Why can’t an army stumble across the bridge?
Historically two events have occurred as follows. The first event occurred when Napoleon led the French army to invade Spain. When an army passed the iron suspension bridge, the commanding officer shouted out the command: “One, two, three, four!” Following the voice, the soldiers strode with vital steps steadily up the bridge. As they were about to approach the other shore, a loud roar suddenly occurred; one end of the bridge collapsed into a large river, throwing all soldiers and officers into the water. Many people drowned.
Another event occurred in St. Petersburg, Russia. When the army rhythmically took a step across the large bridge on the “Fontanka” river, the same deadly bridge break happened. In the end, what causes these events?
It is due to resonance. The girder has its vibration frequency. When a large group of people stroked their footsteps steadily through the bridge, the footstep’s cyclic force also had a certain frequency. If the frequency of that force is close to (or equal to) the bridge’s vibration frequency, resonance will occur. As a result of the resonance, the bridge’s vibration gets stronger and stronger; eventually, when it exceeds the bearing capacity of the bridge, it breaks.
In everyday life, the bridge is for people to walk around and for the use of vehicles. The car’s force generated on the bridge is much bigger than the human footsteps, but the car’s force is not cyclical. There are other vehicles and pedestrians on the bridge; the force generated by them also doesn’t have certain rhythms, eliminating some of the vibrations, not making the sphere generate resonance. There’s nothing dangerous at all. Therefore, all countries have the same thing in common: when the army crossing the bridge is not allowed to step in the footsteps.
In life, the phenomenon of resonance often occurs. For example, playing the Ferris wheel must adjust its lower body’s frequency and turn it on for the swing to produce resonance. If so, the more the new Ferris swings higher. When climbing stairs, sometimes climb quickly, sometimes climb slowly. Doing so prevents the ladder from being born resonant by our footsteps. If resonated, the ladder will shake very hard.