f[v_]  =  9.8 - v*(1/5)

DSolve[v'[t]  ==  f[v[t]],  v[t], t]

<< Graphics`PlotField`

PlotVectorField[{1, f[u]}, {t, 0, 20},  {u, 0, 80},  Axes  -> True, 
      ScaleFunction -> (1 &),  AspectRatio  ->  1]