[ Jani Pallis - 3 - 09:59:38 ]
Hi Michelle, Our chat will start at 10:30. We'll be able to answer your question then.

[ MICHELLE - 4 - 09:59:50 ]
IS TOO MUCH HEAT BAD FOR A TENNIS BALL?

[ yours - 5 - 10:03:30 ]
How does aerodynamics work?

[ Rabi Mehta - 8 - 10:25:19 ]
Hi Jani and others --- I am here.

[ Duane Knudson - 10 - 10:27:30 ]
Hello Rabi, you are getting through fine.

[ Jani Pallis - 11 - 10:27:35 ]
Good Morning Everyone, I'm Dr. Jani Macari Pallis, the principal investigator of the K-8 Aeronautics Internet Textbook and our tennis sport science project. We'll be starting our chat in just a couple of minutes. Can I have all of our experts send a message so that I will know that you are all here?

[ Rabi Mehta - 12 - 10:28:00 ]
Thanks Duane --- hope you are doing well.

[ Jani Pallis - 15 - 10:30:44 ]
Good Morning Everyone and Welcome to our tennis science chat! Today, Dr. Rabi Mehta from the NASA Ames Research Center, Dr. Duane Knudson, from the USTA sport science committee, and Dr. Bill Aronson the USTA's Administrator of Sport Science are with us. Good Morning Rabi, Bill and Duane. We're ready for our first questions.

[ Jani Pallis - 16 - 10:32:07 ]
Rabi, we just finished the wind tunnel test last week. Can you tell everyone about it? How was it different from the previous test wind tunnel test the team conducted?

[ Rabi Mehta - 17 - 10:33:01 ]
Is anyone interested in the wind tunnel tests we just completed --- measuring the drag force on tennis balls?

[ Duane Knudson - 19 - 10:33:59 ]
Rabi, I am.

[ Rabi Mehta - 20 - 10:34:50 ]
In the latest wind tunnel tests we measured the drag (resistance) force on tennis balls. This is the same type of information collected for the resizing (making the ball larger) considered by the Tennis authorities.

[ Jani Pallis - 21 - 10:34:52 ]
RE: [Bill Aronson] Hello!
Bill, you are the new administrator of sport science for the USTA - what are your plans and goals for the group?

[ LANCEHONEAPATHMIDDLESCHOOL - 22 - 10:35:10 ]
Hi Dr. Mehta

[ LANCEHONEAPATHMIDDLESCHOOL - 23 - 10:35:10 ]
Are any of you familiar with animals in space?

[ Jani Pallis - 24 - 10:37:23 ]
Duane - We have had several write in questions on string tension. These came from Herman Wood at West Cobb Middle School. How does string tension affect the ability to impart spin upon the ball? How does string tension affect ball speed?

[ Rabi Mehta - 25 - 10:37:34 ]
We tested a total of 8 balls in this first round --- ranging from balls that are used in the US Open, Wimbledon and the French Open plus some of the larger balls -- also one that was particularly interesting: a ball used in the US Open.

[ Bill Aronson - 26 - 10:38:07 ]
I plan to work closely with our coaches, player development and Sport Science Committee. Our mission is to produce, evaluate, and disseminate sport science information relevant to tennis!!!!

[ Jani Pallis - 27 - 10:38:28 ]
RE: [Rabi Mehta] In the latest wind tunnel tests we measured the drag (resistance) force on tennis balls. This type is the same type of information collected for the resizing (making the ball larger) considered by the Tennis authorities.
Rabi - could you tell everyone what types of balls were tested?

[ Marcie Londono - 28 - 10:38:34 ]
Hi Jani, Bill was unfortunately called out for an emergency meeting. I will be online if there is anything I can answer.

[ Rabi Mehta - 29 - 10:39:05 ]
RE: [LANCEHONEAPATHMIDDLESCHOOL] Hi Dr. Mehta
Hi, I am not the right person to answer questions about animals in space --- however, as you must be aware experiments on life in space and on other planets is a vital part of NASA's mission.

[ Jani Pallis - 30 - 10:39:31 ]
Bill - What types of sport science projects does the USTA conduct?

