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Problem with Waves and bridge

  1. Feb 1, 2005 #1
    I am having trouble with the following question:

    A bridge has a natural frequence of 10 Hz. If a support is placed one-third of the way along the bridge, what is the new resonant frequency?

    How should I start this problem?
  2. jcsd
  3. Feb 2, 2005 #2
    When you place a bridge somewhere between the ends, you alter the way the harmonics are distributed.
  4. Feb 2, 2005 #3


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    Staff Emeritus
    Science Advisor

    Take "L" to be the length of the original bridge. A wave MUST have a node at the two endpoints where the bridge is fixed so knowing the frequency, you can calculate the wavelength(s) of the resonant frequency in terms of L. If a support is placed at at 1/3 L a resonant frequency must have wave length 2(1/3)L= (2/3)L (2 because there is a node in the middle of a wave length). Calculate the frequency from that (again in terms of L) and compare with the original value.
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