Gravitational potential for a flat disk with radius 1 and unit density, obtained by integrating the Green’s function for the Poisson equation from class.

u[r_]:=Piecewise[{{((1-(r^2))/4),r<=1},{((-1)/2)Log[r],r>1}}]

ParametricPlot3D[{r Cos[θ],r Sin[θ],u[r]},{θ,0,2π},{r,0,10},AxesLabel->{"x","y","z"},BoxRatios->{1, 1, 1}]