Answer: r3.
- A r
- B r2
- C r3
- D r4
Correct answer: C. r3
Explanation: Kepler's third law: T2 is proportional to r3 (square of period proportional to cube of orbital radius).
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.