When an outside force with the same frequency as the natural frequency of the object causes the object to vibrate, this action is called resonance, or sympathetic vibration. Resonance can be produced by a small force applied at the right frequency, such as walking in step across a bridge or pumping a swing. When a force close to the natural frequency of an object is applied, the amplitude of the object's vibration can increase substantially. A pendulum is a weight hung from a rod or string from a fixed point in a way that allows the weight to swing back and forth. Tapping the pendulum as it moves backward cancels the motion of the pendulum, and it eventually stops swinging.
Soldiers marching out of step cancel the vibration of a bridge.
Soldiers purposely march out of step when crossing a bridge. As a troop of soldiers walk across a bridge, their movement causes the bridge to vibrate. If the soldiers are in step and if the frequency of their step is the same as the natural vibration of the bridge, the bridge could collapse. If the amplitude of the vibrations becomes high enough, the bridge can collapse. To prevent this, soldiers purposely march out of step when crossing a bridge.