\(\text{dF}\left(z,t\right)=C_{m}\frac{\text{ρπ}d^{2}}{4}\text{\ u}\left(z,t\right)dz+\ C_{d}\frac{\text{ρd}}{2}\text{\ u}\left(z,t\right)\left|u\left(z,t\right)\right|\text{dz}\) (1)
\(F\left(z,t\right)=\int_{z=0}^{z=L}{dF(z,t)}\) (2)