Humans are heading back to the Moon. Tennis, anyone?
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August
30, 2005: They call him the "Rocket Man."
Tennis pro Andy Roddick holds the world's record for fast
serves: 155 mph. By the time opponents realize where the ball
is going, very often, it's already gone. Ace! His
groundstrokes are like rockets, too.
Believe
it or not, this can be a problem. Balls hit so hard want to
go long, flying straight out the back of the court. Out!
It's hard to win a game that way.
Right:
Andy Roddick serves--a 100+ mph blur. [More]
So
what do you do when you're so powerful? Roddick has a trick
up his sleeve: topspin. By hitting the ball obliquely
(at a slant), he causes it to spin. Topspin makes the ball
curve downward. Instead of going out, it drops neatly into
play on the other side of the net.
Now
the Rocket Man is wondering, "What if the US Open were
played on the Moon? Would topspin do me any good?" [video]
It's
a perfectly reasonable question. In 2004 when President Bush
set out the nation's Vision for Space Exploration, he declared
"human beings are headed into the cosmos." He never
said, except for the tennis players. Where people
go, sport follows. Decades from now, tennis might be a popular
pastime on the Moon.
Picture
this: Two space-suited athletes stare at each other across
the net of a tennis court drawn in moondust. Their own reflections
stare back from the faceplate of the opponent. Overhead, the
sun is bright, the sky is black and Earth looks absolutely
beautiful; it's a crescent today.
Above:
An artist's concept of Moon tennis. Credit: Paula Vargas and
Terry Longbottom of NASA/JSC. [video]
One
player tosses the ball up … up … up. Smash! Soundlessly, a
100+ mph serve glides across the net, long. Another serve,
long. And another, long. Groundstrokes go long, too.
Something's
wrong. On the Moon, topspin doesn't work.
Actually,
the ball spins just fine, but spinning doesn't make
it curve downward. The Moon has no air, and spinning balls
only curve when they're flying through an atmosphere. (Note:
Gravity causes balls to curve downward, too, but not enough
to rescue every shot.)
Physicists
call this the Bernoulli Effect: Air pressure on one side of
a spinning ball is higher than it is on the other side. High
pressure pushes the ball toward low pressure--hence the curve.
Swiss physicist Daniel Bernoulli wrote down the equations
describing this curious phenomenon in the 18th century, setting
the stage for Andy Roddick's devastating power game almost
300 years later.
The
Bernoulli Effect is very important in sports. In baseball,
it lets pitchers throw curveballs. In tennis and ping pong,
it helps players ace their serves. In golf, it's responsible
for the dreaded slice.
Right:
The Bernoulli Effect. [More]
Aside:
Apollo 14 astronaut Al Shepard became the first extraterrestrial
golfer on February 6, 1971, when he tried hitting some balls
on the Moon. His club was homemade, consisting of a 6-iron
attached to the end of a geology tool handle. "Unfortunately,
the suit is so stiff, I can't do this with two hands, but
I'm going to try a little sand-trap shot here," said
Shepard. One shot, remarked mission control, "looked
like a slice." Shepard countered "Straight as a
die!" In fact, the ball rolled just 2 or 3 feet. Later,
Shepard did get off a good shot which famously went "for
miles and miles and miles." Practice makes perfect, especially
on the Moon.
Moon
tennis is going to differ from Earth tennis in many ways.
For example: In the Moon's lower gravity, only 1/6th Earth's,
lobs will fly six times higher and hang six times longer.
Thirsty? Hit a lob. While it's in flight you can go get a
drink of water.
Here
on Earth we have clay courts and grass courts. On the Moon
there will be dust courts. The surface affects the style of
play: clay is a slow surface, while grass is faster and prone
to crazy bounces. A dusty surface will probably be slow, until
it packs down a bit.
Moondust
adds an interesting twist to tennis. For one thing, moondust
is extremely dry and insulating, and thus prone to static
cling (like a crackling sock pulled out of the dryer). Balls
hit into this material over and over will soon gather a layer
of electrified dust. Want to shock your opponent? Smash the
ball straight at him!
There's
more: When a ball bounces off a mooncourt, it's going to kick
up a little dusty plume. Spin the ball just right and you
could send that dust jetting toward your opponent, craftily
blinding him. Topspin, or in this case, backspin, might come
in handy after all.
And
don't forget the spacesuits. On Earth, players wear shorts
and light shirts. On the Moon, they'll be climbing into a
bulky full-body pressure suit. As Apollo astronauts discovered,
you don't really "run" in a 1/6-g moonsuit, you
"bound," hopping around like Tigger of the Hundred
Acre Wood. A player might spend more time off the ground than
on it. ("Footwork!" cries your tennis coach. Footwork?
How can you have footwork when your feet don't touch the ground?
Old-fashioned strategies might not work on the Moon.)
Right:
Andy Roddick ponders tennis on other worlds. [video]
Furthermore,
joints at the elbows, knees and especially shoulders of spacesuits
are less flexible than real human joints. Overhead serves
are going to be mighty difficult. Sorry Andy! Forehand or
underhand serves, legal in Earth tennis, might be required
on the Moon. Meanwhile, the backhand stroke could become extinct:
it's tough to reach across the body of a turgid spacesuit.
Eventually,
special suits might be manufactured for moon tennis, designed
to allow overhand serves, backhand shots, and normal running.
The dimensions of courts could change, too, probably made
bigger to accommodate no-Bernoulli serves and low-gravity
volleys.
Low
gravity. No topspin. Bulky spacesuits. Moon tennis is going
to be different. Says Roddick, "…that might be kind of
cool!"
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Authors: Dr. Tony
Phillips & Phil
West | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
More
Information |
Hey,
wait a minute! While this story is grounded
in serious physics, it shouldn't be taken too
seriously. We won't know what it's like to play tennis
on the Moon until someone tries it. Will tennis balls
really accumulate electrical charge as they bounce back
and forth across the court? It depends on what the ball
is made of and how it interacts with moondust. Scientists
do know that the
Moon "crackles" with static electricity--Moon
tennis might crackle, too. Also, by the time people
are actually living on the Moon, advanced space suits
will surely be lighter and more agile than Apollo suits.
This will help astronauts go about their duties: exploring,
building the colony, and acting as field geologists.
Improved suits will help them play tennis, too, making
backhand strokes and over-the-shoulder serves easier
to execute. And, of course, athletes playing inside
a pressurized lunar sports dome could shed their spacesuits
altogether. (Imagine the "bounding" you could
do in there!) There would be no moondust, plenty of
air and plenty of Bernoulli. How different will Moon
tennis be? Time will tell.
All
Things Bernoulli -- a review of the Bernoulli Effect
(and related phenomena) suitable for high school and
undergraduate physics students
The
Magnus Effect -- When applied to tennis and other
ball sports, the Bernoulli Effect is often called "the
Magnus Effect," after 19th-century German physicist
H. G. Magnus who studied the forces on spinning balls
and cylinders.
Bernoulli
vs. Newton -- Topspinning balls sink because they
have "negative lift," the opposite of "positive
lift" generated by an airplane's wing or a backspinning
ball. How is lift created? Scientists are still debating
the underlying reasons.
Follow
the bouncing ball -- a look at the physics of tennis
bounces or, in physics-speak, the "ball-court interaction"
More
physics links:
Alan
Shepard, Out of this World Golfer -- from pasturegolf.com
A
Nice Day for a Game of Golf -- NASA's Apollo 14
Lunar Surface Journal recounts the first-ever round
of extraterrestrial golf.
Daniel
Bernoulli -- a short biography
NASA's
Vision for Space Exploration |
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