1.g=GM/R²
M=gR²/G
2.GM/(R+r)²=v²/(R+r)
v=(GM/(R+r))^(1/2)=R×[g/(R+r)]^(1/2)
T=2π(R+r)/v=2π(R+r)/R×[(R+r)/g]^(1/2)