In
fluid 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 in a direction parallel to the object's surface (primarily along its sides, as friction forces at the front and back cancel themselves out), plus
pressure forces, which act in a direction perpendicular to the object's surface. For a solid object moving through a
fluid or
gas, the drag is the sum of all the
aerodynamic or
hydrodynamic forces in the direction of the external fluid flow. (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.
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