Solution
= Solution
{c}
For the surface, a source element has <mass> $dm=\sigma\,dS'$. Superposing its <Newtonian gravitational potential> gives the <surface integral>
$$
\boxed{\phi(\mathbf r)=-G\sigma\iint_S\frac{dS'}{|\mathbf r-\mathbf r'|}.}
$$
Here $dS'$ is the source <surface area element>, $\mathbf r'$ the source <position vector>, and $G$ the <gravitational constant>. The same zero-at-infinity convention applies.