## College Physics (4th Edition)

Poiseuille's law: $Q = \frac{\pi~P~r^4}{8~\eta~L}$ $Q$ is the flow rate ($m^3~s^{-1}$) $P$ is the pressure ($Pa$) $r$ is the radius ($m$) $\eta$ is the fluid viscosity $L$ is the length of the tube ($m$) We can rearrange the equation: $\eta = \frac{\pi~P~r^4}{8~Q~L}$ We can replace the variables on the right side of the equation with their units: $\frac{(Pa)~(m)^4}{(m^3~s^{-1})~(m)} = Pa \cdot s$ Viscosity has SI units of pascal-seconds.