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Aerodynamics in Sports Technology
Jan. 29, 1998 Event Transcript
NASA Ames: . . . . Thu, Jan 29, 10:09AM . . . [33 ]
: Everyone out there on the Internet, I'd like to welcome you to our first
live video conference from the greater Miami area for the aerodynamics
in sports project. We are here in Key Biscayne which is in the Miami area,
doing some research concerning the flight of the tennis ball. We are going
to talk to you about that today and take your questions. I'd like to introduce
myself and the other members of the team. I'm John yen Dell, the project
manager. I'm a tennis coach and a video producer in California, and with
me, this is Dr. Paul Roeder, the administrator of sports science for the
usTA here in Key Biscayne. Also we have today some people at NASA Ames,
distinguished members of our investigating team who you won't be seeing
over the video, but they are participating by audio, we hope, and I want
to introduce them. And if you guys could just say hi as I introduce you.
First, we have Dr. Ra di met ta, and internal specialist at NASA ams.
NASA Ames: . . . . Thu, Jan 29, 10:10AM . . . [34 ]
: I'm here. This hello kids. And that includes the big kids like yourself.
NASA Ames: . . . . Thu, Jan 29, 10:10AM . . . [35 ]
: Great. And also at NASA ams we have shi sheer pan di a, a computational
fluid die nam my sift. And if you don't know what that is or what computational
fluid DYNAMIC is, you are going to find out in just a few minutes. Shi
sheer, can you read me? SPEAKER1: Yes, I can. Good morning .
NASA Ames: . . . . Thu, Jan 29, 10:11AM . . . [36 ]
: Good morning. And also I think at NASA Ames, my deer friend NASA business
can der, a physicist, a member of our teams, a physics teacher at university
high school in San Francisco where I also coach the girls varsity tennis.
Are you there? SPEAKER1: Yeah, I'm here, John. How are you? SPEAKER1:
I'm feeling good. How is the weather out there in California? SPEAKER1:
It's been raining quite a bit. Had a pretty bad storm this morning.
NASA Ames: . . . . Thu, Jan 29, 10:12AM . . . [37 ]
: We're going to tell you about our problems with the rain here in Miami.
But what we want to do is talk a little bit about our project. It's called
aerodynamics in sports, and what we are doing is studying and hoping everyone
out there understands principles of aerodynamics and the basic physics
of sports by studying tennis, and specifically the flight of the tennis
ball. Before we started this project I never thought of it in quite this
way, but really understanding tennis is all about understanding exactly
how the tennis IS ball flies. And playing tennis is learning how to make
the ball fly in very certain ways. For example, pat your opponent for
an ace on a big first serve. I want to give out the Internet address so
everyone out there can follow our project. It's-> wings.UCDavis.EDU/kpal/TENNIS/.
We'll put that out later in a display screen so you can see it if you
missed it.
NASA Ames: . . . . Thu, Jan 29, 10:13AM . . . [38 ]
That's kind of a long one but you'll be seeing it again in a second. Before
we start we'd like to pose a question for everyone out there and we want
you to email your answers directly to us and we'll see how many people
out there following this video conference know the answer to this fairly
simple question. The answer is, if you are at the us open and you are
center court receiving a serve from Pete Sampras, does that serve speed
up or slow down after the bounce? The answer you might find surprising,
but we'd like to know what you think. So email address us the answer and
we'll let you know what the answers were in a little while. For the topics
of today's conference are going to include various aspects of how the
tennis ball flies, how fast it flies, how it spins in pro tennis, how
the air flows around the ball in tennis, and how we measure that, and
also the biomechanics of what we call strokes in tennis that produce that
flight. Now, I want to start today with my distinguished colleague, Dr.
Paul row ter, he and I are collaborating on this project, and we want
him to answer for you a few basic questions that I'm going to ask. For
example, an obvious place to start is Paul, what is the us T A.
NASA Ames: . . . . Thu, Jan 29, 10:13AM . . . [39 ]
NASA Ames: . . . . Thu, Jan 29, 10:14AM . . . [40 ]
: It's the national governing body for tennis which means this that the
USTA runs amateur tennis in this country. In addition to that and probably
more importantly we run the US open tennis championships held every December
in New York city.
NASA Ames: . . . . Thu, Jan 29, 10:14AM . . . [41 ]
: Very good. And now as administrator of sports science, why don't you
tell us a little about if not exactly about what sports science is and
how it applies to tennis.
NASA Ames: . . . . Thu, Jan 29, 10:15AM . . . [42 ]
: We actually deal with six different sports items. Sports psychology
is one of those sports sciences. Sports psychology, how mentally tough
somebody is as they play tennis. The motor learning component is the second
component. How they efficiently and effectively learn strokes and play
tennis as quickly as possible. Then nutrition is a third area, nutrition
is not only the types of food you take in but also the kinds of fluids
that you require, how your body reacts to that. Another field deals with
the systems within your body such as the muscular system, how hard can
you hit the ball. Then another area is sports medicines which not only
deals with injury rehabilitation, but also prevention. Finally biomechanics
which relate to this project which to put it in simple terms is sports
technique analysis and that's what we've been working on.
NASA Ames: . . . . Thu, Jan 29, 10:15AM . . . [43 ]
: We're are going to see some of that analysis we've done, some of the
incredible video we've shot. Let me ask you another question, Paul. Why
should tennis players, since we are studying the flight of the tennis
ball, be interested in that subject.
NASA Ames: . . . . Thu, Jan 29, 10:16AM . . . [44 ]
: I think tennis has changed over the last 20 or 30 years. Certainly equipment
has changed, technology has improved, rackets have gotten lighter, racket
heads have gotten bigger, strings have changed, string technology. And
also players are being asked to hit on all different sports surfaces.
Grass courts are very soft. Hard courts give an immediate type speed and
slower courts. Players need to know about sports science and how to improve
their game using the very latest in technology to help improve their mental
game.
NASA Ames: . . . . Thu, Jan 29, 10:17AM . . . [45 ]
: Here is a question that just came in over the Internet that someone
e-mailed in for all. Paul, I know you are going to like this one. Is there
a biomechanically best grip for a forehan ground stroke? SPEAKER1: That
is a loaded question. I'm probably not the person to ask that. I'll give
it a shot. With technology changing over the years, really the rackets
are lighter so you can swing them through the air faster. And players
are swinging faster and they've gotten stronger. They also have had to
put more spin on the ball. To be able to put more spin on the ball a lot
of modern players have gotten to what we call a semi western or western
grip which places the racket in a position that you have to ink eight
more spin on the ball as you swing.
NASA Ames: . . . . Thu, Jan 29, 10:18AM . . . [46 ]
: All right. And speaking of forehan grips, it's interesting that although
so many were getting a piece of video, our principal investigator is seeing
that right under that. It's in the video machine even as we speak. It's
interesting although so many players are playing with semiwestern or western
grips, there is one player using more of a classic eastern grip, and we
are going to show you that player now. I'm sure you can guess who I'm
thinking of, it's Pete Sampras, the No. 1 player in the world. And this
is some footage that we shot at the us open of Pete actually playing matches.
Switch it, Jane.
NASA Ames: . . . . Thu, Jan 29, 10:18AM . . . [47 ]
: ECR .
NASA Ames: . . . . Thu, Jan 29, 10:18AM . . . [48 ]
: A PA backhand and you are going to see some more back hands here. We
thought we were going to see a forehan. Paul, why don't you talk a little
bit about his grip.
