Figure 11-36 shows the laminar flow of fluid within a circular pipe seen in cross section. Due to friction, the fluid that’s right up against the inside walls of the pipe doesn’t move at all. This is the \(\textit{no-slip condition}\) that we described in Section 11-9. (The fluid in contact with the lower plate in Figure 11-34 behaved the same way.) The fluid a small distance from the walls moves a little, the fluid a bit farther away moves a bit faster, and so on. The fastest flow is at the very center of the pipe. This flow pattern is called \(\textit{parabolic}\) (since the dashed curve in Figure 11-36 is a parabola lying on its side). A derivation using calculus gives the following expression, called the \(\textbf{Hagen-Poiseuille equation}\), relating the pressure difference between the two ends of the pipe to the resulting flow rate. (Non-French speakers can pronounce Poiseuille as “pwa-zoo-yah.”)