Demos: 1J-13 Large Spool
The demonstration capitalizes on the fact that the static friction force always opposes the tendency to slip. A string is wrapped around the inner shaft of a large yo-yo like spool. If the string is pulled horizontal to the table, the spool will roll backward. If the string is pulled almost straight up, the spool will roll forward. At a particular angle the spool will not roll, but will simply spin in place.
First note that the spool, in the absence of a friction force from the table, would spin in the counter-clockwise direction. The static friction force then is generated to the left to oppose that spin. Now take torques about the point of contact. The torque due to friction is zero about this point because there is no moment arm.
(a) The torque about the contact point due to the tension T is hT and is clockwise. The spool rolls to the right.
(b) The torque due to T is rT and is counterclockwise. The spool rolls to the left.
(c) The torque due to T is zero (The direction of T is such that it passes through the contact point). The spool does not roll. However, with sufficient T, the spool can be made to spin in place.
Directions: None specific. The string should be pulled gently, however, to avoid skidding.
Suggestions for Presentation: Begin by asking what happens to a yo-yo when released. The yo-yo unwinds. So in (a), does the yo-yo unwind? (It actually winds up.) Students will likely be surprised that the spool does different things depending on the orientation of the string.
Applications: None that are obvious.