NASA Ames: . . . . Thu, Jan 29, 10:18AM . . . [49 ]
: I think what Pete is showing here, John, is there is no true one way
to hit the ball. Even though in general players have gone very much to
a semiwestern or western grip, I still see some players out there with
an eastern forehan grip as well. My grip personally which I use which
is a continental grip is probably out there. Although players like ste
fan Ed burg has used in his career.
NASA Ames: . . . . Thu, Jan 29, 10:19AM . . . [50 ]
: Would you say for the average player, Paul, that the grip that Sampras
uses is more applicable and has a greater longevity for playing the game
over the course of your entire life? SPEAKER1: I think so. Pete has very
much an all around game, which is not just forhands and back hands. It's
called an eastern forehan grip. It's easier to change grips because of
the grips. Some of the players that have more extreme grips such as the
semiwestern and western grip are very adapted playing the ground stroke
game but may not be as adept at hitting fouls.
NASA Ames: . . . . Thu, Jan 29, 10:20AM . . . [51 ]
: Very interesting, so maybe for all you younger players out there you
want to think about what in the long run might be the best for your game.
You may very well be a great semiwestern player, but then you may not
be going to the us open and you might want to play tennis for 40 or 50
years and you might have a hard time hitting that heavy top spin when
you get to be, say, my age or even as young as Paul. So now, if we could,
Jane, we'll move on and we'll just see a little bit of what we have done
with some of this video. And one of the most amaze things that we have
done is we have used this video, and this is actual wide footage from
the open , to -- major feedback over the Internet .
NASA Ames: . . . . Thu, Jan 29, 10:20AM . . . [52 ]
: Everybody still there? Can you hear me out at Ames? SPEAKER1: Yeah,
we can still hear you
NASA Ames: . . . . Thu, Jan 29, 10:22AM . . . [53 ]
: Sorry about that. And what you can see here is an actual animated 3D
reconstruction based on some of the video of Pete. It's coming up right
here in a second. I'm sorry about the blockage. Go ahead and play it.
Okay. Right there is a skeleton that we are working on. It's not perfect
yet, but you can see the similarity right there in Pete Sampras's forehan,
and from that, as a result of this project we are hoping, we are hoping
there is a comparison of the skeleton with Pete. To make some biomechanical
measurements that are going to tell us new things for the first time --
I'm going to go to camera now, new things for the first time. Whoops,
let's go back. New things for the first time about the biomechanics of
the strokes in terms of the relationships to the body parts and how fast
the muscles are moving. Okay. So there is Paul.
NASA Ames: . . . . Thu, Jan 29, 10:22AM . . . [54 ]
Paul, why don't you --
NASA Ames: . . . . Thu, Jan 29, 10:22AM . . . [55 ]
: Which mus elts to actually use.
NASA Ames: . . . . Thu, Jan 29, 10:23AM . . . [56 ]
: That's exciting. I want to thank Paul. You may stick around for a couple
of minutes if we get some more questions. Paul is actually getting on
a plane to New York, he's going up there to do a sports science conference
in New York tomorrow morning. We really appreciate him squeezing this
conference into his schedule. If anybody has a question for him or about
the Sampras video that we just saw, now is the time to get that question
in. So I want to move on, if I can, to our fablous team out there at NASA
Ames. I wish you guys could see our team members out there, because the
intelligence is radiating. We have Robbie mad who is one of the leading
arrow die nam my sifts in the country and he has done more research on
sports projectiles than anyone. He's going to talk to us today a little
bit about what a wind tunnel is and how it relates to what we are doing
in our projects. So roby, did you hear any of that? SPEAKER1: Yes, I did.
And I will start right here. Before I --.
NASA Ames: . . . . Thu, Jan 29, 10:23AM . . . [57 ]
: Rabbi, are you there?
NASA Ames: . . . . Thu, Jan 29, 10:23AM . . . [58 ]
: yes, can you hear me? Oh oh. Hello, John? SPEAKER1: Lost him. We just
lost audio.
NASA Ames: . . . . Thu, Jan 29, 10:24AM . . . [59 ]
: Hello, John? John?
NASA Ames: . . . . Thu, Jan 29, 10:24AM . . . [60 ]
: is that you, roby? Take it away, man.
NASA Ames: . . . . Thu, Jan 29, 10:25AM . . . [61 ]
: Okay. All right. Before I get to talking about wind tunnels as John
just said, I want to go through some -- a little bit of background on
what we are doing and why we are doing it, and if you can hold that ball
up, John, that will amuse the audience. There you go. Keep moving it,
by the way. Okay. One of the ams of at least my part of the project is
to study and understand the flight of a tennis ball which means that once
a player is, like we saw Sampras hitting the ball, once it's hit, where
is the ball going to end up? Is it going to end up on the court out of
bounds, hit the net or what? And one way to do this is obviously to shoot
movies like John did, and showed bits of a little while ago. And this
can give you a feel for what the flight of the ball is. But to really
understand the flight in a critical way, we have to actually go in and
study the flow of the ball. There is an illustration there of what the
flow might look like over a ball
NASA Ames: . . . . Thu, Jan 29, 10:26AM . . . [62 ]
and also measure the forces on the ball. Because it is the forces on the
ball that dictate where the ball is going to end up. Now, the ultimate
goal of all this, and you'll hear about this later on, is to be able to
use computers to predict the flight path of a ball, and then it becomes
easier to vary the initial conditions like the speed of the racket, the
amount of spin you put on, what types of balls one should use in terms
of their pressure in the ball or 0 the surface, and so everything becomes
a lot easier once a computer is able to predict the flight path. But before
we can do that we really have to get a grip on what kind of forces are
present on the ball. Now, if you can imagine a ball flying through the
air, there are two obvious forces which I'm sure you've heard about. One
is the force of gravity which is trying to pull the ball down. Anything
that we throw up, within reason is going to come back down to the ground
and that's the gave tagsal force. The other one sf so-called drag force
which slows it down. A good way to experience this although I won't recommend
it, is to put your hand out of a window as you are going down a freeway
in a car. That's the drag force.
NASA Ames: . . . . Thu, Jan 29, 10:27AM . . . [63 ]
Then there is a gravitational force bringing it down. Now, it turns out
that on a ball that is spinning and the ball is almost also spinning in
tennis, there is an additional force which is commonly known as the mag
nus force, that's spelled, M-a-g-n-u-s, and it's named after a German
scientist who first observed this effect. And this magnus force basically
produces an additional force perpendicular to the line of flight and it
can make the ball either go up or dip faster or go sideways. An easy way
at home to experience this is to take a beach ball and propel it with
some back spin and you'll see how it rises as it goes through the air.
If you put side spin on it you'll find that it curves through the air
sideways. This is the same principle that makes a babl curve through the
error a soccer ball or a golf ball lifting through the air. It's all the
same principle, this mag nus effect. So there are these three forces that
we want to try and study. Needless to say it's very difficult to actually
measure the forces on a moving ball in tennis. So what we do is to replace
that with a wind tunnel.
NASA Ames: . . . . Thu, Jan 29, 10:28AM . . . [64 ]
In a wind tunnel the ball is held stationary and we have air flowing past
it. A wind tunnel can be thought of as a large tube, if you will, one
end of which you can have a fan that forces air through the wind tunnel.
Now, in basic concept this is the same as the ball flying through the
air. The advantage of the wind tunnel is the ball is stationary. We can
go in and study the flow over it using, say, smoke, for example. This
is the same smoke they use in theaters, on the stage, and by observing
the flow of the smoke we can see where the flow is going over the ball.
To take it a step further then, we would want to measure the pressure
that I talked about, the forces, rather. And the forces can be measured
easily if the ball is mounted on some sort of a balance. Most wind tunnels
have balance devices that give you the drag force I talked about that
slows it down, also this lift force which is the mag December nus force
that we discovered today. Once we know these forcings and how they vary
with the spin rating, with the flow velocity, we can go into computer
codes and apply those numbers and hopefully predict the flight of the
ball. We'll talk a little bit about this.
