Lift is the upward force of a plane that allows it to take off and fly. Lift varies with airflow passing over and under a wing. Lift is produced by the difference in pressure on the bottom and top of the wing. The air moves faster over the top of the wing which produces a lower pressure than under the wing. As a plane takes off, the change in air pressure on top and below the wing works to counteract gravity. Since the airplane is heavier than the air that surrounds it, a plane relies on the wings to create the lift. The greater the wing area, the greater the lift generated by the plane.

Weight is the natural force of gravity that pulls a plane downward or toward the center of the Earth. Weight opposes lift. The force of weight is created by the mass of the plane and gravity. This is an example of Newton's Second Law of Motion: weight = mass x gravity. It keeps the airplane on the ground and, therefore, must be overcome before a plane can take off and fly. To determine the amount of lift needed, it is necessary to compute the maximum amount of weight needed for a plane to reach its destination. The weight includes the fuel, payload and passengers.

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