TOPIC:

ID Code: D1-84
Purpose: A counterintuitive demonstration of rotational dynamics.
Description: A 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 the tube do after the plastic sheet has been removed? (a) move to the left, (b) move to the right, or (c) stop and remain where it was when it left the plastic sheet.
A: The aluminum tube moves to the left and spins clockwise as the plastic sheet is removed, then very quickly ceases both its spinning motion and its translation after the plastic sheet is gone! One explanation for this uses a conservation of angular momentum argument. As it is yanked out from under the aluminum tube, the plastic sheet applies no net torque to the tube around the point of contact with the plastic sheet, because the distance between the sheet and the tube is zero. Two components of angular momentum around the contact point can be identified: that due to the clockwise rotation of the tube, and that due to the linear velocity of the tube to the left. The net angular momentum of the tube around the point of contact of the tube with the plastic sheet, however, is zero just before the tube leaves the plastic sheet. Likewise, because the plastic sheet is very thin, there is no net angular momentum around the point of contact of the tube with the table just after the tube leaves the plastic sheet. Sliding friction of the o-rings on the tube with the table causes both the rotation and the translation to quickly cease.

Availability: Available

See the Physics Question of the Week for 9/15/2014 for an example of how this demonstration can be used in class interaction and discussion: lecdem.physics.umd.edu/question-of-the-week-archive.html