NASA Ames: . . . . Thu, Jan 29, 10:29AM . . . [65 ]
As you can imagine, spinning a ball in a wind tunnel is not easy. Thinking
about this one has to actually hang it on wires are attached to motors
which can spin it up. We're talking about a spin rate of approximately
60 reves per second. A velocity on a Sampras serve would be over 100 miles
per hour.
NASA Ames: . . . . Thu, Jan 29, 10:29AM . . . [66 ]
: Just to interrupt you one half second, one of the questions that came
in was how do you spin a ball in a wind tunnel.
NASA Ames: . . . . Thu, Jan 29, 10:30AM . . . [67 ]
: I thought I just answered that. One way is to mount it on very thin
wires which do not interfere with the flow, and the wires can then be
attached to motors, say at the top and bottom of the wind tunnel, and
that will then spin the ball. This kind of experiment has been done before.
I was going to say that the fortunate thing is we do not actually have
to use a real tennis ball which is somewhat harder in terms of the testing
parameters, but we could use a bloen up tennis ball. Fortunately John
was able to locate one from the -- it was used I believe as an advertising
piece of the US open. If we use a bigger ball in essence we can test it
at a lower velocity. We don't have to run the tunnel at 100 miles an hour.
That helps out in the testing procedures. That's another way to do that.
The final effect we hope to look at in the wind tunnel is the effect of
the fuzz. As you all know, the tennis ball has this kind of fuzzy outer
layer which eventually wears out. But the question is how does the fuzz
affect the flow of the ball, and we'll be looking at that.
NASA Ames: . . . . Thu, Jan 29, 10:30AM . . . [68 ]
The end product of this experimental study hopefully will be that we have
a lot better understanding of the aerodynamics of a tennis ball in its
flight through the air. I'm done, John.
NASA Ames: . . . . Thu, Jan 29, 10:31AM . . . [69 ]
: I filled out the Web address there on the top of our page on the Web,
for those of you that may not have gotten it the first time. Here is a
couple good questions that came in. What other kinds of balls have been
placed in wind tunnels before, and what kind of sports equipment has been
placed in a wind tunnel? SPEAKER1: That's a good question, and funly enough
I can answer that one. In terms of balls there has been extensive testing
on golf balls which is probably the fastest growing sport in the world
today, apart from tennis, of course. And base balls have been tested to
some extent.
NASA Ames: . . . . Thu, Jan 29, 10:31AM . . . [70 ]
: Of course.
NASA Ames: . . . . Thu, Jan 29, 10:32AM . . . [71 ]
: Cricket balls, that's actually my original favorite and expertise that
I have, and apart from those three or four balls that I talked about,
there hasn't been much research on balls. But I have heard of other sports
articles being tested in wind tunnels, jave a lins come to mind. There
has been extensive testing on ski equipment. Golf club heads, that's the
latest fad now, they are interested in the air' o dynamics of a golf club
because the faster you can swing it the harder you can hit the ball and
the further it will go. Do you have the next one? SPEAKER1: Yeah. One
last question. I don't know if we can answer this. This is one of the
things we are testing down here. But do you have any information on what
the difference might be between a ball and a regular tennis ball? I can
tell you that when we fired the ball out of the machine it seemed to have
a lot greater velocity and possibly spin. But what thoughts do you have
on that.
NASA Ames: . . . . Thu, Jan 29, 10:32AM . . . [72 ]
: Are you talking about a regular ball versus a tennis ball? SPEAKER1:
What if you took the fuzz and the nap off of a regular tennis ball and
you just had the rubber core underneath.
NASA Ames: . . . . Thu, Jan 29, 10:33AM . . . [73 ]
: Okay. My personal feeling is that for the same initial conditions, meaning
the same stroke with a tennis racket, say, the tennis ball should travel,
should have less drag on it, and this is ball the roughness of the fuzz
will actually help to create conditions along the surface which make it
-- which give it less drag.
NASA Ames: . . . . Thu, Jan 29, 10:34AM . . . [74 ]
: I would like to confirm that with an anecdotal evidence. I was hitting
some serves with one of those skinless fuzzless tennis balls, and some
of the biggest, fastest serves I ever hit in my life. Too bad I can't
serve with that ball . The other question, any other sports equipment
that's been placed in a wind tunnel, so far as we know? SPEAKER1: Actually
one thing that came to mind, speed cycling, these are the push bikes,
racing bikes, that's become a very aerodynamic type of sport now and almost
every bike that's built is tested extensively in wind tunnels, especially
the racing bikes. And almost all the -- with the winter Olympics coming
up, all the racing type devices, the Bob shreds, the luj, they are all
tested in wind tunnels very extensively.
NASA Ames: . . . . Thu, Jan 29, 10:35AM . . . [75 ]
: Here is another question that's related that maybe I'll answer part
of. Does the type of racket make a difference in how the ball would fly,
and the answer is yes. The weight, the size of the head, and the flexibility
all make a difference in how hard you can hit the ball, how hard you can
swing the racket, that is, and the amount of spin that you can generate.
But let me ask you this, roby. Do you think we could learn something about
racket design if we were to test some of the various types of rackets
in a wind tunnel?
NASA Ames: . . . . Thu, Jan 29, 10:35AM . . . [76 ]
: most definitely. Off the top of my head the incremental gain in making,
say, a tennis racket frame more aerodynamic may not be that significant
because most of it is quite aerodynamically shaped, meaning the frame
is fairly slim and the actual gut is also fairly aerodynamic in that it's
slim wire, if you will, as opposed to say a golf club where you have this
heavy big bluff mass at the end of it.
NASA Ames: . . . . Thu, Jan 29, 10:37AM . . . [77 ]
: Excellent. So we'll see as more questions come in about the wind tunnel
or the aerodynamics of the ball. But related to that we want to move on
now to shi sheer pan di a. He's a computational fluid die nam a sift.
Shi sheer, why don't you tell us, if you can, exactly what is computational
fluid dynamics, and what is your role going to be in forwarding our project?
SPEAKER1: Okay. The the subject of computational fluid dynamics is broken
up into two parts, the computational part and the fluid dynamics part.
The fluid dynamics is trying to model the air around the tennis. In the
picture up on the screen right now in the assent configuration. By computational
I mean modeling these things on a computer, most of these tools are made
for aerodynamics on airplanes and spacecraft like the shuttle, and we
are going to try to apply those tools onto a tennis ball. Generally conpew
tagsal fluid dynamics is broken up into three parts. One is modeling the
geometry, in our case it would be a sphere call surface like a tennis
ball. Then the second part is solving the flow over the surface.
NASA Ames: . . . . Thu, Jan 29, 10:38AM . . . [78 ]
So once you have a model of the surface you would go around that model
and then you solve at each point you solve for things like pressure and
the speed of the air around the ball. And as you -- once you solve for
these numbers then you have a lot of data, you have a lot of numbers,
and you need to know exactly what those numbers mean. So the last part
of the process is to do what we call post processing, figure out what
the pressures look like on a tennis ball, what the figures look like on
a -- the figure up there right now is a surface model of the tennis ball
we are going to be using, and it's in three pieces. As you can see, what
the main piece is just a sphere call piece, then there are two caps to
remove the balls. That's essentially what fluid dynamics is. And for the
tennis ball, we are going to try to do it in three pieces. The first one
you see right now, these are velocities around the tennis ball, and it's
coming from the right to the left, and as the flow comes into the ball
it comes to a rooftop and as it goes around the ball it accelerates so
the highest velocity is on the top and bottom of the ball, and the lowest
velocity is front and back.
