 So we've been talking about the stream function and the velocity potential function. What we're now going to take a look at is how those relate to one another and it turns out that they're orthogonal to one another. And we can examine this by looking at the definition of velocity for both the stream function and the velocity potential. So that's how they relate to one another. Now we also looked at the total derivative for the potential and the stream function. And we said that on lines of constant potential, that was zero. And therefore subbing in the values for the velocities. We can say that this equation holds along a line of constant phi. And similarly for the stream function, we have a total derivative, again that is equal to zero subbing in the values. And so this line here holds, or this equation holds along a line of stream function, or psi equals constant. So what we can do, we can look at the slope from these two equations. And the slope of dy by dx for a line, a potential line, is equal to minus u over v. And you can show that that is the negative reciprocal of the slope of a stream function line. So they are mutually orthogonal to one another, the stream functions and the potential functions. And let's take a look at the velocity field that we were working with earlier. And that was the one where we came up with the stream function and the potential functions. So on this curve, on the left, is our stream function, psi was x, y. And I've done this with a equals one and the constant equal to zero in the equation. And the potential function on the right, and that turned out to be x squared over two minus y squared over two. So we have those two. Now if we overlay the two, they should be mutually orthogonal and we'll take a look in the next slide and certainly they are. So there you can see an overlay of the stream function and the potential function. And sure enough, the lines are mutually orthogonal or perpendicular to one another. So that gives us a bit of a coverage of the stream function and the potential function. In the next segment, what I'll do is I'll kind of try to wrap it all up and explain where we use the stream function and the potential function in fluid mechanics.