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Question On Resonance- Intuitive feeling required

  1. May 16, 2006 #1
    I am not able to intuitively sense the concept of resonance.
    Let us say we have a building having a fundamental frequency of 5.5 Hertz .
    If the ground moved to and fro(during an earthquake) with a frequency of 5.5 Hertz, the building would vibrate strongly, or resonate.Right?

    I am not able to sense the phenomenon of resonance intuitively---
    IF the fundamental frequency of the ground and building are same, then the building would vibrate strongly.How to sense this intuitively??
  2. jcsd
  3. May 16, 2006 #2


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    Right. If a periodic excitiation (energy source) oscillates at the structures natural frequency, then vibration or oscillation may be excited, i.e. the driving force will excite one of the natural modes of vibration in the structure. This is somewhat like constructive interference.

    Perhaps get a long plastic ruler, hold it upright (vertical) and slowly push (deflect) the top and then slowly release the contact (reduce the deflection). Note the response. The repeat but increase the speed (frequency) of the deflection. Try to match the natural frequency.

    Alternatively, construct a flexible structure with a known natural frequency on a movable platform. Slowly move the platform backwards and forwards (oscillate) and note the response. Start below the natural frequency, then increase frequency up to the natural frequency and not the change in response.
  4. May 16, 2006 #3
    if you have these, use two tuning forks of the same frequency and hit one. then place that one next to, but not touching, the other and listen closely to the sound of the "unhit" one because you will hear the soft tone of it because resonance caused it to vibrate.
  5. May 16, 2006 #4

    Claude Bile

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    When you apply a force to, using your example, a building, it can only respond with a certain speed, which, in the case of a building depends on a whole host of factors, including the dimensions of the building and the material properties such as Young's modulus.

    If the applied force varies at exactly the same speed as the buildings 'response time' then the force and the movement of the building remains in phase, i.e. the force always pushes in the same direction that the building is moving, and the amplitude of the oscillation (assuming the driving force is oscillatory of course) will be a maximum. We can equate this 'speed of response' of the building to a characteristic frequency we call the resonant frequency.

    If we drive the oscillator far from resonance, then the force will spend much of its time opposing the movement of the building, hence the reduced amplitude.

    This argument of course does not just apply to mechanical oscillators, but electrical, optical etc....

  6. May 17, 2006 #5
    jrm: to get a feel for resonance, go down to your local swimming pool. And practice some diving off the springboard.
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