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Homework Help: Moment of inertia qn

  1. Sep 7, 2009 #1
    1. The problem statement, all variables and given/known data
    find I of a horizontal rod, with radius a and length L, with axis through the centre in the z direction

    3. The attempt at a solution
    ok can someone plz help me set up the integral for this?

    i got rho=m/v=m/2pi*a^2*L, I=int(rho*r^2)dV, i used r=rho for cylindrical=L/2

    I=(m/2pi)int from -a to a, 0 to 2pi, 0 to L/2(rho^2/a^2*L)drho dtheta dz, that gave me (mL^2)/12, which apparently is the answer for a thin rod, of infinite thinness... according to wiki... but my answer is supposed to be m((L^2/12)+(a^2/4))... can anyone help me understand this better... because im getting confused with my dimensions.
  2. jcsd
  3. Sep 7, 2009 #2
    sorry, what i did was wrong in the first place...
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