In a solid sphere of mass $M$ and radius $R$, a spherical cavity of radius $\frac{R}{2}$ is made, such that the centre of cavity is at a distance $\frac{R}{2}$ from the centre of the sphere. A point mass $m$ is placed inside the cavity, at a distance $\frac{R}{4}$ from the centre of the sphere. The gravitational pull between the sphere and the point mass $m$ is