Mechanics: Drag

Key Concepts

Drag is a force that opposes motion through a fluid (which can be a liquid or a gas), such as a car moving through air or a swimmer moving through water.
Drag is an example of contact force which undergoes action when two surfaces interact with each other.
There are two main types of drag: viscous drag, which is caused by the friction between the layers of fluid and the object moving through it, and form drag, or pressure drag, which arises due to the fluid pressure on the surface of the object.
The magnitude of drag force experienced by an object depends on several factors: the object’s velocity, the object’s size/shape, and the properties of the fluid (such as its density and viscosity).
Opposite to the direction of motion, the drag always acts.

Most often, the drag force (D) is calculated using the equation D = 0.5 * Cd * A * ρ * v^2, where Cd is the drag coefficient (which depends on the shape of the object), A is the cross-sectional area, ρ is the fluid density, and v is the object’s velocity.
The above formula demonstrates that the drag force is proportional to the square of the velocity - a small increase in speed can lead to a significant increase in drag.
Terminal velocity is achieved when the force due to gravity (Fg = mg) is balanced by the drag force (Fd). At this point, the object’s speed remains constant. Algebraically, this can be expressed as mg = 0.5 * Cd * A * ρ * v^2, and terminal velocity (v) can be isolated by rearranging the equation.

The concept of drag is critical in many fields such as aerospace engineering, automotive design, and sports science. It is always the aim to minimize the drag for enhanced speed and efficiency.
Parachutes work by increasing an object’s surface area, which increases the amount of air resistance (drag), slowing the object’s descent.
The shape of vehicles is designed to be streamlined to reduce form drag. The same concept applies to athletes in sports like cycling or swimming.

The reduction of drag is critical for efficient and fast motion through fluid. Any factor that increases the drag force—by increasing the object’s cross-sectional area, the fluid’s density or viscosity, or the object’s velocity—will increase resistance to motion and decrease the efficiency of movement.
It’s essential to remember that the drag force equation only provides an estimation. In reality, drag is dependent on many other factors like the object’s surface roughness, the fluid’s temperature and pressure, or whether the flow is laminar or turbulent.
Understanding how drag affects motion through a fluid can lead you to understand and appreciate things like why certain shapes are more aerodynamic, why objects reach a terminal velocity, and why moving through water is more difficult than moving through air.