Monday, June 29, 2015

Resonance in Plastics and Metals

The Reddit website has a popular subreddit ELI5, short for "explain like I'm 5", where people can post question that they would to have answered, but answered to someone without the technical background and jargon that is usually used. It doesn't mean to literally explain it as if you were talking to a five-year old (five-year olds wouldn't be asking most of these questions anyway), I take it as if you are talking to the general public who wants to know more.

Yesterday the question was posted as to why when an object is struck, metal objects ring louder than plastic ones. Here's my explanation, albeit it is nowhere near appropriate for ELI5:
A object rings because it has acquired energy in a way that it resonates - it vibrates at a frequency and with enough energy to generate sound waves. As long as the object has sufficient energy, it will continue to ring. It cannot ring forever as the sound waves gradually reduce the amount of energy that the object has. But generating sound waves not the only way that an object can lose energy, One of the ways that plastics differ from metals is that plastics are better able than metals to dissipate energy internally. Any plastic object will show some amount of viscosity dampening since all polymeric materials have a non-zero loss modulus curve. Metals can also have internal dissipation mechanisms, but they are far less prevalent than in plastics.
Another example of this same phenomenon is seen in springs. Both metal and plastics springs exist. Given weight considerations alone, plastic springs should be everywhere. But springs are used because they store energy and return it. The same internal dissipation mechanisms that quiet ringing plastics also result in energy loss when the spring is compressed. A pogo stick with a plastic spring would be no fun at all, and speaking from personal experience, a plastic slinky is nowhere near the fun that a metal one is.

Previous Years

June 29, 2011 - BPA Followup (2/2)

June 29, 2010 - Tapes in Space

June 29, 2010 - Pretzel Logic from the Supreme Court


Anonymous said...

But if this is so, why are so many artificially enhanced women so 'dingy?'

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