1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Gravitational quadrupole moment

  1. Oct 24, 2011 #1
    1. The problem statement, all variables and given/known data

    The surface of a homogeneous body of mass m is the ellipsoid
    (x/a)^2 + (y/b)^2 + (z/c)^2 = 1 with a,b,c>0.

    What is the gravitational quadrupole moment dyadic of this body?

    2. Relevant equations

    This is my first confusion: what is the right equation?

    In my Clssical Mechanics lect notes, it says

    Q = ∫[itex]\rho[/itex](r)(3r.r-r^2.1) dr

    where 1 is the unit dyadic. But there is no hint on how to apply this formula!

    3. The attempt at a solution

    Since I am concurrently doing EM, I remember using Legendre polynomials to solve both electric/magnetic quadrupole expansion-type question. So the question is, is it a must to use that weird integral in 2. above or can the answer be obtained using Legendre polynomials and somehow expanded into a 3 * 3 matrix?

    Thank you very much in advance!!!

    P.S. also I usually solve my doubts by browsing online, but on gravitational quadrupole moment I cant find any links at all that show how to do the integration. What should I search under? GR? Or something else. Thanks once again.
  2. jcsd
  3. Oct 26, 2011 #2
    You dont need GR or anything the like. All you need is to calculate that integral over the volume of the ellipsoid. As the ellipsoid is homogenous the mass density is just a constant.
    So calculate that integral over an ellipsoid which is an exercise of vector analysis. I would suggest switching to ellipsoidal coordinates. It's pretty ugly...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook