(a)[2]
State Newton’s law of gravitation in words.
(b)[3]
Neptune has eight moons (satellites). Each moon moves around Neptune in a circular orbit of radius $r$ with period $T$. Assuming that Neptune and each moon can be treated as point masses, show that $r$ and $T$ are linked by $GM_{N} = \frac{4\pi^{2} r^{3}}{T^{2}}$, where $G$ is the gravitational constant and $M_{N}$ is the mass of Neptune.
(c)
Fig. 1.1 gives the data for the moon Triton, which orbits Neptune, and for the moon Oberon, which orbits the planet Uranus.