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Homework Help: Mass center problem

  1. Jan 22, 2006 #1
    question:
    three uniform disks of the same mass per unit area, and radii a, 2a, 3a are placed in contact with each other with their centers on a straight line. how far is the center of mass of the system from the center of the smallest disk?

    solution:
    m1=pi a^2
    m2=4pi a^2
    m3=9pi a^2

    Xc = (m1/mtotal)Xo + (m2/mtotal)(Xo+3a)+(m3/mtotal)(Xo+8a)
    Xc = 1/14Xo + 4/14Xo + 6/7a + 9/14Xo + 72/14a
    Xc = Xo + 6a

    therefore center of mass of system is 6a from the center of the smallest disk.

    is this correct?
     
  2. jcsd
  3. Jan 22, 2006 #2

    Tide

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    No, that is not correct. Think about what your answer means if a is made arbitrarily small. Also, it appears there is missing information in the way you state the problem. Don't you suppose the thickness of the disks matters?
     
  4. Jan 23, 2006 #3
    i dont really understand whats wrong with it
    but here is the diagram, i guess i did miss this

    http://img63.imageshack.us/img63/9140/phys27rv.jpg [Broken]
     
    Last edited by a moderator: May 2, 2017
  5. Jan 23, 2006 #4

    Tide

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    braindead,

    I misunderstood your original description of the arrangement. I had them stacked one on top of the other - thanks for clarifying with your drawing.

    With the revised configuration, yes, your answer is correct!
     
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