Drag (physics)

In fluid dynamics, drag, sometimes referred to as fluid resistance, is a force acting opposite to the direction of motion of any object moving with respect to a surrounding fluid.^[1] This can exist between two fluid layers, two solid surfaces, or between a fluid and a solid surface. Drag forces tend to decrease fluid velocity relative to the solid object in the fluid's path.

Unlike other resistive forces, drag force depends on velocity.^[2][3] Drag force is proportional to the relative velocity for low-speed flow and is proportional to the velocity squared for high-speed flow. This distinction between low and high-speed flow is measured by the Reynolds number.

Drag is instantaneously related to vorticity dynamics through the Josephson-Anderson relation.^[4][5]