentum?
    Answer in units of kg m2/s

    2. Relevant equations
    Angular momentum=Moment of inertia*angular velocity
    Moment of inertia for a solid sphere=2/5MR^2
    Angular velocity=2π/period
    3. The attempt at a solution
    Angular velocity=2π/1.5= 4.18879rad/s
    Volume of sphere=.2kg/.0076kg/cm^3=26.3158cm^3
    Radius of sphere=[26.3158cm^3/(4π/3)]^(1/3)=1.845197cm =0.01845197meters
    I=(2/5)(.2kg)(0.01845197m)^2=.000027238 kg/m^2
    Angular momentum=(.000027238kg/m^2)(4.18879rad/s)=0.000140973359kg/m^2/s

    I thought my answer was correct but my university's homework page keeps telling me that is it wrong. Anyone know where I am going wrong in this problem?
  2. jcsd
  3. Oct 13, 2014 #2
    Nevermind, I guess I typed in the answer wrong. My solution was correct.
  4. Oct 13, 2014 #3


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    Homework Helper

    The numerical result is correct, but the unit is wrong. The unit of moment of inertia is kgm2. And give the result in normal form (scientific notation) with two significant digits.

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