M-Rot 3: Conservation of angular momentum

PIRA: 1Q40.30

Equipment: rotating platform, with safety rail attached, capable of supporting a man, two heavy cylindrical iron masses, bicycle wheel with rim loaded with lead wire, two or three wooden wedges. The platform, bike wheel and masses are kept in the room between the doors to Rooms 3 and 5.

Procedure: Part I: The demonstrator carrying the heavy masses steps on the platform as a students assistant holds the platform from rotating. When all is stable the demonstrator extends his arms, holding a mass in each hand. The assistant stands outside the reach of the extended hands and by pushing on one of the hands starts the demonstrator rotating slowly about the vertical axis. The assistant steps back. When the demonstrator pulls the masses in toward himself his speed of rotation markedly increases. He slows down by extending the arms again and gets the assistant to stop him by grabbing the extended hand while standing far enough out to avoid any possibility of being struck in the head by one of the moving masses. A slow rotation once around in the opposite direction will usually cure any dizziness that may result from this experiment.

Part II: The demonstrator achieves first a stable motionless position with one of the masses aboard (as in part I). He then passes this mass quickly about his body in one direction. As a consequence his body and platform rotate in the opposite direction. If the mass is close in to the body he has to make many revolutions of the mass in order to rotate himself appreciably, but if the mass is at arm's length his body rotation is considerable. For this to be successful the platform should be well levelled by using the wedges under the base of the platform until a person standing on the rim of the platform has no tendency when started in slow rotation to settle to a stop headed in one preferred direction.

Part III: The demonstrator achieves a stable motionless position on the platform, this time with the specially loaded bicycle wheel aboard. While the assistant continues to hold the platform stationary he starts the wheel spinning about a horizontal axis pointed away from him. The assistant releases the platform and he should remain stationary. When the axis of rotation of the wheel is changed from horizontal to vertical, the platform and demonstrator will rotate in the sense opposite to that of the wheel's rotation on its shaft in its vertical position. Here, you can discuss the components and vector properties of angular momentum.

In all of these demonstrations it is best to stand stably directly over the axis of the platform and to maintain one's balance independent of any safety rail. When students are allowed to experiment with it, and I think they should, be sure to caution them emphatically to stay at all times out of reach of the arms of the person on the platform; also, to avoid horseplay and any attempt to see how fast one can spin another.