Spin Ball: The Physics of Polyester String

TENNIS.COM article published today highlights the "Pros of Poly" and why using Polyester strings is now almost mandatory for the top players of the game.
The modern game of tennis has become so powerful with the advent of racket technology that we are seeing men's forehands regularly approaching and even exceeding 100 MPH! At such high velocities, the forces of gravity are not strong enough to pull a normal groundstroke down into the court fast enough...

Enter Topspin and the Secret of Polyester
With Polyester and Co-Polyester strings, the strings will actually stretch for a few milliseconds with the ball, stop and then quickly snap back like taut bow. This action helps whip the ball and rotate it sideways as it comes off the strings, creating the top spin effect. Normal topspin generated from nylon or multi-filament strings "brush" against the ball but do not have the same recoiling effect. 
TENNIS.com: Racquet experts generally say that string technology has impacted the game more than the racquet technology in recent years. Do you have any thoughts on the next wave of string technology and what it would mean to tennis?

Dr. Cross: The next best thing will be strings that don't break and strings that don't lose tension, but that would put string makers out of business.

Important: "Durability" is a commercial factor with tennis strings, as it is with all other disposable goods such as batteries or even chewing gum - like the commercial goes, if you don't need to ever chew another piece of gum, companies would go out of business. I have a theory that the RPM Blast string has a short shelf life and loses tension fairly quickly because the Babolat string manufacturers were willing to develop a high performance string that would keep customers coming back for more all the time.

Do not fall for TV marketing, instead do your research and look at reviews on reputable sites such as stringforum.net. The key is finding a value Poly String that will last just as long as the most expensive Polys (Luxilons) because you break them or they lose tension...yet over the course of a year of playing, you are saving $7-10 each time or over $100 a year...enough to buy a whole Jumbo Reel (985 feet) of the highly acclaimed, Topspin Cyber Flash 17g.

But you must play test a few types of polys to know which one is the best for you. It should enhance your spin, power and durability without sacrificing too much control. In my case, I had slightly more control/feel with Luxilon Ace 18 - but when you look at match results...I was winning a lot more matches because I was able to keep short balls to a minimum AND keep my opponents honest because of the threat of being able to hit a sharp side-angle or looper to the deep corners - it gave my game a more dimensions and weapons.

"One American racquet manufacturing exec estimates that polyester string accounts for more than 70 percent of string sold in Europe, and the majority of the world’s top juniors are polyester or co-poly string players."