# 5.8.7: Solid Cylinder

Refer to figure \(\text{V.8}\). The potential from the elemental disc is

\[dψ = -2 \pi G ρ δ z \left[ \left( z^2 + a^2 \right)^{1/2} - z \right] \label{5.8.21} \tag{5.8.21}\]

and therefore the potential from the entire cylinder is

\[ψ = const. - 2 \pi G ρ \left[ \int_h^{h+l} \left(z^2 + a^2 \right)^{1/2} dz - \int_h^{h+1} z dz \right]. \label{5.8.22} \tag{5.8.22}\]

I leave it to the reader to carry out this integration and obtain a final expression. One way to deal with the first integral might be to try \(z = a \tan θ\) . This may lead to \(\int \sec^3 θ dθ\). From there, you could try something like \(\int \sec^3 θ = \int \sec θ d \tan θ = \sec θ \tan θ - \int \tan θ d \sec θ = \sec θ \tan θ - \int \sec θ \tan^2 θ d θ = \sec θ \tan θ - \int \sec^3 θ + \int \sec θ d θ\), and so on.