$\displaystyle ma = mg - kv^2

$

I have to find v in terms of t so I used the substitution

$\displaystyle

\frac{dv}{dt} = a

$

$\displaystyle

\int \frac{1}{g - kv^2} \, dv

$

where:

$\displaystyle

k = \frac{0.2D^2}{m}

$

I'm not terribly fond of the "ArcTanh" result... is there another way to do this?