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Natural frequency resonance help

  1. Oct 25, 2009 #1
    I have just started to study resonance and I don't quite understand what it is. My textbook says:
    "All objects vibrate with a characteristic "natural frequency". When an object is forced to vibrate at its natural frequency, the amplitude of its vibration can grow very large. This effect is called resonance."
    This seems to be saying that all objects are constantly vibrating. Is that true?

    It then goes on to say:
    "The seismic waves from an eathquake force the buildings on the sufrace to vibrate. If the frequency of the vibrations is at the natural frequency of the building, then the amplitude of the vibration increases. More energy is transferred to the building and the building will probably fall down."
    Does that mean that the building was vibrating before the earthquake struck, and then the earthquake induced further vibrations onto it which doubled the intensity of its natural vibrations thereby forcing the building to come apart and collapse?
     
  2. jcsd
  3. Oct 25, 2009 #2

    Chi Meson

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    Re: Resonance

    no

    Think of a kid in a swing. When it is just sitting there, it is not vibrating. Give it a push, and you will see that the swing goes back and forth at a specific frequency, regardless of how much weight is on it, and regardless of how high the push is (there are more complications when the "real world" is considered, but the frequency stays close to the same).

    Same thing goes for a car antenna, or a guitar string, or bell, or even a piece of wood or a brick.

    Back to the swing, if you push the swing with a frequency that exactly matches the frequency that the swing naturally has, then the swing gets higher and higher. That's the amplitude getting larger, while the frequency stays the same.

    If you pushed at a different frequency, then every other push might work to stop the swing, rather than push it higher, and the kid will get annoyed!

    no, it's just that the building, like the swing, has a natural frequency. If the earthquake "pushes" the building at the same frequency, the sway of the building will increase in amplitude, and there is a possibility of fracture, since buildings are not known to be very elastic.
     
  4. Oct 26, 2009 #3
    Re: Resonance

    Hi Chi
    Thanks a lot for your help. I understand a little better now although I'm still struggling slightly with the concept of inanimate objects having a natural frequency. One of the questions in my textbook asks, why might a note from an opera singer shatter a wine glass?
    The answer that the book gives is:
    "The glass will vibrate with a particular natural frequency. If the opera singer sings a note of exactly the same frequency, it may cause resonance in the glass. This means that the glass will vibrate with a much larger amplitude, which could break the glass."

    - Under what circumstances does the glass vibrate at its natural frequency?
    - Am I wrong in saying that when the singer produces the note with the same frequency, the vibrations taking place in the glass are increased as in constructive interference?

    Thank you for your patience!

    Steve
     
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