Two wheels and a ball problem confusion When I was preparing for a job interview for chemistry, the following question appeared:

Now, my answer was as such:

If the bigger wheel is rotated clockwise, the smaller wheel moves anticlockwise so the ball will move down and fall off the plank.

However, the answer (which was pretty brief) only mentioned the Centre of Masses being inline, so it will stay level. 
I'm just left quite confused. Why is the Centre of Mass being "level" have any effect on this system?
 A: The drawing is not entirely clear, but I'll try...
Assuming that the black dots are bearings with axis normal to the picture, both blue wheels can rotate freely. The rotational motion of the small blue wheel is generated through an horizontal friction force. This force do not causes motion of the green bar since it is contained in the horizontal plane. The friction force has no vertical component because it is tangent to the contact surfaces. Therefore, there is no mechanism to impose any motion to the ball. If there is no ball motion and the wheels center of mass is in its centers, then the system's center of mass is not changed.
However, if the center of mass of the small wheel is not in its center, and assuming the big wheel is free to rotate but not to translate, then a small centrifugal force will move the ball in the vertical direction.
Note that the ball is not in a stable equilibrium position. Therefore any perturbation in the horizontal plane will make the ball move in this plane.
