1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Density and moment of Inertia

  1. Jun 5, 2005 #1
    A sphere consists of a solid wooden ball of uniform density 800 kg/m^3 and radius 0.20 m and is covered with a thin coating of lead foil with area density 20 kg/m^2.

    How do I calculate the moment of inertia of this sphere about an axis passing through its center?

    ** The equation I'm using for the solid sphere is I = 2/5 * M * R^2
    Here again I know the mass of the inner sphere is the density times the volume of the sphere. They gave me the area density for the layer of lead, so you can find the mass of the lead using the area density times the surface area of the sphere. By definition of moment of inertia, the total moment of inertia of the sphere is the sum of the moment of inertia of the solid wood part, plus the moment of inertia of the lead shell. Is this method wrong? Do you not get the moment of density this way without being given mass but instead given density?
    Please help.
     
  2. jcsd
  3. Jun 5, 2005 #2

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    No, this is a correct method.
    I'm sorry, I don't understand your question.
     
  4. Jun 5, 2005 #3
    The answer= MI of wooden solid sphere+ MI of lead shell
     
  5. Jun 5, 2005 #4
    volume of sphere: 4/3 * pi * r^3
    surface area sphere: 4 * pi *r^2

    mass of lead: 20 * (4 * pi * 0.2^2) = 10 kg
    mass of sphere: 800 * (4 * pi * 0.2^2) = 26.8 kg

    I solid sphere: 2/5 * 26.8 * 0.2^2 = 0.429
    I lead shell: what do I use?
     
  6. Jun 5, 2005 #5
    [tex]I = \frac{2}{3}MR^2[/tex]
     
  7. Jun 5, 2005 #6

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    Incidentally,the sphere has zero volume and its moment of inertia wrt an axis passing through its center is given in post #5.

    BTW,what do you mean by solid sphere,is it a special sphere,i dunno,similar to a plasma or liquid one...?

    Daniel.
     
  8. Jun 5, 2005 #7
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Density and moment of Inertia
Loading...