Dance, dance, revolution: tired of tripping on two left feet? Physics can help you master the dance floor.

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You've laughed at celebrities grooving out of sync on Dancing With the Stars. You've seen contestants answer So You Think You Can Dance? by falling flat on their faces. As these popular TV shows demonstrate, not everyone can be king or queen of the dance floor. What does it take to dance well? Besides talent, some knowledge of physics could help!

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Kenneth Laws, a retired physics professor at Dickinson College in Pennsylvania, began studying ballet 30 years ago. "I didn't want to be left out of my children's activities," he says. So at age 40, he enrolled as the only adult in his kids' ballet class. The scientist became hooked when he realized that every dance move, such as a turn, could be explained through physics. Laws has since written three books on this subject. Science World spoke with Laws to see if we could pick up some dance tips.

A LEG UP

One of the most important things in dance is to learn to balance. When Laws began studying ballet, he was almost floored by this basic move: While standing, try lifting one leg approximately 90 degrees to your side and hold the position--without grabbing onto anything for support. It's an impossible position to hold, Laws thought, because it seems to work against the laws of gravity.

Gravity is an attracting force that pulls objects toward each other. On Earth, it exerts a force that pulls your body vertically downward. At the same time, the floor pushes at your feet, exerting a vertical upward force on your body.

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When you lift one leg high up, "you're likely to lean toward the supporting leg in order to balance," says Laws. your upper body beyond the side of your standing leg causes a shift in your center of gravity. This point is where all your body weight is evenly balanced, so Earth pulls your body on either side of your center of gravity with an equal amount of force. By shifting your upper body way over to one side, your center of gravity is no longer aligned with where the floor is pushing you upward. This imbalance causes you to topple.

Laws' pint-size classmates didn't crash while attempting the balancing act. "They cheated," jokes Laws. As the kids lifted their legs, they instinctively adjusted their hips and upper bodies to keep their center of gravity aligned with the floor's upward force. Laws realized that whether you're standing on one leg or balancing on your head, by constantly keeping your center of gravity directly over your support at the floor, you wouldn't topple.

SPINNING MACHINE

Once the competitors on TV dance shows have mastered the one-legged stance, they might want to impress the judges with fancier moves, such as turns. How come some dancers seem to be able to produce multiple pirouettes, while others tend to sputter out?

As you turn, the foot of your supporting leg continuously rubs against the floor and generates friction. This slowing force causes you to gradually lose rotational momentum, which determines the amount of rotational motion you have. Friction causes a spinning dancer to eventually stop turning. To keep speedy--even as rotational momentum decreases, a dancer uses a trick called the "ice-skater effect," says Laws.

You've seen skaters spin like tornadoes during the Winter Olympics. They usually begin a set of turns with their arms spread wide apart. Then they gradually draw them closer to their axis of rotation, or the vertical line around which they are spinning. This causes a skater's mass to become more concentrated, which helps maintain his or her spin rate. Result: more turns. Dancers move their arms in a similar way to max out on pirouettes.

GREAT LEAP

Twirling on one leg may not look tough, but according to a study by the American Alliance for Health, Physical Education, Recreation and Dance, ballet beats sports like football and wrestling as the most demanding physical activity. Laws agrees. "You need a lot of strength and athleticism to dance. At the same time, you have to show speed, grace, coordination, and many other traits." he says.

For example, when dancers jump, they want to appear more like high-leaping than thumping elephants. To reach lofty heights, you need to fight gravity.

To take off into a jump, your body must exert a downward force against the floor, which pushes you upward. At the same time, that force must be strong enough to help you offset the downward pull of gravity. "If not, you won't go anywhere," says Laws. The greater the force of your push, the larger the net upward force there is to propel you to great heights. And as you're flying through the air, don't forget to put on a graceful smile.

HANDS-ON SCIENCE

(No Lab Required) After reading the article, grab a lab partner and try these two activities.

ACTIVITY 1: BALANCING ACT

OBJECTIVE: Understand center of gravity

1 Stand with your feet planted shoulder-width apart. Then, rise and balance on the balls of your feet.

2 Have your lab partner stand 0.3 meters (1 foot) directly behind you.

3 Your partner will give you a quick push on the upper back. (Caution: For safety, do not push too hard.)

4 Without moving your feet, can you stay balanced on your toes? (Hint: Try adjusting the position of your arms or your upper body.)

ACTIVITY 2: SIT-N-SPIN

OBJECTIVE: Change your spin rate

YOU NEED: 1 swivel chair * 2 similar-size textbooks

1 Place a swivel chair in an unobstructed area. Sit cross-legged on the chair and hold a textbook in each hand.

2 Have your partner push the chair into a slow spin. (Caution: For safety, do not push too hard.)

3 As the chair turns, extend your arms to your sides. Then, bring your arms close to your body.

4 Repeat extending and drawing in your arms. Observe what happens.

web extra

For a dictionary of ballet terms, along with video demonstrations of ballet movements, visit this American Ballet Theatre site: www.abt.org /education/dictionary/index.html

LESSON PLANS

PHYSICAL: Farces and Motion

Dance, Dance, Revolution

DID YOU KNOW?

* Many football players, including Heisman Trophy winner and retired NFL great Herschel Walker, use ballet as part of their athletic training. Dance helps the players gain coordination, flexibility, agility, and balance.

CRITICAL THINKING:

* Show your class two taped performances from a TV dance show. Then have students take on the role of judges and critique the performances. Have them provide reasoning for why they think a performance is good or bad.

CROSS-CURRICULAR CONNECTIONS:

DANCE: Have each student research a different style of dance. Then have him or her give a brief presentation on the selected dance style and give a lesson on a basic dance step.

RESOURCES

* Physics and the Art of Dance: Understanding Movement, by Kenneth Laws, Oxford University Press, 2002.

* Learn about medical issues related to dance at: www.med.nyu.edu/hjd/harkness/

CHECK FOR UNDERSTANDING

DIRECTIONS: On a separate piece of paper, defend or dispute the following statements. (Hint: Defend means to explain why a statement is correct. Dispute means to explain why a statement is incorrect.)

1. When your center of gravity is aligned with where the floor is pushing you upward, you stay balanced.

2. As a dancer turns on one foot, he or she will eventually slow down.

3. To make a lofty jump, you need to fight the pull of your axis of rotation.

ANSWERS

CHECK FOR UNDERSTANDING, TE 5 & 6

DANCE, DANCE, REVOLUTION

1. Defend: Your center of gravity is a point in your body where all your body weight is evenly balanced, so Earth pulls on your body on either side of your center of gravity with an equal amount of force. By shifting your upper body way over to one side, your center of gravity might no longer be aligned with where the floor is pushing you upward This imbalance causes you to topple.

2. Defend: As you turn, the foot of your supporting leg continuously rubs against the floor and generates friction. This slowing force causes you to gradually lose rotational momentum, which determines the amount of rotational motion you have. Friction causes a spinning dancer to slow down and eventually stop turning.

3. Dispute: To make lofty jumps, you need to fight gravity. To take off into a jump, your body must exert a downward force against the floor, which pushes you upward. At the same time, that force must be strong enough to help you offset the downward pull of gravity The greater the force of your push, the larger the net upward force there is to propel you to great heights