July 23, 2014

Physics Of The Sweet Spot

Every tennis player knows when they hit the sweet spot on their racquet, the shot just feels good. Racquet manufacturers design their rackets to maximize the sweet spot, but what’s the explanation for the sweet spot?

Like a guitar string, the strings on tennis vibrate when struck. Depending on how tight the racquet is strung, the frequency of this vibration ranges from 100 Hz (Hz stands for Hertz which is measure of times/second) to 200 Hz. The more tightly a racquet is strung, the higher the frequency will be.

Because a racquet is a semi-uniform beam, there are locations along the tennis racquet were the vibration/energy transferred to the racquet and your hand varies. Below is a diagram showing significant locations on a racquet.


Physics of a Tennis Racquet

CM (Center of Mass)
While you obviously don’t want to hit the ball here, it determines the location of the sweet spot.

Dead spot:
When serving, this is good spot to hit near because the energy from the moving racquet is entirely given to the ball. On the flip side, if you try to return a fast serve near this point, all of the energy of the ball is transferred to the racquet and no energy is transferred back to the ball. Therefore there is no bounce.

Node:
This is the point where no vibration is felt. This means that almost all of the energy of the ball strike will be gone by the time it reaches your hand.

COP (Center of Percussion):
This is spot where the force of the ball strike is balancing with your force applied to parts of your hand and you don’t feel a jolt in your forearm. When the ball hits the tip, more force is applied to the bottom of your hand then top. When the ball hits the throat of the racquet, more force is applied to upper part of the hand.

Best Bounce Area:
This is the area where the racquet returns most of the energy of your opponents shot. This great energy return comes at a price, because you might feel a lot of vibrational shock especially if you are at the business end of a Andy Roddick serve. He holds the record for fastest serve in professional tennis — 154 miles per hour!!

For an even more technical explanation and futher reading, check out a physics web site created by University of Sydney physics professor and tennis fan Rod Cross. The illustration of the racquet used in this article is courtesy of Cross’ site.

Special thanks to guest editor Tim Roth for today’s column. Read more of his fascinating scientific insights at atomstozebras.com

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