[ Jani Pallis - 31 - 10:41:10 ]
Hi Marcie - Can you answer the question that I just submitted to Bill? What types of sport science projects does the USTA conduct?

[ Rabi Mehta - 32 - 10:41:40 ]
We tested 3 new balls: Wilson (US Open), Slazenger (Wimbledon), Roland Garros (French Open) and for the baseline check - a smooth sphere. We also tested two of the larger balls and a ball that was used for about 7 games in the US Open.

[ Duane Knudson - 33 - 10:42:13 ]
RE: [Jani Pallis] Duane - We have had several write in questions on string tension. These came from Herman Wood at West Cobb Middle School. How does string tension affect the ability to impart spin upon the ball? How does string tension affect ball speed?
The tension in the strings is one way a player can dramatically affect the response of their racket. A higher string tension "grabs" the ball and allows the player to create more spin, but less speed of rebound. A lower string tension creates more ball speed, but less spin and accuracy.

[ Rabi Mehta - 34 - 10:42:53 ]
Should the drag on the tennis ball be higher or lower than on a smooth sphere of the same size?

[ Jani Pallis - 35 - 10:43:45 ]
RE: [Rabi Mehta] We tested 3 new balls: Wilson (US Open), Slazenger (Wimbledon), Roland Garros (French Open) and for the baseline check, a smooth sphere. We also tested two of the larger balls and a ball that was used for about 7 games in the 1997 US Open.
Rabi - The two larger balls were the 6.5% and 8.5% larger diameter balls? They are the pre-production prototypes, correct?

[ Bill Aronson - 36 - 10:44:33 ]
Sport Science awards research grants in many areas. These include Sport Psychology, Exercise Physiology, Motor Learning, Nutrition, Sport Medicine, and Biomechanics. Sport Science works closely with our Sport Science Committee on a variety of these topics. We provide fitness testing to our elite athletes. We provide educational resources to our coaches by providing competencies in all areas mentioned above. We provide educational materials to our sections as well. Currently we are working closely with our Coaching Education department!!!!

[ Rabi Mehta - 37 - 10:45:02 ]
That is correct -- the larger diameter balls are the pre-production type with a slightly different felt on them which seems to affect the aerodynamics.

[ Jani Pallis - 38 - 10:45:38 ]
Duane or Marcie this was a write-in question from Cory Kestner. A lot of research has been done on the open-stance forehand, but a shot that I've noticed being hit more frequently is the open stance forehand volley. I've seen more pro level players hitting the shot in the past year than ever before. Is there any research to show whether or not it is a more biomechanically efficient stroke, or what the benefits may be? From my own background in sports science, I understand the similarities between the open-stance forehand and the open-stance forehand volley concerning weight transfer and recovery time. What interests me is whether there is a difference in power between the classic forehand volley and the open-stance forehand volley. Please let me know whether you are aware of any research on this topic.

[ Rabi Mehta - 39 - 10:46:09 ]
Does anyone know why they put dimples on golf balls??

[ Jani Pallis - 40 - 10:46:41 ]
RE: [Jani Pallis] Duane or Marcie this was a write-in question from Cory Kestner. A lot of research has been done on the open-stance forehand, but a shot that I've noticed being hit more frequently is the open stance forehand volley. I've seen more pro level players hitting the shot in the past year than ever before. Is there any research to show whether or not it is a more biomechanically efficient stroke, or what the benefits may be? From my own background in sports science, I understand the similiarities between the open-stance forehand and the open-stance forehand volley concerning weight transfer and recovery time. What interests me is whether there is a difference in power between the classic forehand volley and the open-stance forehand volley. Please let me know whether you are aware of any research on this topic.
Oh Bill's still here ... You are welcome to answer this one as well.

[ Jani Pallis - 41 - 10:47:27 ]
Bill - Tell us what you did as a psychologist and captain in the Air Force? Did you do any sport psychology work there?

