Law of Gravitation

Every two masses pull on each other, along the line joining their centres:

$$F = G\,\frac{m_1 m_2}{r^2}$$

$r$ runs centre to centre, and $G = 6.67 \times 10^{-11}\ \mathrm{N \cdot m^2/kg^2}$. Both bodies feel the same size of force, toward each other.

Equal pulls toward each other; r runs centre to centre.

The force falls with the square of the distance: twice as far gives $\relax 1/2^2 = 1/4$ of the force, three times as far $\relax 1/3^2 = 1/9$. Doubling one of the masses doubles the force.