Ball on string mounts on blackboard and pivots on lower rod.

The Simple Pendulum. The 'bob' (ball) of the simple pendulum is released at 'A', and a rod catches the string at 'B'. It is observed that regardless of the position of 'B', the bob always rises to the level of 'A'. Note: The chalkboards are magnetic. The switch on the clamp rotates an inner magnet, bringing the magnet closer to or farther away from the mounting surface ... attaching or releasing the clamp.