[ Marcie Londono - 42 - 10:47:45 ]
If I may answer that question directed to Bill, the sport science department of the USTA conducts various services. We have established a fitness testing protocol, that is conducted at all of the 151 area training centers in the U.S. We conduct fitness tests on junior nationally ranked tennis players at our Player Development headquarters, using the FASTEX equipment to test speed and agility and Cybex isokinetic equipment to measure muscular strength.

[ Rabi Mehta - 43 - 10:48:01 ]
For a given tennis stroke, would a used tennis ball go faster or slower than a new ball??

[ Jani Pallis - 44 - 10:48:58 ]
Rabi - What's a drag coefficient and how does that differ from drag? How do you calculate drag from a wind tunnel?

[ Bill Aronson - 45 - 10:50:43 ]
I was on active duty in the USAF for three years!! I was involved in clinical work, critical incident stress debriefings, psychological testing, and briefings. My sport psychology training was utilized more at my work at various academic schools, coaching tennis, and wheelchair tennis.

[ Duane Knudson - 46 - 10:51:25 ]
RE: [Jani Pallis] Duane or Marcie this was a write-in question from Cory Kestner. A lot of research has been done on the open-stance forehand, but a shot that I've noticed being hit more frequently is the open stance forehand volley. I've seen more pro level players hitting the shot in the past year than ever before. Is there any research to show whether or not it is a more biomechanically efficient stroke, or what the benefits may be? From my own background in sports science, I understand the similiarities between the open-stance forehand and the open-stance forehand volley concerning weight transfer and recovery time. What interests me is whether there is a difference in power between the classic forehand volley and the open-stance forehand volley. Please let me know whether you are aware of any research on this topic.
Cory, that is a good question. More and more players are hitting forehand groundstrokes and volleys from a open stance (body facing the net). I have done some research with the forehand and we have found the classic square stance allows more of the body to be used to generate racket head speed. The difference is small (5%) but it is there. Racket speed is the primary determinant of the speed of ball rebound in a groundstroke or serve. In a forehand volley, the same forward motion of the body can contribute to racket head speed, but in the volley accuracy of racket placement may be more important that racket speed. I would recommend that players stay with the cross-over step into a square or closed stance for a forehand volley. It gets you closer to the net and allows you to reach farther to the side with one step.

[ Jani Pallis - 47 - 10:51:25 ]
RE: [Rabi Mehta] For a given tennis stroke, would a used tennis ball go faster or slower than a new ball??
Well remember how Andre Agassi felt when Justin Gimblestop threw a used ball into the stands at the 1999 US Open. A new ball would be substituted.

[ Rabi Mehta - 48 - 10:51:59 ]
RE: [Jani Pallis] Rabi - What's a drag coefficient and how does that differ from drag? How do you calculate drag from a wind tunnel?
I will answer this in parts. The drag force on the ball is the resistance force that slows the ball down (much like the force on your hand if you stick your hand out of a car window -- please don't do this!!)

[ Jani Pallis - 50 - 10:53:47 ]
RE: [Bill Aronson] I was on active duty in the USAF for three years!! I was involved in clinical work, critical incident stress debriefings, psychological testing, and briefings. My sport psychology training was utilized more at my work at various academic schools, coaching tennis, and wheelchair tennis.
Bill - How did you get interested in wheelchair tennis?

[ Rabi Mehta - 51 - 10:54:40 ]
RE: [Rabi Mehta] I will answer this in parts. The drag force on the ball is the resistance force that slows the ball down (much like the force on your hand if you stick your hand out of a car window -- please don't do this!!)
In the wind tunnel, the ball is mounted on a "balance" -- this is simply a wing attached to a device that measures "rotational force" --- the ball is attached to the wing via a sting (thin rod) --- as the ball is pushed back by the airflow, the device (load cell) measures the resistance and this is converted to drag force.

[ Jani Pallis - 52 - 10:55:32 ]
RE: [yours] How does aerodynamics work?
Rabi - There was a question submitted at about 10AM .. asking "How does aerodynamics work? I assume the person meant around a tennis ball.

