Q13CQ - College Physics (Urone)

Question

Why are soldiers in general ordered to “route step” (walk out of step) across a bridge?

Step-by-Step Solution

Answer

The soldier’s rhythmic marching can create mechanical resonance with the natural frequency of the bridge and might result in collapsing of the bridge. Hence, soldiers are in general ordered to “route step”.

Step 1: Understanding mechanical resonance

Any mechanical objects or charged particles tend to vibrate at a particular frequency. This frequency is called the natural frequency of the object. When light or sound waves oscillating at a particular frequency strikes the object, it interacts with the object’s vibration. If that frequency happens to match the natural frequency of the object, then the phenomenon of mechanical resonance takes place.

During resonance, the mechanical object responds at a greater frequency. Vibration at higher amplitudes may cause violent catastrophic failures and destructions in mechanical structures.

Step 2: Understanding how to bridge gets affected by soldiers marching steps

Bridges have their own natural frequency of vibration. If soldiers march in unison across the bridge, they apply a force in the bridge which generates a wave with a particular frequency. If that frequency matches the bridge’s natural frequency, it will amplify the bridge’s vibrating amplitude.

 If mechanical resonance is strong enough the bridge may vibrate until it collapses from the movement. That is why Soldiers are ordered to “route step” across a bridge to avoid creating mechanical resonance.