Question

why is the value of g is zero at the centre of the earth

Answer 1

At the Centre of earth the weight of all particles act perpendicularly to each other this net moment of force is zero and thus no acceleration is seen..

Answer 2

Earth is like a magnet. As a magnet, magnetic force is present on its pole as coming center magnetic force decreases and at center it becomes 0. Same as earthenware the gravitational force is 0 at the center