CONCEPTS in Action

Airplane Motion

Four forces act on an airplane in flight. They are thrust, drag, lift, and weight. Lift and drag are known as aerodynamic forces because they result entirely from the plane's movement through the air.

Of the aerodynamic forces, lift is the least familiar. One source of lift is Bernoulli's principle. According to Bernoulli's principle, the pressure of a fluid such as air decreases as its speed increases. Because the air moving over the top of an airplane wing moves faster than the air moving beneath the wing, it creates a pressure difference that lifts the wing and hence the plane. Another source of lift is the slant of the wing. As the wing moves through the air, the wing forces air downward. According to Newton's third law of motion, this action must have a reaction. The reaction force also exerts an upward force on the wing.

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