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Homework Help: Period of standing waves

  1. Oct 26, 2009 #1
    1. The problem statement, all variables and given/known data
    Draw a sketch of of a standing wave created by a string fixed at two ends, the frequency is 60 Hz, the wavelength is 0.4 m. Draw a profile with times t=0, T/4, T/2, 3T/2

    2. Relevant equations

    period= 1/frequency,

    3. The attempt at a solution
    i know that at t/4, you will have a quarter of a wavelength going down the string at 1/240 seconds, but does it get reflected back? or is it just a quarter bump gowing towards the end, and the rest of the string is straight?
  2. jcsd
  3. Oct 26, 2009 #2

    Andrew Mason

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    A standing wave has fixed nodes so the wave does not travel. Hence the name "standing". It just goes up and down in between nodes.

    If it is fixed at two ends, the ends must be nodes. The lowest frequency mode of vibration of this standing wave is with the ends forming the first two nodes of the standing wave - ie. 1/2 wavelength. How long would the string be?

  4. Oct 26, 2009 #3
    This is from a lab, in which i calculated the wavelength to be 0.4 m from this equation:


    therefore wavelength = 2L/n=.4
    L= 1 m
    n = (5 in the non-makeup lab)

    The frequency gotten from the experimental was 60Hz

    which if you plug into the equation v=f* wavelength
    v is given as 23.5, you get about .4 for the wavelength
    Makes sense about the standing wave.

    A period is what it takes for 1 wave to travel a whole wavelength, which here is 1/60 seconds.

    but if you draw something at less than a wavelength, doesn't the pulse need to travel down the whole second to the full reflected wave?

    If it only travels for 1/240th of a second, do you just draw a line at the quarter wavelength mark? How does it look?
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