Two hollow spheres have radii a and b (b > a), and their common centre O is fixed. A rigid ball of radius 1 2 (b - a) can move in the annular space between the spheres and is gripped by them so that it does not slip.
The spheres are made to rotate with constant angular velocities ω1, ω2 respectively. Show that the ball must move in a circle whose plane is perpendicular to the vector aω1 + bω2.