Before we look at how aerodynamics is applied to automobiles, here's a little physics refresher course so that you can understand the basic idea.
As an object moves through the atmosphere, it displaces the air that surrounds it. The object is also subjected to gravity and drag. Drag is generated when a solid object moves through a fluid medium such as water or air. Drag increases with velocity -- the faster the object travels, the more drag it experiences.
We measure an object's motion using the factors described in Newton's laws. These include mass, velocity, weight, external force, and acceleration.
Drag has a direct effect on acceleration. The acceleration (a) of an object is its weight (W) minus drag (D) divided by its mass (m). Remember, weight is an object's mass times the force of gravity acting on it. Your weight would change on the moon because of lesser gravity, but your mass stays the same. To put it more simply:
a = (W - D) / m
As an object accelerates, its velocity and drag increase, eventually to the point where drag becomes equal to weight -- in which case no further acceleration can occur. Let's say our object in this equation is a car. This means that as the car travels faster and faster, more and more air pushes against it, limiting how much more it can accelerate and restricting it to a certain speed.
How does all of this apply to car design? Well, it's useful for figuring out an important number -- drag coefficient. This is one of the primary factors that determine how easily an object moves through the air. The drag coefficient (Cd) is equal to the drag (D), divided by the quantity of the density (r), times half the velocity (V) squared times the area (A). To make that more readable:
Cd = D / (A * .5 * r * V^2)
So realistically, how much drag coefficient does a car designer aim for if they're crafting a car with aerodynamic intent? Find out on the next page.