Mechanical waves are one of the fundamental models of classical physics and have numerous applications in science and engineering. In this work, we are going to analyze graphically what happens to the energy when two pulses traveling in opposite directions approach each other and destructively interfere.

During an introductory lecture on waves, it is a well-known exercise to show pulses traveling in opposite directions along a stretched string and ask the students what happens when the pulses overlap. To explain the situation, the instructor draws snapshots of the string for different instants taking into account the principle of superposition for the pulses.1,2 Notice that if the pulses are of equal amplitude, but inverted, there will be an instant of time in the superposition in which the pulses completely cancel. An interesting question that can arise during class time, and that usually generates confusion in freshman students, is what happens...

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