What exactly is the purpose of the spoiler on a car and how does it work?
Many cars, from drag racers to sports cars to monster trucks carry different kinds of spoilers on them. Some cars, like the Ferrari F1 cars, have them front and back; and since they are probably the most scientifically advanced wheeled transportation, I'll use them for the discussion.
Cars have spoilers to increase their grip on the road. Normally the weight of a car is the only thing that forces the tires down onto the pavement. Without spoilers, the only way to increase the grip would be to increase the weight, or to change the compound the tire was made out of. The only problem with increasing the weight is that it doesn't help in turns, where you really want to grip. All that extra weight has inertia, which you have to overcome to turn, so increasing the weight doesn't help at all. The way the spoiler works is like an airplane wing, but upside down. The spoiler actually generates what's called 'down force' on the body of the car.
The advantages of this can be seen very readily. Instead of having a heavy car, which is slow, or having a very light car, which can slide away easily, you now have a car that sticks better the faster it goes. Sounds perfect, right? There is one catch.
Every time a wing generates lift (or a spoiler generates down force) it also generates drag. Drag is the natural reaction of the fluid (air) to resist motion through it (the car). Drag is bad, because it slows down the car. So, more down force is good... but too much down force = too much drag, which is bad. Very very high performance sports cars, like Le Mans or F1, have a ratio called the 'lift/drag' ratio. The car designers try and maximize this so that the car has just enough force to get around the corners, but not so much that they are too slow. Indy cars, and ones that are designed like that can have down force on the order of 3G's, at 200mph. That means they could hang completely upside down on the track, and as long as they kept going fast enough, they would still stick to the road.
Frank DiBonaventuro, B.S., Physics graduate, The Citadel, Air Force officer.