13.2.2—Newton’s law of gravitation F = Gm1m2 / r2 for the force between two point

Two point masses attract with F=Gm₁m₂/r² along the line joining their centres, where G is the universal gravitational constant.

Use centre-to-centre separation and treat the force as attractive. For several masses, add force vectors.

Doubling one mass doubles force; doubling separation reduces force to one quarter.

The inverse-square law is not inverse distance, and the force pair acts on both masses with equal magnitude in opposite directions.