NASA Ames: . . . . Thu, Jan 29, 10:39AM . . . [79 ]
This flow is not Vis cuss so you don't see a fact in the back of the ball
that if flow would have separated as you would see -- as wus shown in
one of the previous pictures when they were talking about fluid dynamics.
NASA Ames: . . . . Thu, Jan 29, 10:39AM . . . [80 ]
: What exactly is Vis cuss, shi sheer? Viscosity is the -- is essentially
the factor that gives -- that makes a sort of like a boundary area around
any object that the air is moving around, and it's also responsible for
things like turbulence and I guess separation of the flow. In this case
you can see the flow doesn't separate at all because we haven't modeled
the viscosity, so the flow stays attached to the body. In a real flow
which we will model which will be the next step of our computational process,
we will model viscosity as well and you will see separation on the back
part of the sphere.
NASA Ames: . . . . Thu, Jan 29, 10:39AM . . . [81 ]
: And for those of you who need to know, tell us what viscosity is.
NASA Ames: . . . . Thu, Jan 29, 10:40AM . . . [82 ]
: Right. Say you were passing your hand through water and it feels like
the water is sticking to your hand, and that's essentially the viscosity.
It's what makes different air particles stick to the surface as well as
stick to each other .
NASA Ames: . . . . Thu, Jan 29, 10:40AM . . . [83 ]
: Very good. I understood that. Okay. Here is a question that came in.
Does the tension of the strength of the racket affect the flight of the
ball? And that came from a school in North Carolina. Do you want to take
a shot at that? SPEAKER1: I guess the more tension in the string the faster
you can -- the more bounce you get off the ball.
NASA Ames: . . . . Thu, Jan 29, 10:42AM . . . [84 ]
: Well, to a certain extent -- that was kind of a trick question. I set
you up there. To a certain extent that's true. Think of it like a tram
po lien. Think of it like a tram po lien. If the tram po lien is at just
the right tension for your weight, you will come down and rebound to a
maximum height. If, however, the tram po lien surface is too tight, then
you won't be able to get into the air and the analogy to tennis is the
ball won't have as much speed. If the tram po lien surface is too loose,
you would just go down and get tangled in the surface of the tram po lien
and not get into the air at all. So the answer is it depends on how hard
you are hitting the ball and swinging the racket, but there is some optimum
tension in that racket that will maximize the velocity for your game .
All right, then. I guess it's back to me. And I want to remind you guys
that we are still looking to get more answers to our question of the day
which is when Pete Sampras hits this ball right here, this is an official
ball from the US open, and when he hits 120 mile
NASA Ames: . . . . Thu, Jan 29, 10:43AM . . . [85 ]
an hour serve, when this ball bounces, does it speed up or does it slow
down? Okay. Another question, and this skumg in from Eric at high land
school. This is a good one. What is the fastest a tennis ball can travel?
It's interesting you should ask Eric. I wish I could answer that question
right now. It's one of the things we are thinking about. We can assume,
however, that, for example, on a serve, a player's height sets the limit
on how fast the serve can actually be struck and still land in the court,
that still spin would be a factor as well because spin, as we know, someone
out there at Ames correct me if I'm wrong, actually affects the flight
of the ball and causes the ball to drop more accidentally suddenly into
the court. But from a certain height there is some maximum velocity that
any shot in tennis can be struck from. One would have to think with a
record for serves in tennis being now around 140 miles an hour, that we
are getting close to the boundary. Somebody out there at Ames, help me
out on that.
NASA Ames: . . . . Thu, Jan 29, 10:44AM . . . [86 ]
What do you think about that question from Eric? SPEAKER1: Yeah, I think
basically you are right. The -- as far as the aerodynamics are concerned,
the players, apart from serving as fast as you can, which is obviously
an advantage, it is more of an advantage to make the ball curve through
the air and that means putting side spin on the ball, John Mac en row
used to do that quite well. More importantly, once the ball hits the surface
it will then kick off in a new direction because of that side spin. And
so it's a question of not only speed but also making the ball curve through
the air and then off the surface. It's a similar story to babl where you
have the real hard core fast ball pitchers, an if you can throw it at
200 miles an hour, of course that's a great advantage. But for the mere
more tals it's more important to put some deviation on the ball, and so
I think a sensible tennis player will try and combine speed with some
aerodynamic forces added to that serve .
NASA Ames: . . . . Thu, Jan 29, 10:45AM . . . [87 ]
: I think you have the makings of an excellent tennis coach. The guy I
like to see play in tournaments is the guy that hits the 140 miles an
hour serve, that's the one that never goes in. The guy that can serve
with accuracy and move the ball around the box using the spin is usually
more effective, and of course in a player like Pete we decided to show
you a little bit more video of him. We thought that would be more interesting
to you than just looking at my face. The player like Pete, you have an
incredible combination of spin and speed, and as you watch some of these
sequences, you are going to see some of the amazing things he can do with
the ball. Watch, for example, the top spin here on his backhand. This
is a good time to talk about this a little bit. One of the things we were
doing at the open was studying the spin in pro tennis. This is one of
the things we are most excited about in the whole project. We know something
about the speed of the ball in tennis, but very little is really known
about the spin. And with the digital camera that we used to film this
footage here, we are in the process of investigating the spin.
NASA Ames: . . . . Thu, Jan 29, 10:47AM . . . [88 ]
And it's interesting, I've asked many many knowledgeable people in tennis,
including Greg the great John Mc en row, and virtually no one has correctly
estimated the amount of spin these players can generate in tennis. What
we are going to see right here is an actual piece of our analysis, and
you'll be able to see how many times the ball is spinning over the course
of several frames. This camera is shooting at 250 frames per second. So
every time you see the little drop in the ball or the click in the video
frame, that's 250th of a second. In a minute you'll see Pete's racket
come up, you'll see the contact, right there is one frame. Now if you
were to count how many frames it took for that ball to spin one time,
then you can see the logo pretty clearly, you could figure out the actual
revolutions per minute in tennis and equate that to, say, your power drill
at home, or it's in some ways the sames when you rev up the RPMs on the
motor of your car. It's not quite the same, but just to get a rough idea,
that ball that Pete is hitting on for serves is spinning up to 2,000 RPMs
and we recorded second serves up to 4500 RPMs, so that's pretty amazing.
NASA Ames: . . . . Thu, Jan 29, 10:48AM . . . [89 ]
Now, you guys can look at Pete. I much prefer that you look at him than
me, and I want to move on to our next team member, na sif fits gander.
Na sif and I have been involved in this project from is very beginning.
In fact it was our initial work that led to the enlargement in the scope
of this project. What we were very curious about was how fast the ball
was traveling in tennis after it came off the racket. In other words,
what happens to the ball in tennis when it travels through the air towards
the receiver and, do we have any answers to our questions yet? Do we know?
SPEAKER1: Yes.
NASA Ames: . . . . Thu, Jan 29, 10:50AM . . . [90 ]
: Yes. How fast it's traveling prior to the bounce, and that's our question
for the day. What happens to the speed of the ball at the bounce. What
I'm going to do right now is put up on the display -- if I can figure
out this board in front of me here. Now we went to a chart which shows
what we did, some of the research we did in ascertaining the ball velocity.
