Aerodynamics concerns the way air moves through and around a car.

In a car, drag can be further divided into three basic forces: frontal pressure, rear vacuum, and the boundary layer. Frontal pressure is created by the car pushing air aside as it drives. Rear vacuum occurs when air moving around the car creates a hole in the air behind it that cannot be filled. The boundary layer happens where air meets the surface of the vehicle. When engineers understand these three forces, they can describe the majority of car–airflow interactions. Frontal pressure is created when the air attempts to flow around the front of the car. As the car moves, air molecules are pushed to the front of the car; as they compress, they create pressure. Air pressure is higher at the front of the car and lower on the sides of the car, so frontal pressure is a form of drag. As air passes over and around the car, a hole is left at the back of the car, and a rear vacuum forms as the air passes through that hole. This space occurs behind the rear window and trunk. Air molecules are simply not able to fill the hole quickly enough as the car travels down the road, and this creates a continuous rear vacuum that forms in the direction opposite of the car. The technical term for this rear vacuum is flow detachment, and it is another form of drag. As the speed of the vehicle increases, the drag increases two-fold.



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