Write the equation of terminal velocity.

(N/A) The terminal velocity $v_t$ of a spherical object of radius $r$ and density $\rho$ falling through a viscous fluid of density $\sigma$ and coefficient of viscosity $\eta$ is given by the formula:
$v_t = \frac{2r^2(\rho - \sigma)g}{9\eta}$
Where:
$r$ = radius of the sphere
$\rho$ = density of the sphere
$\sigma$ = density of the fluid
$g$ = acceleration due to gravity
$\eta$ = coefficient of viscosity of the fluid