The variation of g with depth d below Earth's surface (assuming uniform density) is

1. Inside a spherical Earth with uniform density, only mass within radius (R − d) attracts the object. 2. That enclosed mass scales as (R − d)³. 3. Combined with the 1/(R − d)² dependence, effective g_d = g × (R − d)/R. 4. So g_d = g × (1 − d/R). At the centre (d = R), g = 0. _Source: syllabus Physics, Ch 7 "Gravitation", §7.4_