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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

1. R/√2
2. R/2
3. √2 R
4. 2R

Correct answer: 2R

Solution

g' = g (R/(R+h))^2 = g/9 -> R/(R+h) = 1/3 -> R+h = 3R -> h = 2R.

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