Translations:One-dimensional waves/23/en

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Let us now apply the superposition principle to the case of two equal symmetrical pulses that have opposite polarity. The two pulses are assumed to have exactly the same shape and size, and each is symmetrical. Suppose that the one that displaces the string upward is the one that travels to the right. The pulse that displaces the string downward travels to the left. There is some moment in their crossing when the addition of equal displacements upward (plus) and downward (minus) leaves us with a net displacement of zero. Thus, at the moment when the pulses pass each other, the whole string appears to be undisplaced. In other words, there is complete cancellation at that particular moment. How is this situation different from the case of a string at rest? In such a case - that is, when the string carries no wave motion - all the various pieces of the string stand still at all times. On the other hand, when two symmetrical equal and opposite waves travel along the string, the string passes through its rest position for only a single instant, yet at that instant, the string is still moving. At that particular instant, all of the moving string’s energy exists purely as kinetic energy.