#### Mechanical Resonance

All objects have a "natural frequency" defined their molecular or atomic structure, physical properties and chemical composition..

If an object is subjected to vibrations from a nearby source that correspond to its natural frequency, it will start to oscillate or vibrate by itself at the frequency of the nearby source.  The internal structure of the receiving object are sympathetic/resonant with the frequency of the vibrations of the nearby source.

For example, Enrico Caruso, an italian opera singer is credited with being able to shatter fine crystal glass simply by reaching certain frequencies of sound with his powerful tenor's voice.  Other singers have achieved this too under special circumstances.

Their voices at certain frequencies and sound levels (typically over 100 dBs) will become resonant with  the internal structure of the the glass .  The frequency of the voice sets the atoms/molecules of the crystal in resonant motion (vibrations).  The vibrations exceed the strength of the bonds of the internal structure of the glass to a point where the entire structure collapses and the glass shatters.

Bridges and buildings have to be designed and built in such a way to withstand the impact of high winds.  High winds may in fact set these structures in resonant frequency and cause them to collapse. Architects and engineers use materials and designs to avoid that these structures could be "in resonance" with the frequencies of high winds and other sources of vibrations such as eartquakes.  A classic example of this is the collapse of the Tachoma Narrows Bridge in the 1940's.

The video below is courtesy of the Prelinger Archives: https://archive.org/details/BridgeLoop

You can see how high winds set this bridge in sympathetic vibrations causing its entire structure to eventually collapse.

#### Lab Exercise:

In the lab you can easily reproduce resonance using tuning forks that are sympathetic.

You strike one tuning fork and place it near a second tuning fork that has the same resonant frequency.

The second tuning fork will start to vibrate on its own.

Here is a YouTube Video that demonstartes the above: