Answer: R.
- A R/2
- B R
- C 2R
- D 3R
Correct answer: B. R
Explanation: g at height h: g' = g/(1+h/R)<sup>2.</sup> For g' = g/4: (1+h/R)<sup>2</sup> = 4. 1+h/R = 2. h = R.
g is maximum at Earth's surface; going up, it falls off as 1/r² (inverse-square); going down, it falls off linearly with depth (since only the mass enclosed within radius r contributes), reaching zero at the centre.
Concept context
Universal gravitation, orbital mechanics, gravitational potential, and escape velocity.