# Arc length constant, despite varying the period via varying amplitude

• I
Gold Member
This is not a school problem, just my own mucking about, but since it has the form of a problem, I am willing to shift it to the "homework problems" rubric.
If there is a theoretical string (no thickness, etc.) that is non-stretchable tied to two endpoints and is long enough to be able to form a (taut) sin wave (say, y= sin x from x=0 to pi), then the same string makes a new sin wave y= A sin (nx) for an integer n, is there any relatively simple closed-form way to calculate A (as a function of n)? For example, a brute force attack to compare n=1, A=1 to n=2, gives the ratio A=
(2/5)½E(½)/E(4/5), where E is the elliptical integral of the second kind with parameter m=k2, making it rather more complicated than desired. If there is no simpler alternative, OK, but it would be nice if there were. Thanks.

Without some approximations I think you are stuck with the elliptic integrals. It is a matter of taste, but you can get a "nice" result assuming ##A## is small. Call the fixed total length of the curve ##L##, and let the curve have the parametric form ##\vec{r}(t)=(t,A\sin(nt))##. I get for the length of the curve:

##L={\sqrt{1+A^2n^2}\over n}\mathbb{E}\left[n\pi,{A^2n^2\over{1+A^2n^2}}\right]##.

Expanding to order ##A^2## gives

##L=\pi+{\pi\over 4}n^2A^2 \Rightarrow A={2\over n}\sqrt{{L\over \pi}-1}##.

It seems like the approximation is self-consistent is some predictable ways: when the length is close to ##\pi##, then curve must be close to straight (i.e. ##A## small), and if you add more wiggles (higher ##n##), then the amplitude must get smaller for a given length of rope.