Resonance in a Hanging Book
To know the reason why soldiers are asked to break steps while crossing a bridge.
Resonance is an example of forced oscillation. If the frequency of the external force is equal to the natural frequency of the object than amplitude of the oscillations becomes very high. This phenomenon is called resonance.
a heavy book suspended by a thread.
Blow air from your mouth on the hanging book. The book will start swing with a little amplitude. Now, start blowing air on the book every time it starts receding away from you i.e., blow air in phase with the swinging book.
The amplitude goes on increasing because you are blowing or applying force on the book in phase with the oscillation of the book. If you blow air when the book comes towards you, the amplitude will decrease.
Every object has its natural frequency of vibration depending on the nature of the material. Hence, if frequency of the soldiers steps becomes equal to the natural frequency of the bridge, then like the swinging book, vibrations of large amplitude may be setup in the bridge and it may collapse. This is the reason why soldiers are asked to break steps while crossing a bridge.