# NASA - National Aeronautics and Space Administration

## Educator Features

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The Four Forces of Flight
 The Four Forces of Flight Keep This Plane Aloft
Have you ever thrown a Frisbee®? It flies because of four forces. These same four forces help an airplane fly. The four forces are lift, thrust, drag, and weight. As a Frisbee flies through the air, lift holds it up. You gave the Frisbee thrust with your arm. Drag from the air made the Frisbee slow down. Its weight brings the Frisbee back to Earth again.

You see them everyday: airplanes, jets, and helicopters, soaring, zooming, and even roaring through the skies. We may take flight for granted; yet, knowing the science behind it gives us a better understanding of the marvels of air travel.

Wings keep an airplane up in the air, but the four forces are what make this happen. They push a plane up, down, forward, or slow it down.

• Thrust is a force that moves an aircraft in the direction of the motion. It is created with a propeller, jet engine, or rocket. Air is pulled in and then pushed out in an opposite direction. One example is a household fan.

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• Drag is the force that acts opposite to the direction of motion. It tends to slow an object. Drag is caused by friction and differences in air pressure. An example is putting your hand out of a moving car window and feeling it pull back.

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• Weight is the force caused by gravity.

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• Lift is the force that holds an airplane in the air. The wings create most of the lift used by airplanes.

The way the four forces act on the airplane make the plane do different things. Each force has an opposite force that works against it. Lift works opposite of weight. Thrust works opposite of drag. When the forces are balanced, a plane flies in a level direction. The plane goes up if the forces of lift and thrust are more than gravity and drag. If gravity and drag are bigger than lift and thrust, the plane goes down. Just as drag holds something back as a response to wind flow, lift pushes something up. The air pressure is higher on the bottom side of a wing, so it is pushed upward.