Answer: -2GM/a.
- A -GM/2a
- B -GM/a
- C -2GM/a
- D -4GM/a
Correct answer: C. -2GM/a
Explanation: Each mass is a distance a from the midpoint. Potentials add as scalars: V = -GM/a - GM/a = -2GM/a.
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.