You can probably rekoznize that. That's actually the front. Do we have
a side view thing too? It's not going to show too well because it's a
transparency but we'll give it a shot and see how it works. It shows how
we measured something about the speed of the ball. What we did was we
went to a professional tournament in the Bay Area in San Jose, and you
can see it there, not really, but sort of. Maybe we'll go back to Pete
and you guys can visualize this as we do . Let's see if we can get back
to Pete here. There he is
NASA Ames: . . . . Thu, Jan 29, 10:50AM . . . [91 ]
. We put two cameras above the court, one shooting down from the rafters
across the line of the net, and the other one perpendicular, we're shooting
down the centerline. What we did was we filmed players including Pete,
and then we reviewed that video and were able to see how far the ball
moved every time we advanced the video one frame. And a frame with conventional
video is 1/30th of a second, 30 frames per second. And that's where na
sif made an amazing contribution to the project. You wrote the software
that allowed uz to analyze the ball speed. Why don't you talk to us for
a while and tell us what you did.
NASA Ames: . . . . Thu, Jan 29, 10:52AM . . . [92 ]
: Okay, John. As you said, when we first started this project we were
interested in measuring the speed of the ball, and not just at the moment
it was served, but as it flies across the court. And also, you know, how
fast the ball is moving after a different kind of hit, not just serves.
And the radar guns which you see at professional tournaments, measure
the speed of the ball immediately after the serve. And the question that
you originally came to me with two years ago was how fast is it moving
as it goes across the court. And as you said, we chose videotape as the
means of measuring this because it was convenient to have these 1/30th
of a second pictures one after another to see how far the ball was moved
in a 30th of a second. One of the problems we had is we need two separate
views of the ball to figure out exactly where the ball is. And you can
understand why if you, like, close one eye and try and touch things out
in front of you with your hand, it's very hard to judge the depth. You
can see where things are but you can't tell how far away they are. You
really need both eyes to get depth information.
NASA Ames: . . . . Thu, Jan 29, 10:53AM . . . [93 ]
And similarly we needed two separate cameras to get three dimensional
information about where the ball was. So I wrote some softwear that took
these two separate views of the court that we shot last winter at the
Sybase open, and put them together to get three dimensional information
about the ball. And we compared the speeds that we measured from those
pads that we saw on the court and compared them to the radar guns and
we found some very interesting things. Some of the results, when Pete
Sampras serves and the radar gun reads about 120 miles an hour, it's going
about 120 miles an hour when he serves. But as it flies across the court
it slows down pretty quickly. And typically just before the bounce it's
going about 90 miles an hour. It's already lost about a quarter of its
speed. Do you want me to talk about how the speed changes during the bounce
yet ? SPEAKER1: Na sif, do you recognize what we put up on the display.
NASA Ames: . . . . Thu, Jan 29, 10:54AM . . . [94 ]
: Yeah, that's a graph. This is graphs of lots of these different incidents
that we measured to compare them all. And I can't quite see the letters
but I'm assuming these are Pete's serves, and the far left point, the
point on the far left at the highest would be the speed right after the
serve. The next point you see, there is a straight line going down to
the next set of points, that would be the speed just before the bounce.
And then the next set of points to the right of that would be the speed
just after the bounce. And the last set of points is the speed when it
gets to the other player. So you can see from this graph that first there
is a drop of speed as it flies across the court going into the bounce,
then there is a big drop in speed as it bounces, and then the speed continues
to drop off but less dramatically following the bounce as it flies across
the court to the other player. One of the reasons it slows down so much
during the bounce is because when it hits the ground it starts to spin
a lot faster. So some of the original energy of the ball in its motion
is converted into rotational energy, and it can't move as fast
NASA Ames: . . . . Thu, Jan 29, 10:54AM . . . [95 ]
.
NASA Ames: . . . . Thu, Jan 29, 10:55AM . . . [96 ]
: So in a sense when you say rotational energy what you mean is that the
ball increases its spin. And why don't you talk about which way it spins
and about what we found about the change in the direction of the ball
and how that almost seems to create an optical illusion. Why don't you
tell them the story of your experience going up on the roof at university
high school and trying to return some serves that I hit to you.
NASA Ames: . . . . Thu, Jan 29, 10:56AM . . . [97 ]
: Okay. One of the questions that you asked me, John, right when we were
starting is why does the ball speed up when it bounces, and I had never
played tennis, and I was surprised that it would speed up. And we went
up to a tennis court at the school where I teach, and you hit some serves
at me, and I would have sworn that the ball was speeding up when it bounced.
And this was quite surprising. When we analyzed the video footage we found
that it really was slowing down, and it's been hard to explain why it
appears to speed up. Many people agree that it does seem to speed up on
a hard court. One of the possible explanations is that when it's going
into the bounce the direction the ball is actually traveling is not directly
towards you. It's toward the ground. Following the bounce if you are standing
in front of the serve, the ball is now coming towards you. So the component
of its speed in your direction increases after the bounce. And this is
happening really really fast. It takes about 2/thirds of a second for
it to get across the court to you. But part of your brain is really focused
on things coming towards you and
NASA Ames: . . . . Thu, Jan 29, 10:56AM . . . [98 ]
you recognize these things immediately. I think it's an evolutionary at
any rate. So one of the possible explanations that the part of the brain
that recognizes things moving towards you is triggered by the sudden change
of the component of the speed that comes towards you during the bounce.
NASA Ames: . . . . Thu, Jan 29, 10:58AM . . . [99 ]
: I thought that was very interesting. We were saying when we start thd
project, he's saying I'm on record in a published work as saying that
I felt the ball did speed up after the bounce, and a lot of experienced
players that I had spoken to felt the same way. This is very very interesting
to see that the reality of that is different. Now, someone say, "Well,
that's very interesting," but are there implications in tennis"? I've
already found tremendous imply indications in tennis having to do with
your ability to watch the ball. Now, for those of you who have had tennis
lessons probably the one thing you've heard more than anything else is
keep your eye on the ball. Part of returning serves going 120 miles an
hour or even 100, is that if you have information about how that ball
is going to move, certainly it is going to help you see the ball and improve
your return. If you know, for example, that the ball is going to be going
only half as fast when you serve, as when the serve is hit, as when the
serve is hit, the serve is hit, then that is good information to know.
And what it helps with is the timing. It means that you have roughly 1/3
of your time after the ball bounces.
NASA Ames: . . . . Thu, Jan 29, 10:58AM . . . [100 ]
I'll tell you one story about a tennis pro friend and one of my practice
partners, I explained this to him, and he hit 20 of my first serves in
a row. Speaking for him, that's a valuable piece of information to have.
Why don't we go back to you, for a second. Why don't you tell them what
we are going to be doing at the Sybase open in San Jose starting the week
of the 9th, what we hope to do there and continuing our work.
NASA Ames: . . . . Thu, Jan 29, 10:59AM . . . [101 ]
: Okay, John. We are going to be going back to the Sybase open this year,
again with our two video cameras, and try and shoot some footage of some
other players this time. John last summer shot images of lots of different
players at the US open, but so far with the ball speed we've only focused
on Pete Sampras. So we want to correlate some of our ball speed information
with some of the spin information that John got from the US open, and
so we'll be analyzing footage from several other top players, not just
Pete. And we'll go through the same analysis with the two cameras and
the softwear that measures the ball position in each frame, and then we'll
get a better idea of how ball speed changes for lots of different players.
NASA Ames: . . . . Thu, Jan 29, 11:01AM . . . [102 ]
: So that's going to be interesting. Roby, sha sheer, any thoughts about
that? Does it surprise you, and does this kind of relate to the point
of the project that so many, I'll have to say seemingly intelligent people,
IE, tennis players, including myself, some of us fall into that category,
we're so clueless about what was really happening in the ball, to the
flight of the ball. Does that say something about the speer it of the
collaboration on this project? SPEAKER1: Yeah, actually I have some experience
with very similar effects, both in baseball and cricket. As na sif said
rightly, the flight of a ball in any sport is a lot easier for the human
brain to figure out if it's not coming straight at you, and that seems
to be the key in all these sports. When the batter goes up or the bats
man, the tennis player, the ball is coming straight at him. It's more
difficult for him to assess how much it's deviating. So the example I
always use is in baseball the coach asks the batter what happened and
the batter says, "Well, it moved 10 feet." And the coach is going, "No
way, I saw that pitch." So this is the same old problem.
