a
$F=\eta A\left(\frac{\Delta V}{\Delta x}\right) ; F=\operatorname{mg} \sin \theta$
$\eta=\frac{F}{A \cdot\left(\frac{\Delta V}{\Delta x}\right)}$
$\eta=\frac{\rho a^{3} \cdot g \sin \theta}{a^{2} v / t}$
$\eta=\frac{\rho a \operatorname{tg} \sin \theta}{v}$