a. Find a potential function for the gravitational field $$\mathbf{F}=-G m M \frac{x \mathbf{i}+y \mathbf{j}+z \mathbf{k}}{\left(x^{2}+y^{2}+z^{2}\right)^{3 / 2}}$$ $(G, m, \text { and } M$ are constants).
b. Let $P_{1}$ and $P_{2}$ be points at distance $s_{1}$ and $s_{2}$ from the origin. Show that the work done by the gravitational field in part (a) in moving a particle from $P_{1}$ to $P_{2}$ is