Gravitational Potential Energy

$$U = - \frac{gMm}{r}$$ (5)

Note:
beginequation5pt] a) U = 0 when $r = \infty$
b)) Escape Velocity:

$$1/2mv^2_E = \frac{GMm}{r}\rightarrow v_E = \sqrt{2GM/r}$$ (6)