Drag, or air resistance, is a force that opposes motion. Since the thrust is a force that provides "forward motion" and, lift one that produces "upward motion", the drag opposes both of these forces. Air resistance is friction between the air itself and the moving object (in this case the aircraft). The calculation of air resistance is far more complicated than that of thrust and lift- it has to do with the material of the aircraft, the speed of the aircraft and other variable factors. However, rockets and airplanes are built with materials and in shapes that minimize drag force, maximizing the force that moves the aircraft upward/forward.