hmmm. mass of earth is M=6e24 kg, radius R=6.4e6 m, G=6.7e-11 m^3 kg^-1 s^-2. so the "energy debt" on the surface (the gpe) is GM/R=6.3e7 J. he's equating th

hmmm. mass of earth is M=6e24 kg, radius R=6.4e6 m, G=6.7e-11 m^3 kg^-1 s^-2. so the "energy debt" on the surface (the gpe) is GM/R=6.3e7 J. he's equating that to an equivalent height H for 1kg in a constant field of 9.8ms^-2, so H=6.4e6 m. so the 6,400 km number is correct (the inset) (curiously, this number is also the radius of the earth, approx). perhaps the value in the main figure is a mistake, or he is folding in some other effect (i can't work out what, though). edit: i see that he gives a formula in the well for jupiter, which is equivalent to what i just did, so i think the main diagram's value for earth is a mistake.