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. When the cord is pulled the spool will roll toward or away from the direction of the applied force or it will slide in the direction of the applied force. If a line along the cord is drawn 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 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. 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."
EQUIPMENT: Spool. Stored in mechanics.
SETUP NOTES: None