When riding a bicycle or simply sitting outside enjoying a summer breeze, we can feel wind on our faces. The wind we feel is the air flowing around us. This air flow results in air friction and thus air resistance. When we drive a car, there is air friction too, which causes a force pushing the car in the opposite direction of the car’s motion. This force is called drag, or air resistance. To experience this drag, try sticking your hand out of a car that is in motion. While driving a car, this force is the reason why you slow down while coasting on even terrain. While driving, the car’s motor compensates for the drag, which in turn requires gasoline.

The origin of the air drag

Unless we are in a vacuum, drag occurs because air has substance. There is nothing in a vacuum, and therefore, no air friction or drag can occur there. But air creates friction and drag because it contains many molecules which have a mass, such as nitrogen and oxygen. There is drag when the molecules are being pushed out of the way of the moving object. Especially when the air molecules are not guided smoothly around the moving object, the air molecules hit the object like little balls, and thus, cause a force on it. The figure below illustrates how the air particles are pushed away from a driving car.

Figure illustrating how the air particles are pushed away from a driving car<br/> which leads to the air drag.Figure illustrating how the air particles are pushed away from a driving car which leads to the air drag.

Even when an object moves in such a way that you’d think little air has to be pushed out of the way, the molecules still come in contact with the object’s surface. Because every surface is somewhat rough, as seen through a microscope, some molecules are being carried along with the surface. That results in a force on the moving object too. The force acting on the moving object creates air drag. The type of flows around moving objects are explained in the Article Flows in and around Objects.

The air drag around driving a car

In the everyday act of driving a car, the air drag is roughly proportional to the square of the velocity. That means if you double the speed, the drag is four times as big (2²). If you triple the speed, than the drag is 9 times as big (3²). This is called a quadratic dependency. Therefore, increasing speed by a small amount will result in a more substantial increase of drag. Let’s say you drive 30 miles per hour. Then you enter the highway, where you drive 60 miles per hour. The air drag is then four times as strong as it was before.

A car’s motor has to work against this drag, which costs energy that comes from the gasoline. Due to the quadratic dependency of air drag on speed, cars need less gasoline at lower speeds than would be necessary at higher speeds. That means that the faster you go, the more gas you will need.

 
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