Airplanes fly due to the interaction of four forces: lift, gravity, thrust, and drag. The opposing forces balance each other, with lift equaling gravity and thrust equaling drag. The following are the four forces that act on an airplane in straight and level flight:
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Thrust: The force that moves an airplane forward through the air. Thrust is created by a propeller or a jet engine.
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Drag: The air resistance that tends to slow the forward movement of an airplane.
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Gravity: The force that pulls all objects towards the earth.
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Lift: The upward force that is created by the movement of air above and below a wing. Air flows faster above the wing and slower below the wing, creating a difference in pressure that tends to keep an airplane flying.
The wings of an airplane are shaped to make air move faster over the top of the wing. When air moves faster, the pressure of the air decreases. So the pressure on the top of the wing is less than the pressure on the bottom of the wing. The difference in pressure creates a force on the wing that lifts the wing up into the air. This is known as Bernoullis principle.
Another theory of lift is based on Newtons third law of motion, which states that a wing keeps an airplane up by pushing the air down. Air has mass, and from Newtons third law, it follows that the wings downward push results in an equal and opposite push back upward, which is lift.
In addition to lift, airplanes need thrust to move forward through the air. This is created by a propeller or a jet engine. Drag is the air resistance that tends to slow the forward movement of an airplane, and gravity is the force that pulls all objects towards the earth.
Overall, airplanes fly due to the careful balance of these four forces. The wings of an airplane are designed to create lift, while the engine provides thrust to move the airplane forward. The airplane must be built so that lift and thrust are stronger than the pull of gravity and drag by just the right amount.
