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Homework Help: Standing Waves on a Guitar String

  1. Jan 11, 2010 #1
    1. The problem statement, all variables and given/known data

    Learning Goal: To understand standing waves, including calculation of and , and to learn the physical meaning behind some musical terms.

    The columns in the figure (Intro 1 figure) show the instantaneous shape of a vibrating guitar string drawn every 1 . The guitar string is 60 long.
    The left column shows the guitar string shape as a sinusoidal traveling wave passes through it. Notice that the shape is sinusoidal at all times and specific features, such as the crest indicated with the arrow, travel along the string to the right at a constant speed.

    The right column shows snapshots of the sinusoidal standing wave formed when this sinusoidal traveling wave passes through an identically shaped wave moving in the opposite direction on the same guitar string. The string is momentarily flat when the underlying traveling waves are exactly out of phase. The shape is sinusoidal with twice the original amplitude when the underlying waves are momentarily in phase. This pattern is called a standing wave because no wave features travel down the length of the string.



    What is the wavelength of the standing wave shown on the guitar string?

    2. Relevant equations



    3. The attempt at a solution

    I answered 120 cm, but it was not correct, someone please explain to me where I went wrong. Thanks
     
  2. jcsd
  3. Jan 11, 2010 #2

    Mark44

    Staff: Mentor

    You don't provide the information in the figure, so it's hard to know where the node is in the string. Assuming that there is a node in the exact middle of the string, the wavelength of each standing wave is half the length of the string, not twice the length of the string.

    When you barely touch a guitar string at the 12th fret and pluck the string you get what is called a harmonic, a tone that is an octave higher than the natural tone of the unfretted string. You can also get another harmonic at the 7th fret, that is another octave higher. In this case you are setting up four standing waves, where the wavelength of each if 1/4 the length of the string.

    By length of the string, I mean the distance from the nut to the bridge, and this length does not include the portion between the nut and the tuners or between the bridge and tailpiece or where the strings are fastened to the body.
     
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