[ Rabi Mehta - 53 - 10:57:32 ]
RE: [Rabi Mehta] In the wind tunnel, the ball is mounted on a "balance" -- this is simply a wing attached to a device that measures "rotational force" --- as the ball is pushed back by the airflow, the device (load cell) measures the resistance and this is converted to drag force.
The drag coefficient is a non-dimensional number that is formed by dividing the drag force by (0.5 X rho X U X U X S) where rho is the air density, U is the flow (or ball) speed and S is the ball projected area. The variation of this nondimensional number with flowspeed is what we measured.

[ Kent - 54 - 10:57:43 ]
I believe they put dimples on golf balls to create lift, thus gaining distance. Is this true?

[ Bill Aronson - 55 - 10:58:11 ]
My first exposure to Wheelchair Tennis was in graduate school at Springfield College. After that I decided to conduct clinics and help promote the sport for wheelchair athletes. I am working closely with our staff to interact and provide players more opportunities in these areas. Marcie has information on the fitness protocol that Sport Science is developing!!! Sport Science hopes to receive research grants in these areas as well.....

[ Jani Pallis - 56 - 10:59:31 ]
Duane - Can you summarize your research results on grips?

[ Rabi Mehta - 57 - 11:00:02 ]
RE: [Jani Pallis] Rabi - There was a question submitted at about 10AM .. asking "How does aerodynamics work? I assume the person meant around a tennis ball?
In terms of the tennis ball, the aerodynamics mainly consists of the flow separating from the surface of the ball at about 90 degrees from the front --- the exact point of separation is greatly affected by the roughness (felt).

[ Jani Pallis - 58 - 11:01:23 ]
Bill or Marcie - What types of research are conducted with pre-college students?

[ Rabi Mehta - 59 - 11:02:27 ]
RE: [Kent] I believe they put dimples on golf balls to create lift, thus gaining distance. Is this true?
Hi Kent, You are half right --- the dimples create a turbulent flow around the ball which reduces the drag and enhances lift on a spinning ball -- the same lift effect also takes place on tennis balls -- that is why you often see the tennis ball swerve sideways.

[ Hans/McNeeseState - 60 - 11:03:39 ]
Any research yet on long rackets - head speed, service angle at contact, etc.?

[ Jani Pallis - 61 - 11:04:37 ]
Hi Hans - Glad you could join us!

[ Duane Knudson - 62 - 11:04:40 ]
RE: [Jani Pallis] Duane - Can you summarize your research results on grips?
There are several styles of grips for holding the tennis racket. I have done a series of studies on two of these grips in the forehand and backhand. Small force sensors were mounted on a racket and I found that the forces on the players hands after impact varied widely. Forces could range from a couple pounds to over a hundred pounds. Another interesting finding was that the patterns of gripping the players naturally created. All players use moderate grip forces to swing the racket, only increasing grip pressure just before impact to values that correspond to only about half of their maximum. The belief of gripping hard to increase the speed of the shot is incorrect for most all groundstrokes. A skilled player only uses the grip forces they need to control the direction of the racket. Increasing hand forces only tends to increase the impulsive loading you experience after impact.

[ Kent - 63 - 11:04:46 ]
What is the other half of the answer to the dimples on golf balls?

[ Jani Pallis - 64 - 11:05:26 ]
We had another question about 10 AM from Michelle who asked: IS TOO MUCH HEAT BAD FOR A TENNIS BALL?

[ Kent - 65 - 11:06:52 ]
Is there a limitation to the tension of tennis racket strings and the type of material used for strings?

[ Rabi Mehta - 66 - 11:07:24 ]
RE: [Kent]What is the other half of the answer to the dimples on golf balls?
The dimples affect BOTH lift and drag by keeping the boundary layers on the ball turbulent --- this is a function of the ball speed AND surface roughness.

[ Bill Aronson - 67 - 11:07:46 ]
We have some interesting research with this population. A member of our Sport Science Committee: Dr. Michael Bergeron is investigating Heat Stress in Junior players during training and competition in the heat. We currently conduct fitness testing at our area training centers throughout the United States. We hope to conduct additional studies in the areas of nutrition and weight training for these populations. This population is a very important part of our mission!!!

