I'm reading a rather old mechanics paper and the author uses the following to derive certain stresses.
Let x + iy = f(u + iv)
Then, J$e^{i\phi}$ = f'(u+iv)
J is the Jacobian, and $\phi$ is the angle between a tangent to the curve v = constant and the x axis
given f is a conformal mapping.
How can this be derived?