Help Me Understand: Why Do Soldiers Out of Step on Bridges?

[emma123]

hi my math teacher ask us this the other day..."in the military, as marching soldiers approach a bridge, the command route step is given. the soldiers then walk out of step with each other as they cross the bridge. explain." i don't understand what she means at all. is it even a physics concept that i don't recognize. please help me .

 

[Lyuokdea]

think of what happens when you stomp on a bridge in terms of forces. Think about the rhythm of marching, and how that could possibly affect a bridge if the timing is right.

The idea is that potentially, the soldiers could accidently march at a speed that is the harmonic frequency of the bridge. If this would happen, the step of many soldiers in time would cause the bridge to vibrate at it's harmonic frequency, and because the frequency is harmonic, the bridge would start to oscillate out of control and possibly break.

Mythbusters did a great episode on this, trying to replicate the effect with piston driven footmen soldiers marching on a harmonic tone of their contstructed bridge, they failed to get the bridge to break up in any harmonic way, although their pistons were a little too powerful and did a great job of breaking the bridge on their own.

However, harmonic frequencies in bridge design cannot be ignored, as they are what led to the Collapse of the Tacoma Narros Bridge in 1940. http://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge A dramatic video of the bridge collapse is available here: http://www.archive.org/details/Pa2096Tacoma

~Lyuokdea