Newton's Law of Gravitation states that two bodies with masses $M$ and $N$ attract each other with a force $$F=G\frac{MN}{r^2},$$ where $r$ is the distance between the bodies and $G$ is the gravitational constant. If one of the bodies is fixed, find the work needed to move the other from $r = a$ to $r = b$. Your answer could contain $G, M, N, r, a,$ and $b$.

Work done =