# FLY LIKE A HAWK.

Skateboard legend Tony Hawk uses physics to land his daredevil skate stunts.

It had never been done before. Not even Tony Hawk, perhaps the world's greatest skateboarder, had ever landed a front-side "900"--two-and-one-half midair somersaults (900 degrees of rotation)!--the pinnacle of all of skate stunts. Hawk, 33, wrestled with the stunt for 10 years--with nothing to show for it but a busted rib.

Then he headlined the 1999 ESPN X Games, the Olympic Games of extreme sports, where 8,000 screaming fans overflowed Mission Bay Park in San Francisco, Calif.

All eyes were riveted on the lanky skate champ as he launched 1.5 meters (5 feet) off the rim of a half pipe--a hollow U-shaped ramp that's flat in the middle and curls skyward with steep walls. Midair he executed two-and-one-half warp-speed flips with the board tight at his feet. He flawlessly sailed back into the half pipe--and became the first skater in history to nail a perfect 900! The crowd went wild. "It was my greatest personal achievement," says Hawk.

"TRICK" PHYSICS

The most awesome force in skateboarding is gravity, the force that pulls all things toward Earth. Gravity helps keep Hawk glued to his board and guarantees every air-bound trick returns to Earth. Gravity also helps him accelerate, or pick up speed, as he swoops down into the half pipe. Speed is critical for Hawk: the faster he goes, the higher he can soar out of the pipe--and the more seconds for his in-flight magic.

What's the first step Hawk must take to gain speed? Overcome inertia, the resistance of an object to a change in motion. According to Newton's first law of motion, an object at rest remains at rest unless acted upon by an outside force. That means Hawk needs to exert enough force to launch his body and board off the pipe's lip or edge. His leg muscles generate the force in the form of kinetic energy (energy of a moving object).

Another way Hawk amps up speed: a technique called pumping. As he glides across the ramp's flat bottom, called the transition, Hawk crouches low on his board. Then, as he nears the steep incline, he quickly bends his knees and straightens his legs. This raises Hawk's center of mass, the point where he perfectly balances the entire weight of his body, and increases his kinetic energy. More energy translates into lightning-speed maneuvers.

TWIST AND SHOUT

The 900 may be Hawk's ultimate claim to fame: "I thought up the trick in 1986, but didn't have the guts to try it for 10 years." To master the stunt, Hawk endlessly practiced midair somersaults--which rely on angular momentum, or the moving power of a rotating object. At 1.5 m (5 ft) above the half pipe rim, Hawk has less than 2 seconds to complete two-and-one-half full flips. Rotating his legs beneath him--as his arms and torso rotate in the opposite direction--Hawk curls his body into a tight ball. He draws his weight closer to his axis of rotation (center point at which Hawk rotates), and hikes up his speed. It's the same principle used by twirling figure skaters when they perform their routines by pulling their arms close to their body. "The most dangerous thing to do is to panic and stop your spin in the middle of a trick," Hawk cautions. "You end up on your back!"

Naturally, what goes up must come down. The second part of Newton's first law of motion states that objects in motion stay in motion until acted on by an outside force. Hawk might soar skyward forever if not for gravity exerting a downward force on his body and board. Gravity's force reconnects Hawk and his board to the ramp.

Grasping the laws of physics may help you become a better skater, but Hawk also suggests using the laws of common sense. "When I try a trick I don't throw caution to the wind," he says. "I figure out the safest way to approach it, then practice until I can say, 'OK, this is the right way to do it.'"

Hawk officially retired from pro skating last year, but his skate career, along with his skateboard company and best-selling videogame (see "Tony Hawk's Pro Skater 2[TM]), has netted the legend more than \$1 million! To join Tony Hawk's fan club, check out his Web site:

www.clubtonyhawk.com

SKATEBOARD LEGEND TONY HAWK rockets off the half pipe to execute what he calls the "Stale Fish"--a midair front-of-the-board grab with one hand. The stunt is one of more than 100 daredevil tricks he's invented. Others include the "Madonna" (Hawk grabs the nose of his board in midair and kicks one leg out) and the "720 McHawk" (in midair Hawk flips twice with the board at his feet).

How does he do it? Practice! The California native started skateboarding at age 9. By 14, he ranked as the world's number one boarder and since then has nabbed more prizes than anyone. "Skateboarding keeps me motivated," Hawk says. "Every time I get on the board I learn something new."

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GRAVITY "DROPS IN"

1 Hawk drops into the half pipe by pushing off its solid rim. In turn, the rim pushes back on Hawk with an equal and opposite force to propel him forward. (The same force rebounds a tennis ball as it smashes against a wall).

2 As Hawk edges over the rim, the upward force pushing beneath his board disappears.

3 Gravity's downward force accelerates Hawk and his board down the ramp.

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MAY THE "FORCE" BE WITH YOU See how the forces of physics help Hawk "ollie" over this bench.

1 On the approach, Hawk bends his knees and lowers his center of mass (balancing point).

2 Hawk straightens his knees and raises his center of mass. He also exerts a sharp downward force on the tail of the board, forcing its nose up.

3 The ground pushes back on Hawk's board with an equal and opposite force. This force and his upward momentum launch him into the air.

