Kill Shot Mechanics
Explained
Squash’s nick shot—where the ball dies instantly in the corner—has long fascinated players. Experiments by Brown University engineers confirm why it happens, revealing the role of rolling mechanics.
Written by Michael Abrams
It’s with elation or frustration, and sometimes both, that squash players execute, or witness, the rare and famous nick shot.
Squash is a racket sport played in a court with four walls and a small ball that bounces off all of them in the course of a game. Occasionally, the ball will hit the corner where a wall and the floor meet and, no matter what its speed, will stop all bouncing and skitter across the floor, entirely unreturnable.
“I think, personally, whenever you see a nick, even if it is against you, it just brings you joy—or the complete opposite, just anger,” said Roberto Zenit, a professor of engineering at Brown University and an avid squash player. “If you’re working super hard and then your opponent hits the nick, there’s nothing you can do—it’s exciting either way.”
High speed video helped the researchers to develop a mathematical model of what exactly happened during a successful nick shot. Video: Zenit Lab/Brown University
Testing the Theory
As fascinating and well known as the shot is, Zenit decided to combine his pastime with his profession and pursue an explanation for the mechanics behind the nick shot.
He started by studying gameplay videos. “They don’t [use] a high-speed camera,” he said. “They have a little bit of slow motion sometimes, but the process that occurs during the nick is very fast.” So to get a clearer view, Zenit took matters into his own hands.
In his lab, he set up an artificial squash-court corner out of two pieces of acrylic and created an air gun with a pressurized PVC pipe. Then he began firing balls at the corner—again and again, changing the angle, speed, and point of impact with every shot.
After a couple days of controlled bombardment, it happened. “The most satisfying thing is that we did see the appearance of the nick in the lab," Zenit said, explaining that the ball needed to hit just right to reach the nick conditions: “It hits to the corner and then just bounces horizontally with no vertical motion.” But he still had no idea why.
Photo: Getty
Modeling the Nick Shot
The simplest model would be to assume that there is compression, decompression, and some loss of energy, Zenit first conjectured. With two walls, the ball might lose double the energy if it hit them at the same time. But that idea doesn’t predict the nick, he explained. “It would always predict a certain amount of vertical bounce. As in most things, you just have to dig deeper.”
Zenit reached out to his colleague Haneesh Kesari, also a professor of engineering at Brown, but an expert in contact mechanics—who had, in fact, done his graduate work on the subject of rolling.
When asked, Kesari had his own ideas. And they were easy to confirm. Before launching the squash ball at his corner mock-up in the lab, Zenit had painted it with short white lines, for the purpose of monitoring spin, in case that might be a piece of the nick puzzle. Instead, they proved Kesari’s conjecture. His thought was that when an object hits a flat surface, there is not only compression but rolling too. And when it hits two surfaces at the same time, the rolling forces from both planes essentially nullify each other.
“That’s when this mechanical frustration occurs. Basically, you cannot roll with two different contact points. And that’s the nick condition.”
—Roberto Zenit, professor of engineering at Brown University
With rolling added to the equation, everything clicked. A bouncing ball always rolls along its path of motion. When it hits the wall first, it rolls downward toward the floor and continues rolling; when it meets the floor first, it rolls upward toward the wall and bounces off. Only when it contacts wall and floor simultaneously is the spin halted.
Though the ball stops rolling when the nick occurs, it’s still compressed against the wall, storing energy until it regains its spherical formation, sending it out across the floor, sans vertical bounce.
Kesari’s thoughts about the importance of rolling in producing the nick shot were borne out in the video—as illustrated by those white lines on the ball (spinning turned out to be irrelevant). “It was really one of those very satisfying moments in research, when the theory and the experiments match perfectly,” Zenit said.
Photo: Getty
Applications and Beyond
Having the answer to a nearly two-century-old sporting problem is its own reward, but the research may have some applications. For one thing, an understanding of the conditions that give rise to nick shots may help athletes reproduce them. “Many high-performance athletes do have the advice of biomechanicians in order for them to improve their performance. And I think this could be another example of that,” Zenit said. “Now they know that they have to hit the ball a little bit harder to get the nick or to angle it within a certain range of angles.”
And it’s possible that a squash ball’s makeup could be tweaked to make it easier—or possibly harder—to make a nick shot. “I would certainly hope that this type of research could be funded by somebody who makes squash balls,” Zenit said.
Regardless of any commercial potential, the research—with its easy-to-explain but nontrivial result—is perfect for pedagogy. “I am proposing to include a module for our introductory engineering class,” Zenit said, “about ball sports to introduce the students to the basic concepts of mechanics, mass, momentum, the conservation of linear movement—things like that.”
But Zenit isn’t done asking and answering questions about what goes on with squash balls. The speed and bounce the ball provides have a lot to do with what makes squash and other ball games fun—and different from one another. Those parameters are largely determined by how much the ball deforms when it hits a wall—the coefficient of restitution. “What is the correct value of the coefficient of restitution to have a fun game?” Zenit asked. “That is something that we’re trying to answer, with mechanics.”
Michael Abrams is a technology writer in Westfield, N.J.
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