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Racquet Science: A Tale of Two Impacts

Tuesday, June 19, 2012 /by

Racquet Science expounds on the surprising realities of what happens when ball meets racquet—and how this knowledge can inform your game. Today, we look deeper into why connecting with the sweet spot feels so good, and why missing it doesn’t. 

*****

Impact LikelyImagine: The ball is fast approaching you. Most days it looks blurry, a whizzing blob of yellow motion. But not today. Today its edges are very hard and distinct. Not that it looks any slower, but it feels slower, today. As it arcs over the net, you’re already halfway to where it’s going to be. A few more steps and you’ve arrived, just before it dive-bombs into the court. Racquet up and back, then everything uncoils—legs, trunk, shoulders, an arm slings toward the rebounding felt. Contact. The ball pancakes; strings deform like a waterbed. And yet? It hardly seems to register. Racquet doesn’t twist; hand and arm don’t jar. And the ball, hissing back over the net, shows it. Sweet.

So what happened? How did you get that one to feel so good? With a stroke of luck and skill, you connected with what tennis physicists call a vibration node, one of several spots on the stringbed that, when hit, doesn’t cause the racquet to vibrate. In fact, these “no-vibration zones” run across the stringbed in a great curved line—a “U”-shape of sorts, extending up from the stringbed’s center to points at 2 and 10 o’clock near the racquet’s tip.

But then again, you didn’t connect with just any vibration node. You nailed the most precious of them all: The sweet spot. True, finding the node line makes for a vibration-free frame. However, being off-center, most vibration nodes cause the racquet (and so your arm) to twist. Not so for the sweet spot, the singular node that intersects the long axis of the racquet. When struck, the stick neither vibrates nor twists. The result, as the authors of Technical Tennis write, is “more an absence of feeling than a sensation of slamming into something solid.” Energy that might be spent on racquet vibrations transfers into the ball, sending it whizzing efficiently off the frame.

 *****

Imagine again: It’s another day, the ball is fast approaching. This time, it’s a streak of yellow, a field of probability you’re struggling to track and hit. Looks no slower, doesn’t feel it, either. It bounces while you’re still trying to catch up. Racquet back late, body out of whack as you just barely connect. Ball whizzes off strings, off target. Frame wobbles, hand stings, arm shakes. Sigh. A shank, right off the edge of the frame.

What went wrong? Why did it feel so terrible? You missed the node, of course. It wasn’t your best effort and, as a result, the frame both twisted and vibrated. It vibrated an awful lot, actually, because your swing was very hard, your miss preposterously bad, and your racquet incredibly flexible. Indeed, the harder you hit, the worse you miss, and the more bendable your racquet, the more jarring the shot will feel.

(Fact: On off-center hits, a flexible racquet will vibrate more than a stiff one. If the racquet is stiff, it’ll vibrate through shorter distances, and thus for shorter periods of time; if it’s flexible, through larger distances for longer periods of time. All things being equal, the stiffer the racquet, the more comfortable—i.e., duller—shanks will feel.)

(Caveat to fact: So does that mean you should use a stiffer racquet? Not necessarily. Just because your racquet doesn’t vibrate, doesn’t mean that your arm isn’t absorbing “shock”—that is, isn't resisting the force of the ball as it rebounds off the racquet. The stiffer the racquet, the more acutely your arm must resist those forces, and so the more your arm is stressed.)

But you felt all that, right? You missed. And then, from the tip all the way to the butt, the racquet bent rapidly back and forth. Vibrations transferred through the handle and into your hand and forearm, causing them to unpleasantly shudder—a reminder that you crashed into not such a sweet spot.

 *****

Just like days, some hits are sweet and others not-so-sweet. And those darling few, those are just sweetest of all. We play for those, don't we?

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