Mechanics

# The frictional force of air on cars

The air resistance force on a car depends on its area, speed, air density and drag coefficient.

When a body moves in contact with the ground or fluid, these means will apply a resistance force that opposes the movement, called friction force.

We can cite several cases in which friction forces act in our daily lives, such as the sound produced by a violin that depends on the friction between the strings and the bow, when we walk we depend on the friction between our feet and the ground, the resistance force of the air that acts on a parachutist at the moment of opening his parachute, etc.

When we are riding in a car, there are two forces that oppose the movement: the friction of the tires with the ground and the air resistance. The role of the vehicle’s engine is to provide enough energy to overcome these resistances and move. In this text we will understand how the air resistance force can be determined.

Four quantities are needed to determine the air resistance force, which will be called FAR. The first quantity is the area (A) measured from the front view of the vehicle, the speed (v) of the car, the air density called ρ, and finally a dimensionless constant called the drag coefficient (C), which depends on of the vehicle’s shape. The most common values ​​for the drag coefficient range from 0.35 to 0.50.

The equation below determines the air resistance force on a car:

AR =    1   .CApv 2
2

Example: Imagine a vehicle that has a drag coefficient C equal to 0.40 and frontal area A of 1.50 m 2 . If this car travels down a residential street at a speed of 10 m/s (36 km/h), what must be the air resistance force on it?

Data: Air density equal to approximately 1.2 Kg/m 3 .

FAR = 0.5 0.40 . 1.50 . 1.2 . 10 2 = 36N

The drag force on this vehicle would be 36N.

Engineering works to make cars more and more aerodynamic in order to minimize the action of the air resistance force. The water drop shape is the one that best presents these results as it eliminates turbulence in the rear of the cars, facilitating displacement.