Every object has natural frequencies of oscillation with which they will respond when deflected from equilibrium. The marching soldiers appear to have been marching at a pace such that their stomping boots drove the oscillation of the bridge at _________.
Pushing an oscillating system at the same rate as it responds is like pushing a child on a swing at just the right time to drive the oscillations_________. If the soldiers break step, the total driving force_________ and will not drive the bridge into large-amplitude oscillations.