Gravitational field

The gravitational field strength at a distance $r$ from a mass $M$ is the acceleration it gives to bodies:

$g=\frac{G\,M}{r^2}$

and the gravitational force between masses $M$ and $m$ (Newton's law of universal gravitation):

$F=\frac{G\,M\,m}{r^2}$

where $G=6.674\cdot10^{-11}\ \tfrac{\text{N·m}^2}{\text{kg}^2}$. Both quantities fall off with the square of the distance. The presets contain the masses and radii of Earth, the Moon, Mars and the Sun — for Earth this gives the familiar $g\approx9.81\ \tfrac{\text{m}}{\text{s}^2}$.