The propeller begins to spin when the wooden stick on which it is nailed vibrates.
This is the "magic" of an old traditional toy, very common in Asia and known all over the world with different names: Notched Stick, Hui game, Girigiri−Garigari, Gee−haw whammy diddle, Bozo−bozo.
Generations of kids had fun with it, but the toy has also been an intriguing object of study for the scientific community, as a number of fundamental physical principles are at play in its functioning. One of the first studies on the stick dates back to 1937, while the most recent was published today in Plos One.
The article, “The Notched Stick, an ancient vibrot example”, was written by Claudio Della Volpe and Stefano Siboni, professors of the Department of Civil, Environmental and Mechanical Engineering of the University of Trento, with Marica Broseghini (PhD candidate at the time of the study and currently researcher at the Institute of Coastal research of the Helmholtz-Zentrum in Geesthacht) and Clara Ceccolini (undergraduate student then, now a Master's student in Energy engineering).
The inspiration for this study came during the "Feste Vigiliane", a local festival which takes place every year in Trento to celebrate San Vigilio, the patron saint of the city. Three years ago a local carpenter, Silvio Menestrina from Aldeno, not far from the city, was there showcasing his works.
The research group achieved results that open the way to new industrial technologies.
The research work involved three stages. The group first carried out a qualitative analysis on different versions of the same toy, then experiments were performed (and video recorded) to demonstrate their functioning, and the processes were finally described through mathematical equations.
The tests prove that the shape of the parts of the toy (the section of the stick can be squared, rectangular, round) or their material (wood, plastic, metal) is not relevant: these aspects do not affect the mechanism for which vibrations turn in rotational motion. The device works more or less like a hula-hoop, another well-known toy which is based on very complex physical principles at work, in that case, on a horizontal axis.
The press release includes comments from professor Claudio Della Volpe.