The Mind-Bending Physics of a Tennis Ball’s Spin
Tennis has been called the game of inches, of kings, of poets, of love, of errors, of endurance, of a lifetime. But those are mostly metaphors. Really, tennis is the game of spin.
Watch Novak Djokovic send arcing yellow streaks from beyond his baseline to the bleeding edge of his opponent’s backcourt. Watch Rafael Nadal’s ground strokes cross a foot or more above the net, then drop like tactical bombs to the competition’s ad corner. Watch Serena’s opponents go crosseyed staring down her barrel-rolling 126 mph first serves. Go to any court in any city and you will find players at every level squatting, twisting, grunting—trying to find that spin.
It’s fairly easy to figure out what spin does: It wins tennis matches. How it works—or rather, how it’s created—on the other hand, is about as complicated a physics question you can set about solving without invoking subatomic particles. The variables include squishy balls, stiff racquets, taut strings, thrusting knees, twisting hips, swinging shoulders, and rotating elbows. But all those mechanics are made possible by a pair of equipment innovations.
In order to generate spin, you have to brush your racquet up across the ball, rather than strike it dead on. The motion looks kind of like you are giving the ball a weird high five. Starting low, with the racquet at your waist, you bring it up and forward, twisting with your hips and elbow so the racquet’s head finishes high above your opposite shoulder.
And that’s not all. “For a good topspin, you have to tilt the racquet at a good angle, too,” says Crawford Lindsey, head tester at the Tennis Warehouse University, a tennis testing website (with some seriously awesome studies). “You don’t present much face to the ball, because everything is slanted.”
The best angle for your racquet’s forward face is around 50 degrees, or less, relative to the surface of the ground. This puts spin on the ball, but also makes it a lot easier to ding the ball with the rim. “Bigger racquets give you more surface area, and therefore safety, so you can swing faster and at a greater angle,” says Lindsey.
But it’s not just about technique—it’s also about the tech. The modern, spin-dominated game of tennis owes everything to an inventor named Howard Head. In the late 1940s, Head was an airplane mechanic, and he was learning to ski. He liked the sport, but didn’t like lugging the heavy wooden planks up the hill between each run. His frustration became the first aluminum skis, which he patented and used to form the Head Ski Company.
What’s that got to do with tennis? A few decades later, Head sold his company, retired, and took up tennis. Like, he went all in: built a court at his house, hired a coach, bought a newfangled ball machine. But again, he found a sport that wasn’t quite designed right. First of all, his ball machine was wack. So he bought the company the made it—Prince—and invented a better version.
Then he went after the personal gear. Like with skiing, tennis was dominated by wood. But that medium restricted the racquets’ head area to about 60 square inches—the frame would break if it got any bigger. “There just wasn’t enough margin of error with a racquet that size,” says Rod Cross, a retired physicist in Australia who studies tennis physics. So Head brought his aluminum expertise to bear on the problem, and invented the sturdier aluminum Prince Classic. His patent covered tennis racquets with heads up to 125 square inches.
Howard Head’s big headed racquets let players attack the ball with more angle on their swings. (He later introduced graphite frames, which are even lighter and stronger.) But frame size is only part of the racquet design equation.
Zoom in to the racquet as a ball encounters the strings, and pause. The racquet face has two types of string. The main strings run from tip to handle, and the cross strings from side to side. For most of tennis history, these were rendered from the intestines of a cow. But today, the best strings for inducing spin are nylon. These have what’s called a low coefficient of friction, which means they slip against themselves. This is huge for generating spin, because it creates a so-called snap back effect.
Cross and Lindsey do some pretty rad experiments.
Push play. The ball, upon meeting the racquet’s upward motion, temporarily bends the mains down. As the ball leaves the racquet, the pressure eases and the mains snap back up into position, imparting their upward energy into the ball. “That stored energy gets snapped up and turned into spin,” says Lindsey.
There are other ways to lower that coefficient of friction. Wilson is just now releasing racquets with fewer cross strings than mains (they call this patented technology Spin Effect). If you’ve been paying attention, you’ll be quick to pick up that fewer cross strings means less friction on the mains.
Wilson sent me one of their racquets to play test. Straight away, I was pretty skeptical, but definitely noticed a difference. So did my hitting partner. And so did the sensor that I attached to the bottom of the handle. Compared to my usual racquet (ironically, a Head graphite XT), the Sony Smart Tennis Sensor showed that my spin on both fore and backhands had nearly doubled. (The sensor’s spin scale is pretty arbitrary, so I don’t have actual RPMs.)
The big head and slick strings are just the basics, technology-wise, that contribute to spin. And spin is just one aspect of tennis that is a physicist’s dream sport. Lindsey and Cross are science buddies, and meet every year to do new experiments—testing things like hybrid strings, racquet vibration, and the way different strings behave under stress. “We’ve tested about 2,000 different strings,” says Cross. “We know the physical properties of them all.”
Which is all super fascinating, but it’s probably better off if you just forget all of it. After all, the best way to ruin your game is by thinking about your game. Just thank your lucky stars for Howard Head, aim for the baseline, and remember to breathe, ace.
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