Problem 1.206

Find the potential energy of the gravitational interaction (a) of two mass points of masses $m_{1}$ and $m_{2}$ located at a distance $r$ from each other; (b) of a mass point of mass $m$ and a thin uniform rod of mass $M$ and length $l$, if they are located along a straight line at a distance $a$ from each other; also find the force of their interaction.