Nothing - there was no energy on the string (or wherever) before the waves arrived and there is none there now that the destructive waves have arrived - there will probably still be none after they pass.
:-)
Bob at PC
________________________________________
From: phys-l-bounces@mail.phys-l.org [phys-l-bounces@mail.phys-l.org] on behalf of Peter Schoch [pschoch@nac.net]
Sent: Friday, June 22, 2012 1:33 PM
To: Phys-L@Phys-L.org
Subject: [Phys-L] A waves question
A fairly inquisitive student came up with the following question, and I thought I'd get the reaction of the list as to the best answer.
The course is "Liberal Arts Physics" -- designed as a survey course for those LA majors that need one semester of a science. The theme was energy conservation (a bit broad, and I would narrow it a bit next time I do it).
We are discussing waves. The student asked the following (more or less)
If a wave can transit energy that is proportional to its amplitude squared, and energy can't be created or destroyed, what happens to the energy when two waves destructively interfere?