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. Several models of the device are available commercially, at modest cost, and homemade versions are not difficult to construct. Advanced-level discussions of the underlying physics have recently appeared in the scientific literature. 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 key role) and using the conditions for translational and rotational equilibrium. A more complete analysis, using Newton’s second law, allows the shape of a stationary loop, lying in a vertical plane, to be predicted with remarkable accuracy. The method requires only a minimal amount of calculus.