Physics of the String Shooter

The string shooter is an intriguing device that can be used to demonstrate a number of important physics concepts. It uses a pair of rotating wheels to continuously propel a closed loop of string at constant speed (see Fig. 1). Remarkably, the loop quickly reaches a stable shape, remaining suspended in the air. This surprising behavior may be viewed in a number of YouTube videos.¹ Several models of the device are available commercially,^2,3 at modest cost, and homemade versions are not difficult to construct.¹ Advanced-level discussions of the underlying physics have recently appeared in the scientific literature.^4–6 Fortunately, the fundamentals of those analyses can be cast into a form that is accessible to students of introductory-level physics. That is the aim of this paper. Conceptually, the device’s behavior can be explained by considering the forces that act on the string (interestingly, air drag plays a...