首先你要知道, gravitational force (萬有引力) 係 F = GMm/R^2 where
R = 物體與地球中心的距離 Distance of the object from the earth's centre
M = 地球的質量 Mass of the earth
m = 自由下墜物體的質量Mass of the falling object
G = 地心收力常數gravitational constant
By F = ma, 物體在自由下墜時,下墜物體的加速度是
a = F/m
= (GMm/R^2)/m
= GM/R^2