What is resonant frequency, and how do things like bridges collapse because of it?
In: 0
Resonant frequency is something that happens when two waves collide. If the distance between the peaks of their amplitude (the frequency) is the same, then when they overlap the amplitudes are added together.
Since bridges are fixed at both ends a wave that hits the end of the bridge orginating from a point on the bridge will bounce back. If the frequency of the pulse that is making the wave is such that the waves reflecting back on the bridge is in sync with the waves geberated by the pulse then you have resonant frequency. The total amplitude will continue to increase until the stress limit of the structure is exceeded and it fails.
Resonant frequency is something that happens when two waves collide. If the distance between the peaks of their amplitude (the frequency) is the same, then when they overlap the amplitudes are added together.
Since bridges are fixed at both ends a wave that hits the end of the bridge orginating from a point on the bridge will bounce back. If the frequency of the pulse that is making the wave is such that the waves reflecting back on the bridge is in sync with the waves geberated by the pulse then you have resonant frequency. The total amplitude will continue to increase until the stress limit of the structure is exceeded and it fails.
Resonant frequency is something that happens when two waves collide. If the distance between the peaks of their amplitude (the frequency) is the same, then when they overlap the amplitudes are added together.
Since bridges are fixed at both ends a wave that hits the end of the bridge orginating from a point on the bridge will bounce back. If the frequency of the pulse that is making the wave is such that the waves reflecting back on the bridge is in sync with the waves geberated by the pulse then you have resonant frequency. The total amplitude will continue to increase until the stress limit of the structure is exceeded and it fails.
If you’ve ever ridden a swing in a playground, then you are already familiar with resonance and resonant frequencies. To get the swing to go higher and higher, you push at the right time (at the resonant frequency of the swing).
By adding more pushes at the correct time, the swing goes higher and higher as the energy of the movement increases each time you push.
But for bridges (and buildings) “swinging” or oscillations are undesirable. And you don’t want to have things that keep adding energy to that oscillation – for example wind or people walking on a bridge. In badly designed bridges, that could (unlikely, but possible) cause the bridge to fail.
If you’ve ever ridden a swing in a playground, then you are already familiar with resonance and resonant frequencies. To get the swing to go higher and higher, you push at the right time (at the resonant frequency of the swing).
By adding more pushes at the correct time, the swing goes higher and higher as the energy of the movement increases each time you push.
But for bridges (and buildings) “swinging” or oscillations are undesirable. And you don’t want to have things that keep adding energy to that oscillation – for example wind or people walking on a bridge. In badly designed bridges, that could (unlikely, but possible) cause the bridge to fail.
If you’ve ever ridden a swing in a playground, then you are already familiar with resonance and resonant frequencies. To get the swing to go higher and higher, you push at the right time (at the resonant frequency of the swing).
By adding more pushes at the correct time, the swing goes higher and higher as the energy of the movement increases each time you push.
But for bridges (and buildings) “swinging” or oscillations are undesirable. And you don’t want to have things that keep adding energy to that oscillation – for example wind or people walking on a bridge. In badly designed bridges, that could (unlikely, but possible) cause the bridge to fail.
Someone’s already mentioned a swing, so I’ll give another basic example.
Find a bottle, the bigger the better. Now half fill it with water, turn it on its side, and try to rock the bottle back and forth to create the biggest wave you can. You should notice some things:
* The size of the waves doesn’t affect the frequency of the waves. In other words, the system (the half-filled bottle) has a natural frequency determined that is independent of the amplitude in each wave.
* When the frequency with which you rock the bottle *matches* the natural frequency of the waves, your wave grows bigger and bigger each period.
Boom, that’s resonant frequency. If you rock the bottle at some other frequency, then that dissonance in the frequencies means your motion is taking away energy from the system just as often as it is adding energy, and so your wave doesn’t reliably grow. It’s only when you time your actions in a way that each action is adding energy that your wave grows.
The exact same phenomenon is apparent, say, when singers break glasses by singing at a high pitch. A wine glass has a *natural frequency* that is determined only by its shape and the speed of sound in glass. When singers match that frequency, each pressure wave (of which there are thousands per second) only adds to the magnitude of the pressure waves within the glass. Eventually, the glass cannot accommodate the stress, and so it breaks.
Someone’s already mentioned a swing, so I’ll give another basic example.
Find a bottle, the bigger the better. Now half fill it with water, turn it on its side, and try to rock the bottle back and forth to create the biggest wave you can. You should notice some things:
* The size of the waves doesn’t affect the frequency of the waves. In other words, the system (the half-filled bottle) has a natural frequency determined that is independent of the amplitude in each wave.
* When the frequency with which you rock the bottle *matches* the natural frequency of the waves, your wave grows bigger and bigger each period.
Boom, that’s resonant frequency. If you rock the bottle at some other frequency, then that dissonance in the frequencies means your motion is taking away energy from the system just as often as it is adding energy, and so your wave doesn’t reliably grow. It’s only when you time your actions in a way that each action is adding energy that your wave grows.
The exact same phenomenon is apparent, say, when singers break glasses by singing at a high pitch. A wine glass has a *natural frequency* that is determined only by its shape and the speed of sound in glass. When singers match that frequency, each pressure wave (of which there are thousands per second) only adds to the magnitude of the pressure waves within the glass. Eventually, the glass cannot accommodate the stress, and so it breaks.
Someone’s already mentioned a swing, so I’ll give another basic example.
Find a bottle, the bigger the better. Now half fill it with water, turn it on its side, and try to rock the bottle back and forth to create the biggest wave you can. You should notice some things:
* The size of the waves doesn’t affect the frequency of the waves. In other words, the system (the half-filled bottle) has a natural frequency determined that is independent of the amplitude in each wave.
* When the frequency with which you rock the bottle *matches* the natural frequency of the waves, your wave grows bigger and bigger each period.
Boom, that’s resonant frequency. If you rock the bottle at some other frequency, then that dissonance in the frequencies means your motion is taking away energy from the system just as often as it is adding energy, and so your wave doesn’t reliably grow. It’s only when you time your actions in a way that each action is adding energy that your wave grows.
The exact same phenomenon is apparent, say, when singers break glasses by singing at a high pitch. A wine glass has a *natural frequency* that is determined only by its shape and the speed of sound in glass. When singers match that frequency, each pressure wave (of which there are thousands per second) only adds to the magnitude of the pressure waves within the glass. Eventually, the glass cannot accommodate the stress, and so it breaks.
Latest Answers