Answer: 8 years.
- A 64 years
- B 16 years
- C 4 years
- D 8 years
Correct answer: D. 8 years
Explanation: By Kepler's third law T2 ∝ r3, so T ∝ r3/2 = 43/2 = 8 Earth years.
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.