Our physics teacher was teaching us about gravitation. He said that the force of gravity in a uniform solid sphere is due to the mass of the smaller sphere inside
(On whose surface it sits)
And that the effect of
Force due to the ring/shell outside is zero
And thus the formulas
$$gd = 4/3 × πρ × (R – d) G.$$
When we asked him why the force due to ring was zero,
It was because the shell exerts equal force on the object in each direction which get cancelled
I am a bit unconvinced by this explanation
For the ring/shell to exert equal force in all directions on the object [that would cancel each other], the object should be placed at the center of the shell which is also the center of the sphere
However the object is only at a depth, d less than the sphere's radius.
This implies inequal force from all directions that do not cancel each other rrsulting in different values
Where am I wrong?