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Homework Help: Frequency and Wavelength Problem

  1. Apr 13, 2008 #1
    1. The problem statement, all variables and given/known data
    A 5.00-m, 0.732-kg wire is used to support two uniform 235-N posts of equal length (see figure). Assume that the wire is essentially horizontal and that the speed of sound is 344 m/s. A strong wind is blowing, causing the wire to vibrate in its 7th overtone. What are the frequency and wavelength of the sound this wire produces?


    2. Relevant equations
    at 7th overtone: L= 8(lamda/2)

    3. The attempt at a solution
    lamda = (2/8)L = (2/8)(5) = (5/4) m

    Is my wavelength correct?
    I'm not sure of which equation to use to find the frequency.
  2. jcsd
  3. Apr 13, 2008 #2
    frequency{n}=(pth overtone+1)xvelocity of sound in air
    2xlength of wire

    therefore n= (7+1) x 344
    --------------- =6880Hz
  4. Apr 14, 2008 #3


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    Homework Helper

    A commonly useful relationship between wavelength and frequency in a wave is

    speed of wave = frequency X wavelength

    Once you have the wavelength of the wire's vibration, you can find the frequency once you know the speed of a wave in this stretched wire. There is a standard textbook formula for the speed of a wave in a stretched string involving the linear mass density (which is mass/length if the wire is uniform) and the tension in the wire, so first you'll need to find the tension by treating the wire and posts as an equilibrium problem.

    Finally, if I'm reading the question correctly, you need to find the wavelength and frequency of the sound wave in the air. In other words how is the wavelength and frequency of the wire's vibration related to the wavelength and frequency of the air's vibration? What will be the same, and how can you find the other?


    Hi Vishal60,

    I don't believe your approach is correct; since all of the strings in a guitar have the same vibration length, your equation would predict that all of the strings have the same frequency and therefore the same sound for their seventh overtones.
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