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Why Cows Hate Natural Gut

Jani Macari Pallis, 
Ph.D. Photo
Jani Macari Pallis, Ph.D.

How many cows does it take to string a tennis racquet? Answer: 3. Read on for the details.

Last year, Professor Howard Brody, who serves as the physicist for the USTA sport science committee, science advisor to the USPTR and on the technical committee of the International Tennis Federation, got a group of us together to speak at a special session on sport science for the American physics teachers convention in Philadelphia. Prof. Mont Hubbard from UC Davis spoke about baseballs, Prof. Alan Nathan from University of Illinois discussed baseball bats, Prof. Stan Johnson from Lehigh University lectured on golf. I spoke about the aerodynamics of cricket, tennis, and golf balls with some racecars thrown in for good measure. And then there was Professor Rod Cross, who in my opinion, "stole the show" with his tennis stories and physics demonstrations.

Rod Cross is associate professor of physics at the University of Sydney, Australia. Along with Brody, Cross is one of the original academics that began serious experimentation and research into the physics of tennis. Rod has written more than 25 science and engineering journal papers on the subject - from racquet dynamics to comparing tennis ball roll versus skid.

Howard Brody, Crawford Lindsey and Rod Cross

Rod also is the only person I know who introduces himself before lectures with "I wrote 50 papers in plasma physics and no one noticed. Then I wrote one paper on sport science and everyone in the news media thought it was terrific." You get the point! When Rod Cross speaks you know you're in for a treat. By the way, one of those papers, on the sweetspots of baseball bats, was voted one of the top physics papers in the US in 1998. Today, Rod's main research interest is the physics of tennis, which he has played competitively since 1950.

Most recently his work has dealt with:

  • Dynamic properties of various ball types. Why does the friction force on a ball reverse direction during the bounce?
  • Vibration characteristics of tennis racquets. Why don't some racquets vibrate at all, regardless of impact point?
  • Measurements of the speed of tennis courts. Why do balls ignore Howard Brody's bounce model and why are clay courts so slow?
  • Evaluation of the sensitivity of players to changes in string tension in a tennis racquet. Why can't most players pick out a 10-lb difference in string tension?
  • Measurement of string tension in a racquet after the racquet is strung. Not as simple as it sounds, Rod tells us.
  • Measurements of the impact properties of tennis strings. Why do professionals still prefer natural gut?

(He answers these questions in his new book, The Physics and Technology of Tennis (written with Howard Brody and Crawford Lindsey).

One of the great things about Rod's research is its straight-forwardness and elegance. Even seasoned researchers smile in genuine admiration at Rod's work and say, "Now that's clever." One of the demonstrations Rod conducted at the physics teachers conference involved sand he brought with him from Australia to Philadelphia to demonstrate how tennis shoes versus tennis balls slide and behave on clay courts. It was simple but elegant, got the point across and created a room full of smiling high school physics teachers.

Rod's the consummate example of Nike's "Just Do It" slogan. Things just seem to grab his curiosity and before you know it he's set up an experiment, tallied results, formulated key observations and written the journal paper. "One can still do lots of interesting and basic physics without a big budget," he says. Of course Rod's idea of "Just Do It" could involve high-speed video or light gates and sensors to determine velocity and acceleration of a sports ball.

Now to the title of this column, which Rod coined. As I said, when a subject grabs Rod's curiosity he starts uncovering the facts like an Agatha Christie sleuth. As he told this story at the physics teacher's conference, all I could do is smile, and say to myself over and over again, "Only Rod would think of doing that."

One subject that has raised Rod's curiosity is why so many top players still prefer to string their racquets with natural gut. The answer appears to be a combination of natural guts soft feel, coupled with high elasticity and the ability to retain tension. Still it is not for everyone since it quite expensive.

Why is it so expensive? Natural gut strings are more difficult and time consuming to produce than synthetic strings. There is a lot of manual time and labor in removing, slitting, washing, twisting, drying and polishing natural gut strings, hence the expense.

However, Rod really had to get his "hands dirty" and into the "guts" of this problem as he began pursuing the answers. He learned natural gut tennis strings are made from the small intestines of a cow (sometimes a bull). A long flexible tube which expands or contracts to accommodate ingested food, the small intestine is part of the digestive tract.

The intestine of a cow or sheep is about 120 feet long. However, only the thin outermost stretchy layer of the intestine is used for making tennis strings. Consequently, it takes roughly 3 cow's intestines to string a tennis racquet - not because the intestine is too short but because the part used to make a string is very thin. "The serosa of sheep and pig intestines would also work however they are used for sausage skins, so the manufacturers prefer to use the more readily available and slightly stronger intestines from cows," Rod conveyed.

The serosa is removed and cut into long ribbons which are cleaned through a series of salt and chemical baths over a period of time. About 18 ribbons are assembled and twisted as a long string and dried under tension in a temperature and humidity controlled room.

The string must be thoroughly dried throughout not just on the outside. After drying, the string has the appearance of ordinary string or rope. The string is polished into a smooth, round and clear string. A protective coating (like polyurethane) is added to reduce abrasion and prevent moisture from entering the string.

Well that certainly explains it. However, that wasn't enough for Rod. He called a butcher shop, got sausage casings and started twisting until he could prove to himself that yes, this would indeed resemble a racquet string.

Rod didn't stop here. Many people think that natural gut is made from cats. However, as Cross reported, "The small intestine of a cat is only 4 feet long and is therefore too short to make a tennis string." According to Cross, "The word 'catgut' appears to have evolved from the use of natural gut in a musical instrument called a 'kit' or perhaps from the name of the town in Germany where the strings were made." So, now you know!

If you ever have the opportunity to hear Prof. Cross speak, go out of your way to do so. As well in his new book, The Physics and Technology of Tennis there are other great tennis science stories, experiment setups and worked out tennis math and physics problems (although the butcher shop and sausage casing story isn't in the book).

Rod's a reminder to us all that learning is fun and that solid research can be conducted without huge grant budgets. My thanks to Prof. Rod Cross for his time and inspiration.

Until next month ... Jani

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This column is copyrighted by Jani Macari Pallis, Ph.D., all rights reserved.

Dr. Jani Macari Pallis is the founder and CEO of Cislunar Aerospace, Inc., an engineering and research firm in San Francisco. In addition to her engineering practice, she has led two collaborations between NASA and Cislunar, creating educational materials on the aerodynamics of sports for pre-college students and educators. As the head of NASA's "Aerodynamics in Sports" project, she has led a team of researchers investigating the aerodynamics, physics and biomechanics of tennis. The group has conducted high speed video data capture at the US Open and research of ball/court interaction, footwork, serve speeds, trajectories and ball aerodynamics. Pallis received a BS and MS from the Georgia Institute of Technology, an MS in mechanical engineering from the University of California, Berkeley and a Ph.D. in mechanical and aeronautical engineering from the University of California, Davis. She is a member of the Executive Committee of The International Sports Engineering Association.

Questions and comments about these columns can be directed to Jani by using this form.


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