Question

explain why the value of g is zero at the center of the earth

Answer 1

hey mate: g means gravity which earth applies on us and you know that earth applied force and attract us towards it if we are already at the centre then why earth will apply that force...
hope it helps

Answer 2

Because at center the force due to upper part is equal to force due to mass of lower part.As force is a vector quantity.Hence net force is zero.So acceleration due to this force is zero.