The way an airplane flies may seem magical. How can something so big and so heavy fly? Several forces act on an airplane. A force is a push or a pull on something. You use force when you throw a ball, pull a wagon, or walk. When you walk, you push backward on the floor, and the resistance from the floor pushes you forward. Forces always work in pairs, on different bodies, in opposite directions, and at the same time.

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. Lift holds an airplane up.

Lift acts in opposition to the weight of an airplane, which acts downward.

Weight is the force generated by gravity. Weight always pulls downward toward the Earth. For an airplane to fly, lift must equal weight. Weight holds an airplane down.

Thrust is the forward force that is produced by the pull of a propeller or the push jet exhaust. A propeller is a small rotating wing that is lifting forward. A jet engine forces exhaust gases out the back of the engine. These exhaust gases push the engine and, thus, the airplane forward. Thrust makes an airplane go.

Drag is a force that slows an airplane down. You feel drag when you ride your bicycle. The air and the wind hold you back. Thrust equals drag for an airplane that is flying at a constant speed. Drag holds an airplane back.