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`HOW & WHY' - FORCES THAT ENABLE OBJECTS TO FLY

Faith took an airplane to visit her grandparents. It was her first flight - and she was sort of nervous about it, especially when she saw how big the airplane was! She flew in a 747, a plane that carries more than 400 passengers, and has wings that stretch nearly 200 feet from tip to tip!

How in the world was that huge machine going to get into the air? And once it became airborne, how was it going to stay up there?The answer has a lot to do with the laws of physics - rules that govern the forces that make the natural world work the way it does. Four forces work on objects in flight: lift, thrust, drag and weight.

Lift does just what you'd expect: It lifts an object upward. Lift happens when air pressure on the underside of an airplane's wing is greater than the air pressure on top of the wing.

An airplane's wings are designed to be curved on top and flat on the bottom - like birds' wings. Each wing has one rounded edge and one flat edge. The rounded edges face forward.

The shape of the wings creates unequal air pressure - greater on the bottom, less on the top. The result: The plane lifts up into the air. Here's how that happens: As an airplane flies, the wings split the air that streams over them. The airstreams flow over the wings, to meet up again on the other side. The air flowing across the top of the wing has to go a little further to get to the meeting place. So it travels faster than the air moving across the bottom. The faster air on top of the wing exerts less pressure than the slower-moving air underneath the wing. Greater air pressure pushes the wing up, creating lift.

There's an interesting name for this phenomenon. It's called Bernoulli's principle and is a law of physics. The principle - which is named after scientist Daniel Bernoulli, who figured it out about 200 years ago - states that the pressure exerted by air decreases as airflow speeds up.

Thrust is the force that propels an airplane forward. The push comes from the energy produced by the airplane's engines. A 747 has four extremely powerful jet engines that can propel it forward at more than 600 miles per hour, for example. Once the engines are roaring, the airplane is pushed forward. It begins to move faster and faster. With the engine creating thrust, the wings create lift, and the plane takes off.

Meanwhile, drag and weight work in opposition to lift and thrust. As an airplane plane moves forward, drag works to slow it down. This happens because the air pushes against the surfaces of the plane, resisting forward motion. In order for the plane to keep moving, its thrust must be greater than the drag created by the air it's moving through. The streamlined shape of an airplane's slanted nose and curving wings helps reduce drag.

An airplane's weight also tends to draw it back toward the ground. Gravity, the natural force that holds objects on Earth, pulls the weight of the plane toward the ground, as thrust and lift work to hold it up. People who operate airplanes are careful to monitor how much weight gets loaded on a plane. If the plane gets too heavy, its engines may not be able to provide enough thrust, nor its wings enough lift, to keep it steady in the air.

The science of moving an object through the air is called aerodynamics. You can experience aerodynamics by riding in an airplane. As you take off, you feel the thrust provided by the engines pushing you back into your seat. Once the plane is airborne, and the forces of lift, thrust, drag and weight are in balance, you'll probably have a pretty smooth ride.

If you don't have a chance to ride in a plane as Faith did, you can watch birds in flight to observe aerodynamics. Birds get thrust by flapping their wings. Most birds are very lightweight creatures, with hollow bones, so it doesn't take a lot of thrust to get them into the air. Their wings are perfectly designed to create lift. And they have aerodynamic bodies - pointed beaks, curving wings and feet that they can pull in close to their bodies to reduce drag. We humans aren't so aerodynamic. We could flap our arms forever and never get off the ground. So we travel in airplanes instead.

-TIPS FOR PARENTS:

Your kids can find out about aerodynamics in action by building and flying paper planes. When they fly paper planes, they provide the thrust by throwing the plane forward. The aerodynamics of the design your child chooses will determine how fast and far the plane flies. The book "Super Flyers" by Neil Francis, an airplane pilot (Addison-Wesley; $6.95), contains blueprints for many different kinds of paper planes, as well as three kinds of kites that can be made at home from plastic bags, paper sacks and typing paper. Or your budding aerodynamic engineers can design their own.

-Do you wonder about your body, your feelings, or how things work in the world around you? Send your questions to Catherine O'Neill, HOW & WHY, Universal Press Syndicate, 4900 Main St., Kansas City, Mo., 64112.