The Moon, whose mass is $7.35 \times 10^{22} \mathrm{~kg}$, orbits the Earth, whose mass is $5.98 \times 10^{24} \mathrm{~kg}$, at a mean distance of $3.85 \times 10^{8} \mathrm{~m}$. It is held in a nearly circular orbit by the Earth-Moon gravitational interaction. Determine the force of gravity due to the planet acting on the Moon.
From the universal law of gravitation
$$
F_{G}=G \frac{m M}{R^{2}}
$$
we get
$$
F_{G}=6.673 \times 10^{-11} \mathrm{~N} \cdot \mathrm{m}^{2} / \mathrm{kg} \frac{\left(7.35 \times 10^{22} \mathrm{~kg}\right)\left(5.98 \times 10^{24}\right)}{\left(3.85 \times 10^{8} \mathrm{~m}\right)^{2}}
$$
which yields
$$
F_{G}=1.98 \times 10^{20} \mathrm{~N}
$$
This is also the force on the Earth due to the Moon, and the force on the Moon due to the Earth.