A uniform ball of mass M and radius a can roll without slipping on the rough outer surface of a fixed sphere of radius b and centre O. Initially the ball is at rest at the highest point of the sphere when it is slightly disturbed. Find the speed of the centre G of the ball in terms of the variable θ, the angle between the line OG and the upward vertical. [Assume planar motion.] Show that the ball will leave the sphere when cosθ=1710.