[ Jani Pallis - 68 - 11:09:16 ]
I want to get back to Rabi's question about the used tennis balls. There's a couple of things to consider: 1) when a ball is really worn down its felt and 2) when the felt or fuzz on the ball has been raised. Rabi - can you tell everyone how this affects drag and speed of a tennis ball?

[ Rabi Mehta - 69 - 11:09:49]
RE: [Jani Pallis] We had another question about 10 AM from Michelle who asked: IS TOO MUCH HEAT BAD FOR A TENNIS BALL?
Too much heat is probably NOT good since the ball will become softer and it will therefore not bounce as much --- severe heat also affect the players, of course.

[ Duane Knudson - 70 - 11:09:56 ]
RE: [Hans/McNeeseState] Any research yet on long rackets - head speed, service angle at contact, etc.?
Hans, good question. Racket technology tends to develop much faster than scientists can get time to research. There are several studies of shorter rackets for young players learning the game. I have only seen one study of the new longer tennis rackets. Rafael Bahamonde and Stan Plagenhoef have done some high speed strobe/film studies of various spin serves. The paths of the racket are pretty consistent with what coaching literature says, but: a flat serve really has some topsin (about 700-100 rpm), slice predominantly sidespin (2000-3000 rpm), and twist has a combination of side and topspin (2000-2500 rpm).

[ Chad - 71 - 11:10:01]
Does the roughness of the surface increase the effect certain types of spin on the ball causing it to curve or slow down more rapidly?

[ Hans/McNeeseState - 72 - 11:10:52 ]
Duane, Did you happen to get a grip pressure reading on a drop volley & if so, was it much lower/higher than you expected?

[ Jani Pallis - 73 - 11:11:22]
RE: [Kent] Is there a limitation to the tension of tennis racket strings and the type of material used for strings?
Bill, Marcie or Duane - Can you answer Kent's question?

[ Rabi Mehta - 74 - 11:12:39]
RE: [Jani Pallis] want to get back to Rabi's question about the used tennis balls. There's a couple of things to consider: 1) when a ball is really worn down its felt and 2) when the felt or fuzz on the ball has been raised. Rabi - can you tell everyone how this affects drag and speed of a tennis ball?
Our recent measurements show that, at say 100 mph, the drag on tennis ball REDUCES as the ball becomes worn -- in the extreme case when ALL the felt is worn, the drag can be lower by a factor of 5!!

[ Rabi Mehta - 75 - 11:14:27 ]
RE: [Rabi Mehta] Our recent measurements show that, at say 100 mph, the drag on tennis ball REDUCES as the ball becomes worn -- in the extreme case when ALL the felt is worn, the drag can be lower by a factor of 5!!
If the felt is raised, the drag goes up --- it seems the drag of the Wimbledon and French Open balls is slightly higher.

[ Duane Knudson - 76 - 11:15:11 ]
RE: [Kent] Is there a limitation to the tension of tennis racket strings and the type of material used for strings?
Kent, my 1985! rules of tennis say nothing about the kind of strings or their tension. There is a comment on the old "spaghetti" stringing that was outlawed. The tension you string the racket is up to you and is only limited by the strength of the frame and stringing machine.

[ Jani Pallis - 77 - 11:15:37 ]
So Rabi - can you tell everyone what happened with the larger tennis balls in the wind tunnel test?

[ Rabi Mehta - 78 - 11:16:24 ]
RE: [Rabi Mehta] If the felt is raised, the drag goes up --- it seems the drag of the Wimbledon and French Open balls is slightly higher.
If a worn ball has less drag, why do players prefer serving with new balls ?? I believe that the advantage of the extra bounce from a new ball probably out weighs the aerodynamic advantage --- do you agree, Jani??

[ Rabi Mehta - 74 - 80 - 11:18:59 ]
RE: [Jani Pallis] So Rabi - can you tell everyone what happened with the larger tennis balls in the wind tunnel test?
The 6.5% larger ball had a slightly lower drag than a standard ball --- this was expected because of the felt with less nap that was used on these larger balls --- however, the 8.5% larger ball had a slightly higher drag --- unfortunately these balls were used in play for some time and their felt (roughness) characteristics were therefore probably affected -- we hope to test new larger balls soon.

