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Question:

The height at which the acceleration due to gravity becomes g/9 (where g = the acceleration due to gravity on the surface of the earth) in terms of R, the radius of the earth, is:

2 R

R/2

R√2

√2 R

Solution:

GM/9R² = GM/(R+h)²
→ 3R = R+h
→ h = 2R