Suppose a holes is drilled completed through the earth along a dimeter. Mass and radius of Earth are `M` and `R`. What is the force acting on a body of mass `m` at a distance `r` from the centre of Earth?

Explanation

If `d` is the depth of the body from the surface of
Earth then `r = R - d`
and `mg' = mg (1 - d//R) = mg (R - d) //R`
`= mg r//R`