[ Kent - 81 - 11:20:10 ]
Wouldn't a player be able to induce more curve into a ball with rapid spin if the ball were new vs. an worn ball?

[ Rabi Mehta - 82 - 11:21:45 ]
RE: [Chad] Does the roughness of the surface increase the effect certain types of spin on the ball causing it to curve or slow down more rapidly?
Hi Chad. Yes, the roughness affects both the drag and lift characteristics of the ball -- the dimples on a golf ball and the felt or fuzz on a tennis ball have the same overall effect on the flow very close to the surface, but the forces on the balls are different.

[ Rabi Mehta - 83 - 11:22:49]
RE: [Kent] Wouldn't a player be able to induce more curve into a ball with rapid spin if the ball were new vs. an worn ball?
Kent, Yes, you are right, but don't forget that in tennis matches balls are replaced every 7 games or so -- so there is not much chance of the felt coming off completely.

[ Jani Pallis - 84 - 11:24:00 ]
There was another write in question - Duane or Marcie: I was looking for a reference on the axis of rotation for a "kick serve". My group supervisor thinks the axis must have a component perpendicular to the net. I don't see how I could be putting that spin on the ball without wrenching my shoulder.

[ Jani Pallis - 85 - 11:25:15 ]
And yet another write - in questions from Brian Lamore: Subject: String Tension vs. Ball Energy Return Efficiency As I understand the subject, tighter tensions cause the ball to deform more and lose efficiency in the transfer of energy between the racquet and ball. Looser strings deform the ball less and, thus, have a greater efficiency. Is this correct, and is the notion of "tight for control" and "loose for power" really a matter of efficiency? I would also be interested in any string "trends" you have discovered.

[ Rabi Mehta - 86 - 11:25:31]
RE: [Rabi Mehta] Kent, Yes, you are right, but don't forget that in tennis matches balls are replaced every 7 games or so -- so there is not much chance of the felt coming off completely.
Also, a new ball probably "bites" more off the racket strings and court surface.

[ Duane Knudson - 87 - 11:26:59 ]
RE: [Jani Pallis] There was another write in question - Duane or Marcie: I was looking for a reference on the axis of rotation for a "kick serve". My group supervisor thinks the axis must have a component perpendicular to the net. I don't see how I could be putting that spin on the ball without wrenching my shoulder.
I tend to agree with the writer of this question, and the not the supervisor. Most of the spin on a twist serve is a combination of topspin and sidespin.

[ Rabi Mehta - 88 - 11:29:31 ]
By the way, if anyone is wondering, the drag coefficient of a new tennis ball is about 0.6 in the speed range, 50 to 150 mph.

[ Jani Pallis - 89 - 11:31:27 ]
I want to thank everyone for there participation! Rabi, Bill, Marcie, Duane - thanks very much again! I want to thank all the people who came to chat with us too! And I want to thank NASA's Learning Technology Project and Quest group for hosting us! Thanks everyone!

[ Duane Knudson - 90 - 11:31:37 ]
RE: [Jani Pallis] And yet another write - in questions from Brian Lamore: Subject: String Tension vs. Ball Energy Return Efficiency As I understand the subject, tighter tensions cause the ball to deform more and lose efficiency in the transfer of energy between the racquet and ball. Looser strings deform the ball less and, thus, have a greater efficiency. Is this correct, and is the notion of "tight for control" and "loose for power" really a matter of efficiency? I would also be interested in any string "trends" you have discovered.
Brian, your observations are correct. No object is perfectly elastic, meaning all the energy stored in the object as it is deformed can be recovered as it returns to its normal shape. Note how the tennis ball when dropped does not rebound to the height you dropped it from. Tennis balls are less elastic than the string bed, so lower tensions store more energy in the strings (ball deforms less) that can be recovered. String at lower tensions for more ball speed, and string at higher tensions for more accuracy and ball spin.

[ Jani Pallis - 91 - 11:34:08 ]
We will be posting the chat this afternoon. You'll be able to view it at http://wings.avkids.com/Tennis/Howto/chatFeb29.html Take Care Everyone!