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Drag (physics)

In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) refers to forces acting opposite to the relative motion of any object moving with respect to a surrounding fluid.〔http://www.merriam-webster.com/dictionary/drag〕 This can exist between two fluid layers (or surfaces) or a fluid and a solid surface. Unlike other resistive forces, such as dry friction, which are nearly independent of velocity, drag forces depend on velocity.〔French (1970), p. 211, Eq. 7-20〕
Drag force is proportional to the velocity for a laminar flow and the squared velocity for a turbulent flow. Even though the ultimate cause of a drag is viscous friction, the turbulent drag is independent of viscosity.
Drag forces always decrease fluid velocity relative to the solid object in the fluid's path.
==Examples of drag==
Examples of drag include the component of the net aerodynamic or hydrodynamic force acting opposite to the direction of movement of the solid object relative to the Earth as for cars, aircraft〔 and boat hulls; or acting in the same geographical direction of motion as the solid, as for sails attached to a down wind sail boat, or in intermediate directions on a sail depending on points of sail.〔Forces on sails#Relationship of lift coefficient to angle of incidence: polar diagram〕 In the case of viscous drag of fluid in a pipe, drag force on the immobile pipe decreases fluid velocity relative to the pipe.〔(【引用サイトリンク】url=http://galileo.phys.virginia.edu/classes/152.mf1i.spring02/RiverViscosity.pdf )〕〔(【引用サイトリンク】url=http://www.ce.utexas.edu/prof/kinnas/319LAB/Applets/Viscous/viscous.html )

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