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Homework Help: X value closest to origin - phases difference - answer does not match teachers

  1. Sep 16, 2009 #1
    1. The problem statement, all variables and given/known data

    Two sinusoidal waves in a string are defined by the
    y1 " (2.00 cm) sin(20.0x # 32.0t)
    y2 " (2.00 cm) sin(25.0x # 40.0t)
    where y1, y2, and x are in centimeters and t is in seconds.
    (a) What is the phase difference between these two waves
    at the point x " 5.00 cm at t " 2.00 s?

    (b) What is the positive
    x value closest to the origin for which the two phases
    differ by (& at t " 2.00 s? (This is where the two waves
    add to zero.)

    3. The attempt at a solution

    My teacher went over this problem and gave us this answer...

    -5x+16= +3.14 -> X=2.57cm -> choose this answer because it is closest to the origin
    -5x+16= -3.14 -> X=3.83cm

    book answer is X=.0584

    He mentioned why it was different, but I could not understand what he said... he mumbles a lot... lol.
  2. jcsd
  3. Sep 17, 2009 #2


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    Homework Helper

    Your symbols are either quite nonstandard, or my browser doesn't interpret them properly. So, just in case others are having the same problem (and this may be why your post has gone unanswered), I will rewrite it for you using standard notation for what I think you meant. However, I'm pretty sure I can tell you why the book disagrees with the teacher ...

    The key is in the part that I bold-faced, italicized, underlined, and capitalized. There is an ambiguity. I took what was in parenthesis (basically, the requirement of destructive interference) as the actual condition. So, you just have to figure out how many odd multiples of π, and you can do this by trial and error fairly quickly. (You can also make a plot of 16-5x, and then follow the line from x=0 until it hits an odd multiple of π to get an idea of which odd multiple of π to use.)
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