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Homework Help: Hollow Sphere

  1. Dec 14, 2006 #1
    How do I get rid of infinitesimal mass element dm?
  2. jcsd
  3. Dec 14, 2006 #2


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    I assume you're trying to derive the moment of inertia of a hollow sphere, but you should really be more specific when posting questions.
  4. Dec 14, 2006 #3
    I am not determining the moment of inertia. I didn't specify what I was doing because I just wish to know of to solve this particularly problem. How to get rid of dm?
  5. Dec 14, 2006 #4


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    Your differential mass is the rate of change in the mass. It will depend on the object's density (g/cm^3, kg/m^3, etc).

    In your case, you have a hollow sphere, so the mass will change in relation to the area (assuming the sphere has an infinitely small thickness). That would be g/cm^2, kg/m^2, etc.

    That should allow you to change your variable to dr, the differential radius, since the volume and/or the area will depend upon the radius.
  6. Dec 14, 2006 #5
    But I need the constant sigma M/A (equivalent to density) which I do not know.
  7. Dec 14, 2006 #6
    Maybe Integrate? I don't really understand your problem.
  8. Dec 14, 2006 #7
    I need to [tex]\int dm[/tex] for a spherical shell.
  9. Dec 14, 2006 #8


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    Look at the thread that started this.
    Last edited: Dec 14, 2006
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