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Homework Help: Understanding Inner and Outer Radius

  1. Jun 20, 2011 #1
    Im new to these forums and i'm looking forward to becoming a regular contributor. I'm attempting to prepare for physics in the fall and I have a textbook problem that i understand the process of solving but not understand the concept.

    Heres the question:
    What mass of a material w/ density p is required to make a hollow spherical shell having inner radius r1 and outer radius r2?

    I understand everything besides the inner and outer radius part. What is the textbook trying to say with that statement. How do you know to subtract them? So if you understood to subtract the radii can you please explain your thought process?

    Again thanks a alot.
  2. jcsd
  3. Jun 20, 2011 #2


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    'Hollow' means 'It ain't dere no mo.'

    If I have a 'hollow spherical shell', it means the outer surface of the sphere has a radius 'r2' for instance and the inside surface of the hollow spherical shell, which isn't there, has a radius of 'r1' for instance.
  4. Jun 21, 2011 #3


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    The difference between the two radii is the thickness of the shell. You can determine the volume of the shell by subtracting the volume of a sphere of radius r1 from the volume of a sphere of radius r2. Once you know the volume of the shell, you can calculate its mass with a given density or vice-versa.
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