Solution
= Solution
For $C=0$, the increasing branch with $W(0)=0$ satisfies $W'=\sqrt{2W}$, hence
$$
\boxed{W(Z)=\frac12Z^2,
\qquad A(Z)\propto\frac2{Z^2}.}
$$
The thread thins algebraically, and the tensile vertical stress is proportional to $W'=Z$.