- #1
jamesdocherty
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Homework Statement
Let R be the solid region that is bounded by two spheres x^2 + y^2 + z^2=1 and x^2 + y^2 + z^2=2. Determine the moment of inertia of R around the x-axis if the mass density per unit volume of R is u=sqrt(x^2 + y^2 + z^2).
Homework Equations
Moment of Inertia around the x-axis: triple integral (y^2+z^2)*u(x,y,z) dxdydz
using (p,@,theta) for spherical coordinates as it will be faster to type, sorry for any confusion.
spherical coordinates:
x=psin(@)cos(theta)
y=psin(@)sin(theta)
z=pcos(@)
The Attempt at a Solution
Using the Moment of Inertia around the x-axis equation, i first got the equation:
triple integral (y^2+z^2)*sqrt(x^2 + y^2 + z^2) dxdydz
now converting to spherical coordinates i got:
triple intergal (p^2*sin^2(@)*sin^2(theta) + p^2*cos^2(@))*sqrt(p^2*sin^2(@)*cos^2(theta) + p^2*sin^2(@)*sin^2(theta) + p^2*cos^2(@)) dpd@dtheta
After some simplification:
triple intergal p^3(sin^2(@)sin^2(theta) + cos^2(@)) dpd@dtheta
where 1<p<sqrt(2) and 0<@<pi and 0<theta<2pi
the 1<p<sqrt(2) is because of the two radius and the other two because its a sphere and its centred at the origin, i know 0<theta<2pi is correct but I'm not sure if 0<@<pi or 0<@<2pi.
solving this integral i got 9*pi^2/4 which I'm not sure is correct as I'm not sure if I'm even able to do it this way, any tips or help would be much appreciated !