An elementary derivation that appears in a number of introductory textbooks is as follows [1]: Let an element of
the unstretched string (in equilibrium) be dx, and the same element when displaced from equilibrium, is of length ds.
Therefore, the lengthening of the string is by ds − dx =
p
dx2 + dy2 − dx ≈
1 + (1/2)( ∂y/ ∂x)
2 + · · ·
dx − dx ≈
(1/2)( ∂y/ ∂x)
2 dx. The potential energy in the string is related to the restoring force by F = dU/( ds − dx). The
restoring force F is just the string tension τ. Therefore,