Four main forces act on airplanes. Lift acts upward and allows the airplane to fly. The weight of the airplane acts downward. Thrust is the force that moves the airplane forward. The force that holds an airplane back is called drag .
Lift is generated as a result of the special shape of an airplane's wing.
The top of the wing is curved more than the bottom of the wing. The air that flows over the top of the wing moves faster than the air moves along the bottom. The air that is moving faster exerts less force than the slower moving air. Thus, the air pushes down on top of the wing less than the air pushes up on the bottom of the wing. The greater push upward is the lift of the wing.