In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow vilocity. Unlike other resistive forces, such as dry friction, which is nearly independent of velocity, drag forces depend on velocity.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 the 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 a sails on a down wind sail boat, or in intermediate directions on a sail depending on points of sail. In the case of viscous drag of fluid in a pipe, drag force on the immobile pipe decreases fluid velocity relative to the pipe
