PURPOSE: A counterintuitive demonstration of rotational dynamics.

DESCRIPTION: An six-inch diameter thin-walled aluminum tube with O-rings tightly wound around each end lies on a long plastic sheet. The plastic sheet is then quickly pulled horizontally out from under the tube. Q: What will be the motion of the tube after the plastic sheet has been removed? A: The aluminum tube spins as the plastic sheet is removed, then stops spinning after the plastic sheet is gone, but remains at the same position!

One explanation for this uses a conservation of angular momentum argument. Because the plastic sheet is very thin, as it is yanked out from under the aluminum tube it actually applies no torque to the tube around the point of contact, so the net angular momentum of the tube around the point of contact of the tube with the plastic sheet, and thus the table, is zero. Therefore, while friction with the table causes the rotation to cease after the plastic tube has been removed, there is likewise no torque applied to the tube around the contact point. Hence, at no time can the tube begin to roll!

SUGGESTIONS: Yank rapidly. Do it by an edge of the lecture table to enhance the excitement.


EQUIPMENT: Thin-walled aluminum tube fitted with O-rings, long plastic sheet.


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