Gravitational Potential

I'm not understanding this integral quite fully. Of course G is the constant and rho is also a constant since the sphere is homogeneous. And pi*r'^2 dr' gives the area of circle with respect to r' with limits of a and b. However, I'm not understanding how theta comes into it and the ignoring of phi. I understand the rest of the derivation.

Of course, the integral from a to b of pi*r'^2 dr' gives an area for the annulus in the plane. I guess I'm having trouble seeing the three-dimensional derivation. [Broken] [Broken]
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They use spherical coordinates.
A unit of volume in spherical c. is
V=\int \int \int \rho^2\ \sin\phi\ dp\ d\phi\ d\theta


phi and theta may be swapped in your text.
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Crap. Duh. I forgot about the transformation. Thanks.

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