17.1

Constructive and Destructive Interference of Two Pulses

Student Notes

Two pulses are sent along a string by a driver at the left end. They reflect from the ends, which are either both free or both fixed, and interfere with each other. The way the pulses reflect, and how the pulses interfere, are really independent, and should be considered separately. How do the pulses reflect when the ends are fixed? What happens when the ends are free?

Self-test

1. | At the end of the string, can the amplitude of the string ever exceed the amplitude of the pulse reflecting from that end? In what situation?
2. | When a pulse reflects from a fixed end, as well as when destructive interference occurs for identical pulses, the string is flat at one instant in time. What causes the pulses to re-emerge from the flat string? Note that the string is flat during reflection only when the pulse is symmetric, as it is here, and is during destructive interference only when the pulses are mirror images, as they are here in the case of identical pulses.

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