Drag

Drag is the force a fluid exerts against an object's motion relative to it. Its magnitude and direction depend on the object's shape, speed, and the fluid's properties.

Skin friction: Fluid resistance caused by viscous shear along a surface. Viscous shear contributes to drag on surfaces exposed to flowing fluid.
Fluid: A substance that flows and continually deforms under applied shear stress. Drag arises from an object's relative motion through a fluid.
Aerodynamics: The study of air flow and its forces on moving bodies. It uses drag analysis to shape aircraft, vehicles, and structures.
Lift: The component of a fluid force perpendicular to the incoming flow direction. Lift and drag are perpendicular components of the same aerodynamic force.
Form drag: Fluid resistance caused by pressure differences around an object's shape. Separated flow and uneven pressure create this major component of drag.
Viscosity: A measure of a fluid's resistance to shear and flow. Viscosity governs frictional stresses that contribute to drag.
Streamlining: Shaping an object to reduce resistance from fluid flow. A streamlined shape delays flow separation and can lower pressure drag.
Buoyancy: The upward force a fluid exerts on an immersed or floating body. Buoyancy comes from displaced-fluid pressure, unlike resistance to relative motion.
Boundary layer: A thin region near a surface where viscosity strongly affects fluid motion. Its growth and separation shape the pressure and shear forces that produce drag.
Fluid density: The mass of a fluid per unit volume. Density affects inertial pressure forces and the scale of many drag estimates.

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