What is the gravitational force on a particle at the centre of the Earth?

Acceleration due to gravity at a height $h$ is equal to that at a depth $d$ below the surface of the earth,if

The diameters of two planets are in the ratio $4:1$. Their mean densities have the ratio $1:2$. The ratio of gravitational acceleration on the surface of the planets will be:

Considering Earth to be a sphere of radius $R$ having uniform density $\rho$,the value of acceleration due to gravity $g$ in terms of $R$,$\rho$,and $G$ is:

If it is assumed that the spinning motion of the Earth increases,then the weight of a body on the equator

$A$ body weighs $200 \; N$ on the surface of the earth. How much will it weigh halfway down to the centre of the earth (in $; N$)?