How To Cross A Bridge - Answer

How To Cross A Bridge - Answer = B

Yes, it does make a difference. When troopers march in cadence across a bridge, the marching may match a natural resonance frequency of the bridge.  Although only a small amount of energy is added with each step, because of the resonance effect this energy will be stored. As a result, the bridge will cumulatively absorb energy from the marching men, increasing the oscillation amplitude in the bridge (just as pushing someone on a swing, in cadence, increase the amplitude of the swing). Enough energy may be added this way to damage or destroy the bridge. In fact, a concrete suspension bridge, the Tacoma Narrows Bridge, was destroyed by resonance vibrations set up by winds acting on the bridge. Today's bridges are constructed to prevent or minimize this type of wind resonance effects. (Some information from The Flying Circus of Physics by Jearl Walker)

Return to question