Law of Gravitation

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

F = G m1 m2 / r²

r runs centre to centre, and G = 6.67×10⁻¹¹ N·m²/kg². 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 1/2² = 1/4 of the force, three times as far 1/3² = 1/9. Doubling one of the masses doubles the force.