4 Midair over the bench, Hawk pulls his knees to his chest. His front foot exerts a downward force on the nose of the board. This raises the board's tail and changes Hawk's velocity, or his speed and direction.

5 Gravity's downward pull balances out the upward forces acting on Hawk. With both feet on the board, Hawk lands and bends his knees to absorb the impact.

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CRASH PHYSICS

Tony Hawk wipes out on the half pipe. Why? His center of mass was improperly balanced over his skateboard. If Hawk shifts too much weight over the board's nose, for example, gravity's downward force will send him tumbling. Don't try this at home!

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HANDS-ON SCIENCE GRAVITY GAME

How does Tony Hawk balance with one hand on the lip of a half pipe (see photo on p. 16)? He positions his arm directly beneath his center of gravity, the point at which the entire weight of his body balances. At this point, gravity pulls equally on all part of his body. Check out the experiment below to see how a saltshaker's center of gravity helps it balance on its edge.

YOU NEED: a saltshaker full of salt * two pinches of salt

TO DO:

1 Sprinkle two pinches of salt onto a table and push the salt into a mound.

2 Tilt the saltshaker halfway toward the tabletop and place one of its edges in the middle of the salt pile; crush the salt on which the edge is resting by pushing on the saltshaker.

3 With your finger or a pencil point, tightly pack some salt under outside edge of the shaker.

4 Using both hands, slightly tilt the shaker back and forth until you find the point at which the saltshaker balances on its edge. (If you have trouble, wedge more salt under the shaker and try again).

5 Once you balance the shaker, put your mouth at table level. Gently blow the salt away from the shaker. Be careful: If you blow too hard, the shaker could fall over.

6 Your shaker appears to be balancing by itself!

CONCLUSION: What keeps the saltshaker from tipping over? How does salt help support the shaker's center of mass? What happens to the shaker if you remove all of the salt? Why?

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Anatomy of a Video Game: Tony Hawk's Pro Skater 2 [TM]

Check out Tony Hawk's smash-hit video game Pro Skater 2. This interactive adventure puts you in the virtual shoes of Tony Hawk and is among the most advanced video games ever created. But without these basic computer parts the game would "crash."

* Hardware: Mechanical parts you can see and touch, like the game console, the box-like computer that plays video games.

* Software: Programmed instructions inside the video game that interact with the hardware.

* Central processing unit (CPU): An electronic device that allows the game to perform multiple tasks. Each button you press transmits a command to the CPU in the form of an electronic signal that travels near the speed of light (186,000 miles per second)!

* Computer codes: The "beef" of a video game: written instructions that the computer translates into a series of numbers called binary code (a two-digit computer language made of zeros and ones). Each code commands the CPU to perform a different task, like displaying 3-D images.

* 3-D graphics: Images with height, width, and depth made up of thousands of tiny pixels, or small bits of information that form shapes.

* Motion capture: A technique used to fuse real-life video footage into a video game's animated images. To help make the reality-based graphics in Pro Skater 2, Hawk stuck green Ping-Pong balls to his body. Then he performed skate stunts in front of a video camera. The footage was fed into a computer, and animated characters were created to trace the movements of the green balls. Learn more about videogames at: www.howstuffworks.com/video-game4.htm

CONTEST

Be a Skate Stunt Scientist!

GRAND PRIZE: An autographed Tony Hawk skateboard.

10 FINALISTS: Tony Hawk's new video game Tony Hawk Pro Skater 2.

YOUR MISSION: Tony Hawk has created more than 100 skateboard tricks. Now it's your turn--and you don't even need to know how to skateboard. The only thing you need to enter our contest is imagination and physics know-how. Design, describe, and name a skateboard trick. (It can be any combination of flips, twists, and jumps.) Then explain the physics principles that would make your new skate stunt possible.

SUBMIT TO SCIENCE WORLD:

* a written description of your skateboard trick (include a creative name) on an 8 1/2-by-11-inch sheet of paper

* an illustration or diagram of your trick

* a written explanation of the physics principles that make it possible.

An official entry form, along with the contest's official rules, can be found in this issue's Teacher's Edition.

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Cross-Curricular Connection

History: Trace the history of skateboarding to its origins in California during the early 1960s. Name the first three professional female skateboarders.

Did You Know?

* At age 33, Tony Hawk is one of the oldest elite skateboarders. The average age of pro skaters is 20.

* During a formal half pipe competition, a skater has 45 seconds to perform variations on three types of acrobatic tricks: lip tricks (performed on the edge of the pipe), aerials (performed above the pipe), and plants (performed with the skater's hand or foot).

* Most skateboards are made of seven layers of maple wood--a material that allows for maximum elasticity, flexibility, and toughness.

Directions: After reading the story "Fly Like a Hawk," fill in the blanks.

1. The force that pulls all things toward Earth is --.

2. Gravity helps Hawk --, or pick up speed, as he swoops down the half pipe.

3. To gain speed the first thing Hawk must do is overcome --.

4. An object at rest will remain at rest according to --.

5. Hawk's -- is the point where his weight is perfectly balanced.

1. gravity

2. accelerate

3. inertia

4. Newton's First Law of Motion

5. center of mass