PALAISEAU - Who would have thought you could find a slingshot-type weapon in a research laboratory? And who would have though you could use it to study a sport -- squash -- that hasn't even been elevated to Olympic status? Yet that is exactly what has been taking place these few weeks at the hydrodynamic laboratory at the Polytechnic school (dubbed LadHyX) near Paris, France.

Sometimes, for your research to stand out, you need to identify the question that no one else has asked before. In this case the question goes like this: "In squash, why does hitting the ball near the corners make its trajectory unpredictable?" Squash is played in a small closed room; the goal is to hit a ball against a wall with a racket and to take advantage of the adjacent walls to destabilize the opponent.

"There have been fewer than ten articles on squash physics in the past 30 years and none have focused on the behavior of the ball that's hit against the "nick" where the sidewall and the floor meet," says Philippe Brunet, a hydrodynamics specialist from the CNRS at the university of Paris-VII. "Most notably, no one can explain the "nick" kill shot that only the greatest can pull off. The ball, after a first rebound, hits a nick at a downward angle and bounces back with a slow velocity along the wall or rolls parallel to the ground, making it impossible to hit."

Science to the rescue

For the researcher, who also plays the sport, this mystery had to be solved. He found two partners, Caroline Cohen and Baptiste Darbois-Texier, who are so passionate about sports and physics that they are writing a thesis on the subject.

Instead of playing on a real court with rackets and trying to hit balls into corners, they favored a more practical protocol: throwing balls with a slingshot into a Plexiglas corner and filming the experience with a high-speed camera at 3,000 frames per second.

An incubator enables them to heat the balls for better bounce, like they would be in real game conditions. At 70 °C, the balls bounce back with 70% of their initial speed. If they were cold, this percentage would be approximately 35%. The researchers sometimes heated up the projectiles so much that their volume doubled, causing them to bounce back as superballs - a property that wasn't relevant for this experience.

More than 100 shots later, the results were in.

Lessons learned

Lesson number one: the ball's slow rebound, measured as the ratio of the rebound speed and the initial speed, depends neither on the angle nor on the velocity. Everything depends on the impact of the ball on the first wall and then on the second wall when it bounces back. The ball slows down the most when it simultaneously touches both walls by hitting exactly at their intersection. At equal initial speed, around 100 kilometers-per-hour, the final velocity can be twice as slow.

Second lesson: it is possible, independently from speed and angle, to perform the perfect move by hitting slightly above the corner. The ball then sharply slows down and barely bounces back, rolling on the ground (if it hits the ground first, it hugs the wall). All it takes is a centimeter between both walls for the perfect rebound. Not an easy shot…

The explanation is in ball deformation. By hitting the wall before the ground, the ball flattens and deforms a lot more vertically than it does horizontally. It therefore rubs the wall more than it does the ground; the speed is lowered more in one direction than in another and it bounces back skimming the floor.

This was confirmed by another series of experiments that consisted in coating the balls with glycerol, a liquid that lowers object-friction. After a few stains on the lab coats and the Plexiglas, the team found that this coating made the perfect move even more difficult.

For the sport to remain spectacular, the court walls have to be the least slippery possible and the referees have to make sure the players don't wipe their sweaty hands in the corners, which would make the balls slide.

Read more from Le Temps in French.

Photo - The World Squash Federation