The physics of why basketball shoes are so squeaky
The noise is down to the base of the shoe forming wrinkles that travel at near supersonic speeds The post The physics of why basketball shoes are so squeaky appeared first on Physics World .
The persistent squeaking of basketball shoes during games has long been a familiar sound to fans and players alike. While the bounce of the ball is often highlighted, the squeaky noises produced by the shoes as players move across the court have remained a mystery. However, recent research by physicists from France, Israel, the UK, and the US has shed light on this phenomenon, revealing that the squeaks are caused by a previously unobserved mechanism involving the rapid deformation of the rubber sole.
In a study published in Physics World, Katia Bertoldi from the Harvard John A. Paulson School of Engineering and Applied Sciences, along with her colleagues, conducted experiments to recreate the squeaking noise in a controlled laboratory setting. They used a basketball shoe, or a sample of rubber, which was slid across a smooth glass plate. High-speed imaging and audio measurements were employed to analyze the resulting squeaks.
Previous research on similar phenomena, such as the scraping of chalk on a blackboard or brakes on a bicycle, suggested that "pulses" were created when two materials "stick and slip." However, these studies focused on slow movements, which do not produce the sharp squeaks associated with basketball shoes. Bertoldi and her team discovered that the noise was not caused by random stick-slip events but rather by deformations of the rubber sole pulsing in bursts, or rippling, across the surface.
The key to understanding the squeaks lies in the rapid deformation of the rubber sole. Small parts of the sole change shape and lose and regain contact with the surface, with the "ripple" traveling at near supersonic speeds. The pitch of the squeak even matches the rate of these "bursts," which is determined by the stiffness and thickness of the shoe sole. This means that the squeak's frequency is directly related to the physical properties of the shoe material.
Interestingly, the researchers found that the type of surface on which the shoe is worn also plays a role in the sound produced. If a soft surface is smooth, the pulses are irregular and produce no sharp sounds. In contrast, ridged surfaces тАУ like the grip patterns on sports shoes тАУ produce consistent pulse frequencies, resulting in a high-pitched squeak. This explains why basketball shoes, which are designed with specific grip patterns for traction, tend to produce more pronounced squeaks.
An additional twist to the story comes from the fact that in some instances, the slip pulses are triggered by triboelectric discharges тАУ miniature lightning bolts caused by the friction of the rubber. These discharges, which occur due to the buildup of static electricity between the rubber and the surface, can further contribute to the squeaking noise.
The physics of these pulses share similar features with fracture fronts in plate tectonics, where the movement of tectonic plates can also produce rapid, localized deformations. By studying the squeaks of basketball shoes, researchers have gained a deeper understanding of the dynamics of such rapid, nonlinear processes, which can have applications in various fields, from material science to geophysics.
In conclusion, the squeaky sounds of basketball shoes are not merely an annoyance but a fascinating example of the complex physics that governs everyday phenomena. The research not only explains the source of the noise but also highlights the intricate relationship between material properties, surface interactions, and the resulting acoustic effects. As the study of such phenomena continues, it is likely to reveal even more fascinating insights into the world of physics.
