PURPOSE: To illustrate a counterintuitive problem in rotational dynamics.

DESCRIPTION: The cord is wrapped around the smaller radius of a spool and placed on a horizontal surface such that the cord emerges over the top side of the spool, as shown at the left above. When the cord is pulled the spool will move toward the direction of the applied force, the forward direction. Q: When the cord emerges from the bottom side of the spool, as shown at the right, how will the spool move? A: Forward, just as in the previous example. Click your mouse on the photographs above to see mpeg videos of what happens in each case.

This problem can be varied as follows. Imagine a line along the cord such that when the cord is held up at some large angle this line intersects the floor along the line of contact of the large radius rims of the spool with the floor. If the cord is held below this line, the spool will move toward the applied force, in the forward direction. If the cord is held above this line, the spool will move away from the applied force, in the backward direction. If the cord is pulled along this line, the spool will remain in place and spin. Click here to view the action. By holding the cord at this angle while you walk, the spool will slide along the floor as you move, so you can "walk the spool."

SUGGESTIONS: See Question of the Week #50 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 1K10.30) See Demonstration Reference File for further information.

EQUIPMENT: Spool with cord, as photographed.

SETUP TIME: None.