A popular demonstration involves placing a yo-yo on a level table and gently pulling the string horizontally when it is wrapped to come out below the center of the yo-yo's axis. Students are then asked to predict which way the yo-yo will move.1 A similar demonstration is performed with a tricycle by pulling forward on a pedal with the pedal down in its lowest position.2,3 As well as pulling the yo-yo horizontally, often the string is lifted until the angle it makes with the table causes no motion. This occurs when the line extended from the string intersects the point of contact of the yo-yo with the table.4 This paper describes an apparatus that extends these demonstrations to the situation where the force pulling the yo-yo is still horizontal yet is below the level of the table.

Topics
Point contacts, Friction, Educational aids
1.
Many yo-yo discussions can be found on the web. Point your browser to: “Horizontal yo-yo pull.”
A long discussion (without illustrations) can be found at: www.physicsforums.com/ archive/index.php/t-279432.html. Eventually, the correct answer emerges.
2.
“
Little Stinkers: The tricycle problem
,”
Phys. Teach.
 
1
,
193
(
Oct. 1963
).
4.
The following two URLs include demonstrations and discussions of the situation where the string is held at an angle to the horizontal: www.wired.com/wiredscience/2010/01/yo-yo-rolling-sliding-pulling/ and www.physicsforums.com/ showthread.php?t=420430.
© 2013 American Association of Physics Teachers.
2013
American Association of Physics Teachers
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