Solution
= Solution
Constant translation speed gives $\boxed{\ell(t)=vt}$, since the initial displacement is zero. Substituting into the <Fourier series> gives
$$
\boxed{g(x,t)=\frac K\pi\sum_{n=1}^\infty\frac{(-1)^{n+1}}n
\sin\left[\frac{2\pi n}{L}(x-vt)\right]}.
$$
The periodic ramp moves in the positive $x$ direction for $v>0$. At the moving jumps, this series gives the average of the two one-sided values.