An aircraft in straight and level flight is acted upon by four forces: lift, gravity, thrust and drag. The opposing forces balance each other: Lift equals gravity, and thrust equals drag.

Thrust: The force that moves an airplane forward through the air. Thrust is created by a propeller or a jet engine.

Drag: The air resistance that tends to slow the forward movement of an airplane.

Gravity: The force that pulls all objects towards the earth.

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.