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Homework Help: Simple density law

  1. Sep 26, 2010 #1
    1. The problem statement, all variables and given/known data
    Show that a spherical body with density
    p (r) =Pc(1−x)
    where x = r/R and R its radius, has a central density pc that is 4 times larger than the mean density (< p > =mass/volume).


    2. Relevant equations

    not sure

    3. The attempt at a solution

    im not sure how to start this problem... can someone give me a hit on doing this question. please!
     
  2. jcsd
  3. Sep 26, 2010 #2
    help?plz
     
  4. Sep 26, 2010 #3
    can some1 tell me how to approch this problem ...
     
  5. Sep 26, 2010 #4

    Dick

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    If you know the density of the object at each point, do you know how to compute its mass?
     
  6. Sep 26, 2010 #5
    nop.
     
  7. Sep 26, 2010 #6

    Dick

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    Not knowing any relation between the density function and the mass might be the root of your problem. Could you try and look something up?
     
  8. Sep 26, 2010 #7
    sure
     
  9. Sep 26, 2010 #8
    i know that p=m/v and v=(4/3) pi R^2

    umm could you post some links b/c im googleing it not much tho
     
  10. Sep 26, 2010 #9

    Dick

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    It might be better to check your notes or the textbook. In general you find the mass by doing the triple integral of density*dV. I'll give you a hint that if the density is spherically symmetric then the mass is the integral from 0 to R of density*4*pi*r^2*dr. Does that sound familiar? If you can show that form follows from the density*dV formula by using spherical coordinates, I'll give you extra virtual points.
     
  11. Sep 26, 2010 #10
    let me give it ago and get back
     
  12. Sep 26, 2010 #11
    OMG!!!:surprised thank you soo much... i figured it out! and showed it too..


    RESPECT TO YOU :smile:
     
    Last edited: Sep 27, 2010
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