NASA Ames: . . . . Thu, Jan 29, 11:02AM . . . [103 ]
In tennis there are two things happening. The ball bounces, comes at the
player or towards his head, and also the time that the ball takes to get
to the bounce is a lot longer than the time it takes for the ball to reach
him after it bounces, and that producess yet another, if you like, red
flag in the brain. So I'm not surprised that the common perception is
that it speeds up. If you remember, the first time we met I was not convinced
at all that it could speed up. If not, I'd have to go back to high school,
like I told you, because I obviously didn't understand my physics. So
basicly it cannot speed up and it does not, as you've seen. The other
point that does surprise me a little bit is what he said, is that the
ball always generates more spin after it bounces. Does it pick up spin
always after it bounces or in some cases? SPEAKER1: So far I haven't been
able to look at --.
NASA Ames: . . . . Thu, Jan 29, 11:02AM . . . [104 ]
: That's a very interesting question and I'm going to talk a little bit
about what we are doing down here. I think we can safely say that except
in the case of underspin where the spin is reversed and maybe you could
talk about it a little bit with the energy there, that a top spin ball
or a flat ball will always pick up rotation, and we think that may be
true on every serve, so we're going to talk about that. By the way, that's
up here for your viewing enjoyment.
NASA Ames: . . . . Thu, Jan 29, 11:03AM . . . [105 ]
: I want to cover one final thing here. I don't want to hog the time here.
The one question we've been asked a lot of times, including today, was
the effect of the fuzz on the ball, and we've done a little bit more thinking
on that, and really the critical question is how fast is the ball traveling.
If you are going to serve at 100 miles an hour, 120 miles an hour at your
pace, wherever that is, John, then it is believable that the bald ball
without the fuzz will have less drag on it. In the aerodynamics of a ball,
it's critical speed at which the flow field over the ball changes completely,
and amongst other things that critical speed is a function of the roughness,
if you are a good player, very good player where you are serving very
fast, then it's beneficial to have a bald ball. If you are serving at
very nominal speeds like I do, then you are better off with a fuzz ball.
So think that's a question that we need to address more carefully in the
future.
NASA Ames: . . . . Thu, Jan 29, 11:04AM . . . [106 ]
: That's interesting. Now, here is a question that came in, and this is
from Bradley at high land middle school -- high land school. Bradley wants
to know, does the weight of the ball really affect the speed? Why don't
you take that out there at Ames.
NASA Ames: . . . . Thu, Jan 29, 11:04AM . . . [107 ]
: I'll take that, John. The weight of the ball will have an impact on
the speed. The heavier a ball is the harder it is to get it moving. So
the harder it will be for a player to get it going fast. But, on the other
hand, it will be harder for the air to slow it down. So the heavier the
ball, like if it was wet as you are experiencing out there and it picks
up water and gets heavier, you will probably see lower high speeds, and
higher low speeds, so less range in the speed of the ball as it travels.
NASA Ames: . . . . Thu, Jan 29, 11:05AM . . . [108 ]
: Let me just add to that. Hello, John? Let me just add to that. A good
way to look at it is that there is an optimum mass off the ball as well.
Think of two extremes. Think of a tennis ball being made of lead and try
to serve that, how fast do you think you can serve that? Not very fast.
The same time take the other extreme where the tennis ball is made of
just a plastic shell and it's very light, say a couple of ounces, again,
the same thing has, there is a lot of drag on it. It won't go very far.
So there is an optimum mass of the ball that you need to have in order
for it to go at a reasonable speed.
NASA Ames: . . . . Thu, Jan 29, 11:06AM . . . [109 ]
: I liveg that. Okay. Here is another one. This is -- is that Whitney?
Whitney. And this is a good question. I don't know if we've answered this
yet but maybe we can talk hypothetically about this. Let's get shi sheer
involved in this one from the simulation aspect, too. If the lines on
the tennis ball, or do the lines on the tennis ball have anything to do
with or impact upon flight and the speed? SPEAKER1: We are going to study
that. But my initial response to that is that they have minimal effect
on at least the air flow around the ball. So the speed changes due to
it will probably not be that much but it will have some affect, some minimal
effect. We'll know more as we get into the project a little more.
NASA Ames: . . . . Thu, Jan 29, 11:07AM . . . [110 ]
: So roby and sha sheer, would you say that the lines which are bay sablg
seems that bind the cover and the fuzz of the ball are less important
factors than the fuzz itself, and then that the seams in the tennis ball
would have less of an effect on the aerodynamics than say the seams on
a baseball which are actually stitches and which protrude from the surface?
SPEAKER1: Let me take a crack at that. That's kind of hard to say. The
seams, the stitching on a baseball is very pronounced, especially in rehabilitation
relation to the rest of the surface which is very smooth. On a tennis
ball we have a situation where the fuzz is relatively rough. Then you
have this continuity which is a seam going through it. If I had to guess
I would say that on a spinning tennis ball that probably increases the
roughness slightly. So if you took a fuzz ball without this hourglass
seam on it versus a regular tennis ball, I would venture to say that the
regular tennis ball is slightly rougher, and so it would have a different
implication. But the difference probably isn't all that great .
NASA Ames: . . . . Thu, Jan 29, 11:07AM . . . [111 ]
NASA Ames: . . . . Thu, Jan 29, 11:08AM . . . [112 ]
: Okay. Let's go to another question here. This is an interesting question
relates to aerodynamics. Does it matter if you play tennis inside or outside?
And I would say that if it reins you are going to want to play inside.
But assuming that you've got a nice day outside, what would be the difference
in those two conditions? I want somebody else to answer that besides me.
NASA Ames: . . . . Thu, Jan 29, 11:09AM . . . [113 ]
: I'll take a crack. The only time, apart from the obvious things like
rain and srb and what have you, that the differences will be relevant
is if there is a strong breeze at the -- at the outside court, typically
at least the more famous courts are covered by these huge grand stands,
so I think that prevents strong winds from blowing across the courts.
But I can see situations where you might have a gusting wind, and sometimes
you see tennis players hold up on their serve, they toss the ball up and
won't hit it because of the wind. And I think in those situations the
wind can affect the flight of the ball, especially the slower head shots
which are like the sliced backhands or ones where they are just trying
to lob it over a player up at the net. I think those shots are often affected
by wind. And in fact you even see the pros being fooled by that more often
than that. You probably know more about it than I do, John .
NASA Ames: . . . . Thu, Jan 29, 11:10AM . . . [114 ]
: Yeah. I mean I think that most tennis players feel that indoor conditions
are the ideal test. And one of the things that we marvel -- sorry, sorry
about that. One of the things that we marvel about -- I'm showing you
right now pictures of the U.S. open stayeddiums, but one of the famous
matches that was played at the open several years ago was one between
Pete Sampras and Andre ar gassy and there was a famous point made in a
Nike commercial. The point lasted a year and it seemed like the people
that were watching that was it was the best games that was ever played
and it was played in 20 to 25 mile an hour winds. My question to you guys
out there at Ames, as the speed of the shots increased, would the affect
of the wind be lessened? Because I know myself that I would have found
it impossible to play my best tennis in that much wind.
