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Mystery #2Tier 2-3forces resonance bridges

Why are marching soldiers ordered to fall out of step when crossing a bridge?

Soldiers are trained to march in perfect step, every boot striking the ground as one. But there is one place their officer orders them to stop and walk any old way they like: a bridge. Break step, comes the command. Why would keeping in time suddenly be dangerous on a bridge?

An old photograph of a long column of soldiers marching across a wooden bridge over a river, with a town behind.
Photo: Unknown photographer · Public domain · via Wikimedia Commons

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Why might marching in perfect time be risky on a bridge?

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