Beads on Rotating Arm

When the arms (rods) are at rest, the beads remain stationary at the bottom of the arms. On setting the arms into rotation the beads slide along the arms and remain stable at some height depending on the speed of rotation. The beads rise until their displacement from the central shaft, their axis of rotation, is a maximum. Speed Governors that are used to limit the maximum speed of heavy duty vehicles work on this principle.

In this exhibit there are two types of arms, one linear and the other curved connecting a loop and the central rod. The beads are placed such that they can be moved along the arms. The whole system can be set into rotation using the shaft provided.

When gravity alone is acting on the beads, they lie close to the central rod i.e, bottom of the arms. When the system is set into rotation, they get pushed away from the central rod. But they are not free to fly away. Rather they are constrained to move on the arms. The friction between the surface of the beads and the arms also acts as a driving force and the beads take the direction of the net force acting on them.

The beads move until they reach a position of maximum displacement with respect to their initial resting position and remain there as long as the system is rotating. Once the rotation is stopped, they drop back to their initial position.

Centrifugal governors too work on the same principle. The governors have balls that rise up and move away from the rotation axis as the speed of the vehicle increases. When the speed of the vehicle reaches its maximum limit, these balls reach a position that is maximum displaced from their rest position. Apart attached to these moves up and obstructs the fuel flow and thus automatically restricts the speed of the vehicle from going beyond the limit.