Why do soldiers march disorderly while crossing a bridge?

Analysis of the Question

This question tests your understanding of Resonance in Physics. It asks why a specific behavior (marching disorderly) is required when soldiers cross a bridge.

Concept Explanation: Resonance

Resonance occurs when an external force or a vibrating system forces another system to oscillate with greater amplitude at a specific frequency, known as its natural frequency.

Every physical structure, including a bridge, has a natural frequency at which it vibrates. If a rhythmic force (like soldiers marching in perfect unison) matches the natural frequency of the bridge, the bridge will begin to vibrate with increasing amplitude. If these vibrations become too large, the structural integrity of the bridge can be compromised, potentially leading to a collapse.

Step-by-Step Solution

1. The Problem: When soldiers march in step, they create a periodic, rhythmic force on the bridge. If the frequency of their steps matches the natural frequency of the bridge, the bridge enters a state of resonance.
2. The Effect: During resonance, the bridge undergoes large oscillations (vibrations). These vibrations exert immense stress on the materials of the bridge.
3. The Solution: By marching "disorderly" (breaking the rhythm or step), the soldiers ensure that their footsteps do not have a single, consistent frequency. This prevents the buildup of resonant vibrations, keeping the bridge stable and safe.
4. Evaluating the Options: • A: This is correct. Breaking the rhythm prevents the frequency of the footsteps from matching the natural frequency of the bridge. • B: This is incorrect. Setting the bridge into resonance is exactly what they want to avoid. • C: This is incorrect. Soldiers aim to cross the bridge safely, not destroy it. • D: This is incorrect. Marching disorderly does not change the total weight or the distribution of weight; it only changes the frequency of the force applied.

Conclusion

Exam Tip: Whenever you see a question about soldiers on a bridge or a car vibrating at a certain speed, think Resonance. The goal is always to avoid matching the "natural frequency" of the object.