What is the purpose of curved surfaces, called spoilers,placed on the rear of sports cars? They are designed so that air flowing past exerts a downward force.

The spoilers are mounted on the rear of the sports car so that the air flowing past exerts a downward force on the car. This increases the magnitude of the normal reaction generated by ground and also the friction between the tires of the car and ground.With large friction the fast moving car can be better controlled, which would have been impossible in the absence of the spoilers. It is important to note that in the fast moving car, the pressure/force from the air is significant enough unbalance it, thereby making it harder for the driver to attain a controlled motion.