Teaching Physics with the Physics Suite

Edward F. Redish

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Cancelling pulses?

Two almost rectangular pulses approach each from opposite directions on an elastic string. The pulses have the same symmetric shape, but the one on the right is reversed. A video tape of the string is taken of the motion of the pulses. A few frames before the pulses reach each other (frame number 1237), they look like shown in the figure below at the left. At the instant when the pulses are right on top of each other, a frame from the video tape (frame number 1287) looks like the figure below at the right. This frame shows that the they cancel perfectly at that instant.

The video tape keeps rolling. What do you think frame number 1337 looks like? Draw a sketch and explain why you think so. Make sure that your explanation includes an "analysis of the mechanism," that is, your explanation includes a discussion of how the various bits of string "know what to do" to get to the state you have described.

[Note: In actual fact, a string cannot support sharp edges. The rises and falls of the pulses will actually be somewhat rounded off. We have described the pulses as sharp-edged in order to simplify the analysis. If you prefer, you may assume that the pulses are slightly trapezoidal rather than rectangular, but assume that the width of the rise and the fall is narrow compared to the width of the overall pulse.]

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