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Asked Dec 23, 2019
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Why is it a sensible procedure for soldiers to break step when marching over a bridge?

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While soldiers are passing over a suspension bridge are always asked to break their steps because if the frequency of their march coincides with the natural frequency of bridge then at each step of the force is in phase with the suspension bridge. Hence successive impulses given by the soldiersto the bridge are added up and increase the amplitude of oscillation continuously. But with the increase of amplitude, the air resistance and internal friction also increase so that the loss of energy from the bridge also increases. Finally, astage is reached when the energy supplied by the soldiers becomes equal to the energy lost by the bridge.This stage of the equilibrium is reached a slightly earlier the amplitude becomes very large. If there were no loss of energy, the amplitude would have become infinity and ultimately bridge will collapse. To avoid such happening there is a sensible procedure for the soldiers to break the steps while marching over a bridge.

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