Also referred to as Drag
An object moving through a gas or liquid experiences a force in direction opposite to its motion.
Terminal velocity is achieved when the drag force is equal in magnitude but opposite in direction to the force propelling the object.
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 parallel to the object's surface plus pressure forces, which act in a direction perpendicular to the object's surface. For a solid object moving through a fluid, the drag is the sum of all the aerodynamic or hydrodynamic forces in the direction of the movement.
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.
In astrodynamics, depending on the situation, atmospheric drag can be regarded as inefficiency requiring expense of additional energy during launch of the space object or as a bonus simplifying return from orbit.
Types of drag are generally divided into three categories:
Parasitic drag, lift-induced drag, and wave drag.
Parasitic drag includes form drag, skin friction, and interference drag.
Lift-induced drag is only relevant when wings or a lifting body are present, and is therefore usually discussed either in the aviation perspective of drag, or in the design of either semi-planing or planing hulls.
Wave drag occurs when a solid object is moving through a fluid at or near the speed of sound in that fluid.
The overall drag of an object is characterized by a dimensionless number called the drag coefficient, and is calculated using the drag equation.
Assuming a constant drag coefficient, drag will vary as the square of velocity. Thus, the resultant power needed to overcome this drag will vary as the cube of velocity.
The standard equation for drag is one half the coefficient of drag multiplied by the fluid density, the cross sectional area of the specified item, and the square of the velocity.
Wind resistance or air resistance is a layman's term used to describe drag. Its use is often vague, and is usually used in a relative sense (e.g., A badminton shuttlecock has more wind resistance than a squash ball).