Editing Drag equation (section)

== Discussion ==
The equation is easier understood for the idealized situation where all of the fluid impinges on the reference area and comes to a complete stop, building up [[Pressure#Stagnation pressure|stagnation pressure]] over the whole area. No real object exactly corresponds to this behavior. <math>c_{\rm d}</math> is the ratio of drag for any real object to that of the ideal object. In practice a rough un-streamlined body (a bluff body) will have a <math>c_{\rm d}</math> around 1, more or less. Smoother objects can have much lower values of <math>c_{\rm d}</math>. The equation is precise – it simply provides the definition of <math>c_{\rm d}</math> ([[drag coefficient]]), which varies with the [[Reynolds number]] and is found by experiment.

Of particular importance is the <math>u^2</math> dependence on flow velocity, meaning that fluid drag increases with the square of flow velocity. When flow velocity is doubled, for example, not only does the fluid strike with twice the flow velocity, but twice the [[mass]] of fluid strikes per second. Therefore, the change of [[momentum]] per time, i.e. the [[force]] experienced, is multiplied by four. This is in contrast with solid-on-solid [[Friction#Kinetic friction|dynamic friction]], which generally has very little velocity dependence.