# The Significance of the Air Resistance Formula in Automobile Design

### Key Takeaways

• The force exerted by air friction on the motion of a body is generally referred to as air resistance or drag.

• When drag is equal to the weight of a falling body in the earth’s atmosphere, then the acceleration of it is zero, making the velocity constant.

• When an automobile shape is efficiently streamlined to move forward easily in the atmospheric viscous air with minimum resistance, then its drag coefficient is a low value. Air resistance is a critical factor in achieving good mileage in automobile design

When an object moves in the air, certain forces oppose the motion. Most aerodynamic applications deal with one such force–the force exerted by air due to friction. The air offers resistance against the body motion and is generally referred to as air resistance. It is essential to overcome air resistance to accelerate the body. The air resistance formula is used for determining the impact of the air resistance force on aerodynamic systems externally and internally.

Computational fluid dynamics (CFD) simplifies the study of air resistance by offering simulation models. The structure of an aerodynamic system can be optimized for air resistance, otherwise called drag, using CFD tools. Let’s take a look at air resistance for the CFD-based study of aerodynamic systems.

## The Air Resistance Formula

The knowledge of air resistance is of great importance when working on aerodynamic systems. The force exerted by air friction on the motion of a body is generally referred to as air resistance or drag. The unit of air resistance is Newtons. Air resistance is one of the main reasons for the deceleration of bodies in motion. The faster the body motion gets, the higher the air resistance is exerted on it. All moving objects are impacted by air resistance.

Consider an airplane as an example of a body in motion. The speed of an airplane is influenced by air resistance; the air resistance opposes the airplane movement. The force exerted by air is in the opposite direction to the airplane movement and decreases the speed.

The air resistance formula is: Fair resistance is the force of air resistance, also known as aerodynamic drag, k is the force constant, and v is the velocity of the body. The negative sign indicates the opposition offered to the motion of the body.

The force constant ‘k’ brings the effect of density, drag, and area on the air resistance formula and can be expanded as: p is the density of the air through which the the body moves, CD is the drag coefficient that takes into account hard to measure effects, such as surface roughness and turbulence, and A is  the area of the moving object where the air exerts force.

## The Air Resistance Formula and Acceleration

According to the first equation, drag is proportional to the square of velocity. For a body moving in the atmosphere, there are two external forces:

1. Gravitational force, expressed as weight of the body (W)
2. Drag (Fair resistance)

The net force acting on the moving body is the difference between its weight and drag. Replacing the force term (F) in Newton’s second law of motion (F=m.a) by the net force acting on the moving body, acceleration can be obtained with the following equation: When drag is equal to the weight of a  falling  body in earth’s atmosphere, then the acceleration of it is zero, making the velocity constant.

## Air Resistance and Automobile Design

The air resistance formula is used for calculating the air resistance, force constant, and velocity of a moving body when the remaining parameters in the first two equations are known. The air resistance formula finds wide application in aerodynamics. Aerodynamics engineers design the shape and size of objects, such as cars and airplanes, such that it lowers the drag force.

In the case of automobiles, air resistance is a critical factor in achieving good mileage. It is observed that on highways, 50% of the vehicle power is utilized for overcoming the air resistance or drag force. By streamlining the shape of automobiles, the air resistance against the forward motion of the vehicle can be reduced, thereby increasing the vehicle’s fuel mileage.

When an automobile shape is efficiently streamlined to move forward easily in the atmospheric viscous air with minimum resistance, then its drag coefficient is a low value. A high value of drag coefficient implies that the automobile is poorly streamlined, and there is high air resistance exerted against its forward motion.

The air resistance formula is not only applicable to air but also to any type of fluid. The functionality of the drag force or air resistance becomes complicated and influenced by the size, shape, and velocity of the moving body as well as the fluid it is in.

To mitigate air resistance effects on your design, you can rely on Cadence’s suite of CFD simulation tools. Cadence’s robust CFD suite makes it easier to analyze the impact of air resistance on astreamlined aerodynamic body shape.