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Question
Explain why are soldiers asked to walk out of step while crossing bridges?
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Solution
When the soldiers walk in step, they produce some fixed frequency. If this frequency corresponds to the natural frequency of the bridge, the resonance can take place. This, in turn, will make the bridge to vibrate violently, and hence make it collapse. To avoid such a situation, the soldiers are asked not to march in step.
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In the diagram below, A, B, C, D are four pendulums suspended from the same elastic string PQ. The lengths of A and C are equal to each other, while the length of pendulum B is smaller than that of D. Pendulum A is set into a mode of vibrations.
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| P | Q | R |
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