Aerodynamic drag is the force of air along the length of the traveling car, opposing the car's force. As the car cuts a path through the air, some air molecules collide with the front bumper, producing resistance.
Other molecules flow along the hood, only to come up against the windshield -- another source of drag. The air that glides smoothly over the roof grows turbulent above the rear window and behind the car, exerting a backward force on the vehicle.
Speed, air density, and car size, shape and design all determine the magnitude of a car's drag force.
"A faster car experiences more drag because it has to push air molecules out of the way faster," Diandra Leslie-Pelecky explains in her book, "The Physics of NASCAR." "Dense air increases drag because there are more air molecules hitting each area on the car. A larger cross-sectional area increases drag because more air molecules have to be moved out of the way" [source: Leslie-Pelecky].
Drag is the major obstacle to acceleration and racing speed. A passenger car driving on the highway spends an estimated 60 percent of its energy overcoming air drag, a far greater percentage than tire friction and the energy needs of the drive train itself [source: Beauchamp].
Defeating drag was the first major focus of automotive aerodynamics, beginning in the 1960s. It is still the most important variable in racing conditions that place a smaller premium on downforce, such as longer tracks with more straightaways.
The sleek lines, tilted windshields and rounded corners of modern race cars -- and passenger cars for that matter -- are designed to minimize drag. But the quest to engineer racecars with high net downforce sometimes leads to additional drag. The rear spoiler found on NASCAR vehicles is a case in point: It increases drag by distributing weight from the front to the back of the car [source: Circle Track]. Aerodynamics remains a vibrant and young field of engineering, with many innovations still to come down the road.
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