Unpacking the unreturnable squash nick shot

[Music] Welcome to Science Sessions, the podcast of the proceedings of the National Academy of Sciences, where we connect you with Academy members, researchers, and policymakers. Join us as we explore the stories behind the science. I'm Paul Gabrielson. Our everyday activities and motions are governed by laws of physics, so we can use physics to understand the interesting phenomena we experience in the course of those activities. For example, the game of squash shows off plenty of fascinating physics as players hit a rubber ball around a walled court. In a recent PNAS study, Roberto Zenet of Brown University and colleagues used physics to understand a singular phenomenon in squash, the Nickshot. The results show how curiosity and experimentation can yield insights into everyday events. Roberto, can you briefly describe what the game of squash is? Yes, so squash is a racket sport. It's played between two or four players, normally two players. It's played in a walled court, and the objective of the game is to hit the ball, such that your opponent cannot hit it back after two bounces. And of course, there's some boundaries, but that's the essence of the sport. Do you play squash? I do. Yes, that's one of the reasons why I became interested in this problem. I've been playing squash for many, many years, and this is one of the things that I do to keep in shape. It's a very good sport, it's very dynamic, and it's super fun. Introduce us to the Nickshot. Yes, what happens is that when the ball hits the Nick, the Nick is with a vertical wall on the floor meat, when the ball collides near that junction on their certain conditions, instead of bouncing, the ball just rolls on the floor. Winning shot because basically your opponent cannot pick it up, even if you're very good, if it doesn't bounce, then the point is over and you score a point. But it's a shot that nobody can answer. What led you to this study? I'm a scientist of the way I am. It's a try to understand things that I see in nature, and this is something that I see often in the game of squash, while playing or while watching professionals. It's not obvious why this is the case. It's peculiar. I've been thinking about this problem for a long time, trying to understand it, you know, using my background in mechanics and in physics, but it was far from obvious. You have a ball that has kinetic energy, it becomes towards a wall, and some of it will be dissipated, but it's never completely dissipated. It has to bounce up. It has to bounce from the floor, except in this particular shot. It doesn't bounce. It just rolls down. So it was the need for understanding this shot that led me to study it a little bit more seriously. How did you study the next shot? Again, I'm a scientist, I'm an engineer. I do experimental science. So in my lab, we had a high-speed camera. It's a camera that is capable of capturing photographs many times per second for these studies, 4,000 frames per second, and that allows you to unveil phenomena that occurs in a very short time span. And this is exactly what happens in the case of the next shot. The details of the process are captured in a very, very brief time duration. We design and build an air cannon. It's a chamber that has compressed air that is connected to a piece of tube, and then we placed a squash ball inside this tube. And we have a valve that can be opened quickly. So the ball is deployed at a certain speed close to that encounter when you place a squash. We directed that flying ball towards Enic. And we varied the location of collision. We varied the angle, and we also varied the speed. And by that, we were able to basically replicate this amazing shot. We would aim it near the NIC, and under certain conditions it rolled. We mounted this cannon. We mounted a wall and a floor configuration. And we observed that with the high speed camera to slow down significantly in time. And from that we were able to view the process with more measurements to locate when it happens, and then proposed a mechanical model to understand it. What is happening mechanically with a NIC shot? For the NIC shot to occur, first you have to use a squash ball that is deformable. And then it has to hit the vertical wall first, but it has to hit it at a certain distance away from the floor. So it's not exactly at the NIC. It's not exactly at the corner. It's a little bit above. And while it's deforming, if it makes contact with the floor, that's when you observe the NIC shot. Then that leads to the occurrence of this rolling motion in the bouncing process of the ball. Under those conditions, the ball reaches what we call a mechanical frustration condition. The ball basically gets stuck in the corner. But in order for it to get stuck, it needs to be compressed, it needs to touch the floor, and then under those sets of conditions, the momentum of the wall gets cancelled by the momentum that the floor is exerting on it. The ball gets stuck and it rolls horizontally because he has some kinetic energy stored during the compression phase. How does knowing how a NIC shot works helps squash players? Perhaps it's not going to make squash players play better. They will understand why this happens physically. But what it validates is all these empirical knowledge that squash players have. Squash players know that they have to hit the ball really hard. They know they have to hit it towards the NIC. And they know that it is more likely to cure if the ball is warm and deformable. It gives you an explanation in mechanical terms, in physical terms. But this muscle memory, this repetition that professional athletes have, anybody who performs a repetitive task is irreplaceable, even with the physical understanding of the process. If you try to be a better athlete, or to appreciate how athletes perform their job, if you know mechanics, you can actually make more sense of how they can accomplish their task or their goals. Everything has to obey the laws of physics. And if you have this basic understanding, you can say, yes, that makes sense. That's why that shot is played in a certain way. Or that's why sprinters can run so fast, or swimmers can swing so elegantly and swing fast. It's all about mechanics. And it's something that we try to teach our engineering students to view the world in terms of mechanical terms, to understand it better, and hopefully eventually make better engineering designs. What are the applications of these findings beyond squash? One of the ways I conduct my research is just trying to understand the work around us. Now I understand the nickshot, and it's already a fulfilling sentiment. If he has applications, great, but that's not necessarily the motivation of these sort of studies. The idea of understanding this mechanical frustration condition, when things get stuck, could be used if you're trying to develop a mechanical system, a device that uses that principle. So for instance, dampeners, these devices that dissipate the motion vibrations, for instance, they can use the same principle. One example would be, for instance, if you want to design better helmets for sports or for safety, one of the problems in traumatic brain injury is this whiplash motion that you experience when you have a collision. If you have a helmet that is designed to reproduce this mechanical frustration condition, then you can reduce that whiplash effect. What are the caveats and limitations of this study? The explanation that we found is limited to very small deformations or very limited range of speeds. So these mechanical frustration condition would not apply if the velocity of the impact is very high, or if the deformability of the ball is very large. So it would only apply for the limited range of conditions for squash plane. You could not extend what we understand to rigid objects like miller balls. So the limitations are for the type of deformations that one observes in squash balls, which are very deformable, the rubber-like materials, and for the limited range of impact speeds. Thanks for tuning into science sessions. You can subscribe to science sessions on iTunes, Spotify, or wherever you get your podcasts. If you like this episode, please consider leaving a review and helping us spread the word. (upbeat music)