NASA Ames: . . . . Thu, Jan 29, 11:11AM . . . [115 ]
: I'll take a crack at that, I guess. As the speed of the ball increases,
you will see some more aerodynamic drag, and it will -- I guess the speed
of the ball to some extent will overtake the wind problems. But I think
you will still see the effect of the wind substantially, especially at
22, 25 miles per hour winds .
NASA Ames: . . . . Thu, Jan 29, 11:12AM . . . [116 ]
: Yeah, I think, John, the aerodynamics of a tennis ball, I guess we should
have said this up front, is quite complex, and everything we talk about
is a function of that initial velocity. So depending on what you assume
to be the initial velocity and then you say okay, let me add 20-mile-an-hour
tail wind or head wind to that, how will the ball be affected? You have
to look at what is the regime the ball is to start with and how will it
be affected. We are trying to generalize a lot of these effects but in
truth we shouldn't do that, ball it all depends on what that initial,
especially the velocity of the ball is, and perhaps spin rate, too. So
really when talking about these effects we should take a typical serve
or shot, say, that Pete Sampras hit, and then take it from there and see
what the effects would be. In fact that would be one of the goals of this
computer cord, hopefully we'll be able to predict all this in the end.
NASA Ames: . . . . Thu, Jan 29, 11:12AM . . . [117 ]
: So you are saying we need to turn the US open stadium into a laboratory
for our further investigation.
NASA Ames: . . . . Thu, Jan 29, 11:12AM . . . [118 ]
: Yes. And I have to be there at all times, of course.
NASA Ames: . . . . Thu, Jan 29, 11:13AM . . . [119 ]
: I think our entire team could well profit sign tiveth tivethally, scientifically
from a week at the US open. Okay, scientists, these are good questions.
Here is another one. Is the ball hot or hotter in tennis after it bounces
? SPEAKER1: I'll take that one, John. Every time the ball bounces, some
of its energy that it was moving with gets converted into heat. And.
NASA Ames: . . . . Thu, Jan 29, 11:13AM . . . [120 ]
: Can you hear me? Okay. Am I still here? All right; you can see this
by dropping a tennis ball and it will never bounce back to the same height
that you dropped it from. Some of its speed gets -- some of the energy
of its motion gets converted into heat energy, so the ball will get warmer
with every bounce. And actually robly --.
NASA Ames: . . . . Thu, Jan 29, 11:14AM . . . [121 ]
: And just coincidentally, that would be true, if we dropped the ball
straight down on the court. Now, explain to us dprbs and I'm going to
rewind that and show that to you on an end less loop. Explain to us --
explain to us what we are seeing here, the ball went backwards now, don't
think that. But as we see the ball travel here on this arc it appears
to be bouncing higher after the bounce. What do you have to say about
that? SPEAKER1: Well, I can't see how high it was coming from, but if
it's slowed down in its horizontal motion, then it would climb more steeply,
and look like it's actually going to go higher. I doubt it's actually
going to go higher.
NASA Ames: . . . . Thu, Jan 29, 11:15AM . . . [122 ]
: Yeah, I guess I stated the question wrong. It would be, I'm talking
about the angle that it comes in and out, rather than the bounce itself.
It appears to be leaving the court at a steeper angle than it entered
the court.
NASA Ames: . . . . Thu, Jan 29, 11:15AM . . . [123 ]
: Yeah. Well, when it bounces it's going to lose horizontal speed faster
than it loses vertical speed because of that contact with the court. So
if it slows down horizontally but it's still traveling vertically at the
same rate, it's going to take a steeper vertical path. So I think what
you are seeing there is that just that the horizontal component of its
velocity has been reduced by the bounce, but it's still losing energy
I would guess .
NASA Ames: . . . . Thu, Jan 29, 11:16AM . . . [124 ]
: Yeah, it's got to be losing energy. But I think that what we see there
speaks to what you were saying and shows exactly what you were talking
about about the change in the direction and what roby was mentioning about
the ball coming directly at you or more directly at you, and that creating
the illusion of increased velocity. Am I in the ball park with that hypothesis?
SPEAKER1: Yeah, that sounds right. That sounds like the same thing we
were talking about before.
NASA Ames: . . . . Thu, Jan 29, 11:16AM . . . [125 ]
: And John, in terms of the bounce, don't forget that the surface is also
very relevant. We found in cricket, for example, that if the ground is
soft, that would relate to the course at women bell ton, for example,
in tennis, and the ball can actually dig a little divity, if you will,
in the ground and that will cause the ball to come up more vertically
on the softer ground, so that will come into play as well, not so much
on the hard surfaces. That's something you could probably look at down
there in Florida.
NASA Ames: . . . . Thu, Jan 29, 11:16AM . . . [126 ]
: Yes. I'm going to talk about that in just a second here. Here is a question
that's coming in. There are some people out there that want to know, what
is cricket?
NASA Ames: . . . . Thu, Jan 29, 11:18AM . . . [127 ]
: well, if I -- I don't know how much time we have. Not enough, obviously.
What I would suggest to start off is that they get on the Web and just
type in the keyword "cricket," and there is a whole bunch of stuff there
that will keep them busy until Christmas of the year 2,000. Cricket in
some ways is similar to baseball. It's a sport, team sport where you have
the pitcher and he'll hurl the ball at the batter, and again, the basic
goals are the same. The batter tries to hit the ball. And then runs are
scored by running between two wickets as they are called, they are like
three sticks stuck in the ground about 66 feet apart, a similar distance
to baseball also. Some of the rules very are very similar, the fly ball
is out. The sticks are relevant because if the ball hits the stick then
you are also out. The one rule I love to quote to Americans in particular
here, in cricket once you are out you are done and you go and sit down.
You don't get another chance, so it's a lot more critical than in baseball
where you say, oh, well, I'll be back in one or two innings and try it
again.
NASA Ames: . . . . Thu, Jan 29, 11:18AM . . . [128 ]
NASA Ames: . . . . Thu, Jan 29, 11:18AM . . . [129 ]
: Roby, can you see can you see the picture display I've got up there?
SPEAKER1: Yes. The tennis ball? Oh, no. There we go.
NASA Ames: . . . . Thu, Jan 29, 11:18AM . . . [130 ]
: Did you see it.
NASA Ames: . . . . Thu, Jan 29, 11:18AM . . . [131 ]
: Yup, I can see it. I don't know if you can see it well enough, but that
is not -- it's a green court but that is not a hard court.
NASA Ames: . . . . Thu, Jan 29, 11:18AM . . . [132 ]
: Okay. Is it grass?
NASA Ames: . . . . Thu, Jan 29, 11:18AM . . . [133 ]
: it's grass. It is.
NASA Ames: . . . . Thu, Jan 29, 11:18AM . . . [134 ]
: Okay.
NASA Ames: . . . . Thu, Jan 29, 11:18AM . . . [135 ]
: That is a very rare court that we are going to be talking about in a
minute, and that's in Florida and that's more of a surface you play cricket
on. Is it not? How similar are the grasses in the two sports?
NASA Ames: . . . . Thu, Jan 29, 11:19AM . . . [136 ]
: quite similar if you look at a well maintained tennis court, then that's
the kind of surface that they would try and obtain on the cricket field.
The cricket field, the outfield is just a grass field like in baseball,
but they especially prepare a strip in the middle of that, say, circular
ground where the ball is expected to bounce, and that pitch, as it's called,
is prepared in a special way. The role is to can you tell the grass quite
shore, and then you can have, like I said, depending on the weather and
the type of grass that they've grown, it could be a hard wicket where
the ball would bounce a lot. Like, for example, in the west in dis, in
the crib Ian, they would have very hard wickets as opposed to those in
England which by definition and the weather tend to be quite soft. So
there is a lot of care taken to maintain the grass.
