When a satellite in a circular orbit around the earth enters the atmospherice region, it encounters small air resistance to its motion. Then

A cavity of radius$\frac{R}{2}$ is made inside a solid sphere of radius $R$. The centre of the cavity is located at a distance $\frac{R}{2}$ from the centre of the sphere. The gravitational force on a particle of mass $m$ at a distance$\frac{R}{2}$ from the centre of the sphere on the line joining both the centres of the sphere and the cavity is (opposite to the centre of cavity). [$\mathrm{Here}, g=\frac{GM}{R^2}$ where $M$ is the mass of the solid sphere.]

A solid sphere of uniform density and radius 4 units is located with its centre at the origin $\mathrm{O}$ of coordinates. Two spheres of equal radii 1 unit, with their centres at $\mathrm{A}(-2,0,0)$ and $\mathrm{B}(2,0,0)$ respectively, are taken out of the solid leaving behind spherical cavities as shown in figure. Then :-