C.17-C.20

$${\begin{align*} \text{jõud} = \mathrm{\frac{energia}{vahemaa}} \bigg |_\text{ideaal}&= \frac{1}{2} c_d \rho A_p v^2 + \frac{1}{2} \frac{(mg)^2}{\rho v^2 A_s} \\ &= c_d \rho A_p v_\text{opt} ^2\\ &= c_d \rho A_p \frac{mg}{\rho (c_d A_p A_s)^\frac{1}{2}} \\ &= \bigg( \frac{c_d A_p}{A_s} \bigg )^\frac{1}{2} mg\end{align*}}$$