A ring of mass m and radius R and a sphere of mass M and same radius R are separated by a distance √8R as shown in the figure. The force of attraction between the ring and the sphere is
2√227GmMR2
The sphere will behave as a point mass placed at its centre. So, effectively we have to find the force exerted on a point mass M at a distance √8R on the axis of the ring.
Now, electric field at point B because of the ring, E=Gm√8R[R2+(√8R)2]32=2√2Gm27R2
Now, force on the sphere, F=ME=2√2GmM27R2
Hence, the correct choice is (a).