In flui dynamics, drag (sometimes called resistance) is the force that resists the movement of a solid object through a fluid (a liquid or gas). Drag is made up of friction forces, which act parallel to the object's surface plus pressure forces, which act in a direction perpendicular to the object's surface. For a solid object moving through a fluid, the drag is the sum of all the aerodynamic or hydrodynamic forces in the direction of the movement. Forces perpendicular to this direction are considered lift. It therefore acts to oppose the motion of the object, and in a powered vehicle it is overcome by thrust.