text: what happens when soldiers march on the bridge? i heard that bridges collapse 'cause of that.why!!??? every object has a definite frequency with which it can oscillate. when the soldiers march they march with a definite frequency. if it so happens that this frequency of marching is same as that of the intrinsic fequency of the bridge then the bridge picks up the motion of the marching soldiers, ie.e it starts oscillating and eventually breaks. this is called resonance. so generally soldiers are asked to break their steps when crossing a bridge.