A point mass `m` is released from rest at a distance of `3R` from the centre of a thin-walled hollow sphere of radius `R` and mass `M` as shown. The hollow sphere is fixed in position and the only force on the point mass is the gravitational attraction of the hollow sphere. There is a very small hole in the hollow sphere through which the point mass falls as shown. The velocity of a point mass when it passes through point `P` at a distance `R//2` from the centre of the sphere is