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A question about resonance

  1. May 16, 2015 #1
    1. The problem statement, all variables and given/known data
    Which of the following situations is a result of resonance?
    a. Hydro wires "hum" as a mild breeze blows over them.
    b.A child pushes herself on a swing by moving her legs back and forth appropriately.
    c.Windows in your house rattle as a large truck drives by.
    d. Building begin to sway when an earthquake occurs.
    e. all of the above.

    2. Relevant equations

    3. The attempt at a solution
    I choose B because I thought the resonance happens when the frequency of the wave is the same as the frequency of the object, and the hydro wires "hum" is the sound caused by vibration. I don't think that the frequency of an earthquake could be equal to the natural frequency of a building.
    But the answer my teacher told me is E, it was a quiz so I didn't have time to ask him why so I came here.
  2. jcsd
  3. May 16, 2015 #2


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    The humming and swaying only occur if resonance is hit, otherwise you just have "noisy" vibration. It's not that the wind or the earthquake is "at the resonant frequency" of the object, it's that it sets up a motion that is harmonic in the object. I suspect that if the wind and the earthquake WERE at the harmonic frequency, MUCH more radical swings would occur. The building for example would like collapse very quickly instead of just swaying.

    The window rattling feels much less intuitive to me, but I suppose it must be the same phenomenon that I'm describing above.
  4. May 16, 2015 #3


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    For a) see http://en.m.wikipedia.org/wiki/Vortex_shedding.
    Did you not think c) would be from resonance?
    d) is a bit tricky. The clue is in the word 'sway', which implies a more or less regular oscillation. I.e the swaying is down to the natural frequency of the building, and could result from a single sudden displacement.
  5. May 16, 2015 #4
    Thanks for helping.
  6. May 16, 2015 #5


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    Staff: Mentor

    I used to live under the flight approach to the city's airport. Sometimes when a jet flew over its engine sounds would coincide with the natural resonance of my large-glass-paned timber door and the sound would be hugely magnified as the door rattled in synchrony.

    When things are bumped, hit, struck, or jarred, if they are able to vibrate they will vibrate at their resonant frequency.

    If this were a public exam the examiners may be taken to task over that word "is" in the question statement, which I think carries an implication that only one situation satisfies the criterion.
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