why soldiers go out of step while crossing a bridge?
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Every object "prefers" to vibrate at a certain frequency when it absorbs energy.
For a bridge this preferential frequency depends both on the materials used for its construction as well as the type of bridge it is -suspension, cantilever etc.-
Every time, a person walks over the bridge, he/she transfers a portion of energy from every step to the bridge. This causes the bridge to absorb the energy and vibrate. Of course for a single person the energy imparted to and absorbed by the bridge is very very small. Consequently, the amplitude - the maximum displacement from the resting position - of vibrations is negligible.
The important factor to note here is that there are 2 frequencies at play here.
The frequency at which the person is transferring energy and the frequency at which the bridge is vibrating.
If these two frequencies are different, part of the energy that is transferred from each step actually "fights" or "cancels out" the vibration of the bridge, keeping its amplitude low.
For a bridge this preferential frequency depends both on the materials used for its construction as well as the type of bridge it is -suspension, cantilever etc.-
Every time, a person walks over the bridge, he/she transfers a portion of energy from every step to the bridge. This causes the bridge to absorb the energy and vibrate. Of course for a single person the energy imparted to and absorbed by the bridge is very very small. Consequently, the amplitude - the maximum displacement from the resting position - of vibrations is negligible.
The important factor to note here is that there are 2 frequencies at play here.
The frequency at which the person is transferring energy and the frequency at which the bridge is vibrating.
If these two frequencies are different, part of the energy that is transferred from each step actually "fights" or "cancels out" the vibration of the bridge, keeping its amplitude low.
Answered by
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That’s basically because of energy propagation and the harmonic oscillator principle.
In this case the energy released by the soldier's feet propagates through the bridge and since they are all marching at the same frequency, the bridge absorbs all that energy and it can become a really big harmonic oscillator.
That means the bridge would vibrate like a really big guitar string, and if the soldiers kept marching at the same frequency, the vibration would increase until a theoretical point where it the bridge would collapse.
This happened once in the Angers Bridge incident, in 1850. Now a days, however, it would take a really big (and fragile) bridge and a really big army to put the bridge down that way, since some of that energy is lost during propagation.
Mythbusters tested this "myth", just google Mythbusters Breakstep Bridge and you'll have a really good explanation
In this case the energy released by the soldier's feet propagates through the bridge and since they are all marching at the same frequency, the bridge absorbs all that energy and it can become a really big harmonic oscillator.
That means the bridge would vibrate like a really big guitar string, and if the soldiers kept marching at the same frequency, the vibration would increase until a theoretical point where it the bridge would collapse.
This happened once in the Angers Bridge incident, in 1850. Now a days, however, it would take a really big (and fragile) bridge and a really big army to put the bridge down that way, since some of that energy is lost during propagation.
Mythbusters tested this "myth", just google Mythbusters Breakstep Bridge and you'll have a really good explanation
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