NASA Ames: . . . . Thu, Jan 29, 11:20AM . . . [137 ]
: I hope that that helps some of you guys out there that are interested
in cricket. I'm going to shift it you now over to what we are actually
doing here in the Miami area in Key Biscayne. The purpose of for our trip,
in addition to collaborating with Paul rotor, was to investigate the differences
in the way the tennis ball bounces on the tennis court. I have a question
here. Someone wants to know how you can treat a variable so you know exactly
what you are really analyzing when a tennis ball hits a tennis court.
And that's exactly the question we are attempting to answer. And you can
see that in this grass court setup here. I'm going to show you a little
bit of the different court surfaces, too. You can see our cameraman. That's
mark, he's a professional TV camera. He filmed at the us open. He travels
around the world filming tennis. He's looking through the monitor of our
digital camera. And I think you can see there if you back it up just a
little bit, that light block in the upper right-hand corner, those are
some special blocks
NASA Ames: . . . . Thu, Jan 29, 11:22AM . . . [138 ]
we cut. We know the measurements of those blocks, we know the distance
of the camera from those blocks. We know the length of the lines on the
court and we know the height and angle at which the camera is pointing.
Now, we have a state-of-the-art machine ball machine on the other side
of the court. That machine is equipped to throw the ball according to
certain settings. There it is . That's our ball machine, and you can see
the knobs there. That allows us to vary the spin and to throw a ball either
flat with no spin, to throw it with top spin, or to throw it with underspin,
and to throw it with varying amounts. So what we were trying to determine
was with all those types of balls, flat balls or spin balls, this actually
happened when the ball bounces on the court, and how does that vary when
you go from one court to another. Again, players have perceptions about
the bounce of the court. But as we saw with the perceptions of the speed
of the ball, those aren't always correct. Okay. So that was the grass
court we were seeing right there.
NASA Ames: . . . . Thu, Jan 29, 11:23AM . . . [139 ]
What I'm going to hold up now, don't change it yet, yeah, right here,
this is a unique ball. This ball right here is actually a sla zin jer
ball from wim bell Don. These are the exact balls that they play with
on the grass courts at wim bell Don. We've got a -- okay. I'm sorry. Sorry
about that, guys . We are learning. We are learning here even as we progress.
What we are seeing up there, we are going to see there now in a minute
is. Yeah. Now we are going to see one of the other very common types of
courts in Europe and in south America, and that is a red clay court. Now,
the two courts we just saw, the grass court and the red clay, those are
two courts that aren't very common in America. But two of the biggest
tournaments in the world are played on those same
NASA Ames: . . . . Thu, Jan 29, 11:26AM . . . [140 ]
courts. The grass court tournament I'm talking about, roby is very familiar
with, that's wim bell Don. And the French open is played on the red clay
court you are looking at there. And you can see our blocks and you can
see a little bit of our setup. So that's what we've been doing here for
the past week is we've been checking various spins and various speeds
along a straight line and we are using our camera in order to study exactly
what happens when the ball bounces off the court. Now, you might have
said well, why haven't people done this before? And it's a function of
the same digital technology that we used in that video we showed you with
Pete Sampras. For the first time now within the last two years we have
had cameras available to us in sports science that are filming at very
high rates and also allowing us to record what they see on tape. Like,
for example, at the open we filmed about 130 hours of footage, and down
here in Key Biscayne we filmed close to 20 over four days of just the
ball bounces. And that's what they could see.
NASA Ames: . . . . Thu, Jan 29, 11:26AM . . . [141 ]
If you would look at that U.S. open sign, you can see us in the camera
position fill mg at the US open the spin. So that's what we are going
to be doing for the next few weeks is digitizing the video we shot down
here. I'll show you in one second, the other two balls. I'll give you
a hard court shot now. These are more common balls in the United States.
This one right here, this is the open ball. This one right here, this
is a ball that's played on green clay courts. So those are the four courts
we analyzed, the grass, the red clay, hard courts and green, and hopefully
we are going to be able to share with you in the near future what we found
out about that. Now, two more things, we are getting to the very end.
We've had a request. People want to Seymour biomechanics. Before we do
that, yes, there I am adjusting the block. That's on the stadium court
at Key Biscayne where we are doing a hard court test right before the
torrential downpoor that forced us inside for a day. That's one of the
things you encounter in research, that you might know
NASA Ames: . . . . Thu, Jan 29, 11:26AM . . . [142 ]
what you want to investigate, but sometimes you have to be patient to
get it done. Now, a few things. One, I want to answer the question, I
have to say that most of you guys out there knew more about how the tennis
ball bounces than myself or some other pros because the majority of the
people e-mailed in that they thought the ball would slow down after the
bounce, so you are off to a very good start in understanding the flight
of the tennis ball and our project.
NASA Ames: . . . . Thu, Jan 29, 11:28AM . . . [143 ]
: They think it's a perceptual problem which was the problem I had as
to why the ball would speed up. And now we are getting down to the last
three or four minutes here. What we want to do, we had to request to show
some more of the graphics. People e-mailed in saying we'd really like
to see the skeletal stuff. I want to show, one thing that we haven't shown
before, this is, we're about to show it here There it is. This is a animated
conception of a tennis player, and we are going to be in the future, fitting
this animating model to the skeleton that you saw previously, and God
willing here, we may be able to fill the last couple of minutes by looking
at the moving skeleton. You've been watching -- we've got the VCR . I'm
attempting to make the VCR play . We're going to attempt to show the skeleton
that goes underneath this. Basically what we'll be doing is fitting this
animatinged picture with the an mation of Pete Sampras or whoever it is
we want to see, actually moving. And then what we will be able to do (garbled.)
VCR display that -- yes
NASA Ames: . . . . Thu, Jan 29, 11:30AM . . . [144 ]
. Video . We are trying to send you a picture out of the Internet . There
it is . More coming up here. That's really interesting. There is the skeleton
in kpar comparison to Pete himself . As I said, we are eventually going
to use that skeleton and we will be able to work -- there is the skeleton
again on a grid. We will be able to look at that skeleton from actually
any viewpoint, a complete three dimensional, a 360 degree viewpoint, and
that's really going to help us understand exactly what is happening with
the biomechanics of the strokes. We were talking to some other people
at the USTA today, Ron woods, the former director, there are a lot of
great coaches out there, and they are saying some interesting things about
how the ball is hit in tennis. But those things -- here we go again. Now,
there is where it was actually fitted the animated guy to the video model,
an example of that. You can see this is not a simple process. It takes
a lot of hours, and, you know, we are still working to refine the
NASA Ames: . . . . Thu, Jan 29, 11:31AM . . . [145 ]
fit. If you look at the two closely you'll see at some points where they
don't correspond. There is more coming up. But we hope in the very near
future to have it correspond better, and time will tell what the value
of that is going to be for biomechanics in tennis. So so far as I can
see, we must be getting pretty close to the end of our hour and a half,
and my voice is horse. I like to talk, but normally not for an hour and
a half. I want to thank everybody for coming and participating. I want
to thank everybody out there at NASA ams. You guys did a fablous job.
NASA Ames: . . . . Thu, Jan 29, 11:31AM . . . [146 ]
: Thank you, John.
NASA Ames: . . . . Thu, Jan 29, 11:31AM . . . [147 ]
: Everybody is still out there at Ames? Say good-bye on the Internet.
NASA Ames: . . . . Thu, Jan 29, 11:31AM . . . [148 ]
: So long, guys. Bye.
NASA Ames: . . . . Thu, Jan 29, 11:31AM . . . [149 ]
: Bye, everybody.
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