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