Drag (physics)

In fluid dynamics, drag is a force that acts against the motion of any object moving through a fluid, like air or water. This force works opposite to the direction the object is moving and can happen between two layers of fluid or between a fluid and a solid surface. Because of drag, objects moving through fluids often slow down over time.

Drag is different from other resistive forces because it changes depending on how fast something is moving. At slower speeds, the drag force is directly related to the speed of the object. But at higher speeds, the drag force becomes related to the square of the speed. This difference in behavior is measured using something called the Reynolds number, which helps scientists understand and predict how fluids will flow around objects.

Examples

Drag can be seen in many everyday situations. For example, when a car, airplane, or boat moves, it feels a push back from the air or water — this is called drag. This same push back can also happen to fluid moving through a pipe, slowing it down.

In sports, drag plays an important role. It affects how balls, javelins, arrows, and frisbees move through the air. Even runners and swimmers need extra energy to overcome the drag from the air or water around them.

Types

There are several types of drag that occur when an object moves through a fluid like air or water. Two main types are form drag and skin friction drag. Form drag happens because of the pressure on the object as the fluid flows around it, and it depends on the shape of the object. Skin friction drag is caused by the friction between the fluid and the surface of the object.

For aircraft, two other important types of drag are lift-induced drag and wave drag. Lift-induced drag happens when wings or other lifting surfaces create lift, and it increases as the angle of attack goes up. Wave drag appears at higher speeds when shockwaves form around the aircraft.

Shape and flow	Form Drag	Skin friction
	≈0%	≈100%
	≈10%	≈90%
	≈90%	≈10%
	≈100%	≈0%

The drag equation

Drag is the force that slows down objects moving through a fluid, like air or water. The drag equation helps us understand this force: F_D = ½ ρ v² C_D A, where F_D is the drag force, ρ is the fluid’s density, v is the object’s speed, C_D is the drag coefficient, and A is the object’s cross-sectional area.

The drag coefficient, C_D, depends on the object’s shape and the Reynolds number, which compares the object’s speed to the fluid’s thickness. At low speeds, drag increases with speed. At high speeds, drag increases with the square of the speed.

At high velocity

Main article: Drag equation

An object falling through viscous medium accelerates quickly towards its terminal speed, approaching gradually as the speed gets nearer to the terminal speed. Whether the object experiences turbulent or laminar drag changes the characteristic shape of the graph with turbulent flow resulting in a constant acceleration for a larger fraction of its accelerating time.

When something moves really fast through a fluid like air or water, it feels a force pushing back on it. This is called quadratic drag because the force gets bigger very quickly as the speed increases. The force depends on how dense the fluid is, how fast the object is moving, and the shape of the object.

For example, a car going at 100 mph needs a lot more power to push through the air than the same car going at 50 mph. This is because the force pushing back gets four times bigger when the speed doubles. To keep moving, the car has to work much harder!

Low Reynolds numbers: Stokes' drag

Main article: Stokes' law

Trajectories of three objects thrown at the same angle (70°). The black object does not experience any form of drag and moves along a parabola. The blue object experiences Stokes' drag, and the green object Newton drag.

When objects move slowly through a fluid, they feel a force called drag. This force works against the motion of the object. For very slow speeds, this force is directly related to how fast the object is moving.

Scientists have found equations to describe this force. For tiny particles or slow-moving objects, the drag force depends on the object's speed, its size, and the fluid it moves through. This helps us understand how small things, like tiny particles or bacteria, move in fluids such as water or air.

d'Alembert's paradox

Main article: d'Alembert's paradox

In 1752, a mathematician named d'Alembert showed that a certain way of understanding how fluids move could predict that there would be no drag on an object moving through a fluid. This idea did not match what experiments showed, and this mismatch became known as d'Alembert's paradox.

Later, new equations were developed to better describe how fluids move when they stick to surfaces. These helped explain why drag happens, especially when the fluid moves very fast compared to the object. A key idea was the boundary layer, a thin layer of fluid near the object's surface where the fluid's stickiness plays an important role.