We require to check the derivative thus let's use v = 60. Plugging it in (2) provides the slope of the tangent line as -1.96, or negative. Thus, for all values of v > 50 we will have negative slopes for the tangent lines. When with v < 50, by looking at (2) we can notice that as v approaches 50, all the times staying greater than 50, the slopes of the tangent lines will approach zero and flatten out. As moving v away by 50 again, staying greater than 50, the slopes of the tangent lines will turn into steeper. We can here add in several arrows for the region above v = 50 as demonstrated in the graph as in following.

This above graph is termed as the direction field for the differential equation.