These are tow pulleys, and a plastic plank is placed above them which can slide freely. The plank has uniform density and it touches both the pulleys. Both pulleys are placed on the same plane, and the rod lies on them horizontally. The pulleys can be rotated in such a way, that they wove in opposite directions with the same angular velocity.
If we move the A pulley in the anticlockwise direction the plank falls off the apparatus in the left direction. But if we rotate the A pulley in the clockwise direction the plank keeps on moving to and fro. What is the reason behind this?
You can watch the video here, if you need more clarity of the question. http://www.youtube.com/watch?v=GlrreXLkzrk