13.3.2—G = GM / r 2

For a spherical source outside its surface, g=GM/r² in N kg⁻¹ or m s⁻².

Use M for the source and r from its centre. The inverse-square relation is a scaling law, not a claim that g changes linearly with height.

At twice the centre distance, a satellite experiences one quarter of the original field strength.

Do not use altitude alone when the planet’s radius is significant; r is radius plus altitude.