At a depth d below the Earth's surface (d ≪ R), the acceleration due to gravity is approximately:

1. For a uniform-density Earth, only the mass enclosed within radius (R − d) attracts an object at depth d. 2. Mass enclosed scales as the cube of the radius: M(R−d) = M · (R − d)³ / R³. 3. g(d) = G M(R−d) / (R − d)² = G M (R − d) / R³. 4. Simplify: g(d) = g · (1 − d/R). 5. So gravity decreases LINEARLY with depth, while it falls off as (1 − 2h/R) with height — different from below. _Source: syllabus Physics Ch 7 "Gravitation", §7.6 g below the surface, p. 133_