If things were that simple...

Cd in the above formula can only be assumed constant under certain set of circumstances. In general though Cd is a function of so called "Reynolds number" (http://en.wikipedia.org/wiki/Reynolds_number_)

"At a low Reynolds number, the flow around the object does not transition to turbulent but remains laminar, even up to the point at which it separates from the surface of the object. At very low Reynolds numbers, without flow separation, the drag force is proportional to v instead of v^2; for a sphere this is known as Stokes law."