Let, gd be the acceleration due to gravity at depth d from the ground.
We know that gdg=(1−dR)
Here, d=R2
∴gdg=⎛⎜
⎜
⎜⎝1−R2R⎞⎟
⎟
⎟⎠=12
If W be the weight of the body on the surface of Earth and Wd be the weight of the body at depth d
then, WdW=mgdmg=gdg=12
or Wd=12W=12×250=125N