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krrazypassions's avatar

Why does a plastic bag make crunching sounds when we crush it? What energy is getting converted to noise here?

Asked by krrazypassions (1337points) May 13th, 2011

Are some molecular bonds breaking to give out such a noise? And why don’t we see tiny light sparks (scintillations) instead of the crunching noise? it would look nice :) The noise can be very irritating at times!

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13 Answers

cazzie's avatar

It’s not molecular. It is more likely the stiffness of the material and its resistance to being folded. If you take a plastic cup and squeeze it in your hand, you will hear a cracking sound and it may split in parts. It is the stiffness of the material and resistance to being crushed that create the sound and damage. If you want to induce change on a molecular level of a plastic, you can heat it.

mattbrowne's avatar

I’m not sure about your first question. Crushing requires kinetic energy to overcome the repulsive electromagnetic force. Like when using a hammer to get a nail into a wall. The electrons of the wall don’t like the approaching electrons from the nail.

cazzie's avatar

I agree, @mattbrowne. The transfer of kinetic energy creates the sound. The material being moved transfers the motion into sound through its movement. The stiffness of the material creates the specific sound. If you hit a drum, it returns with the sound of a beat. It is not based on a ยจ‘molecular change’ but simply structural. A motion, or vibration that moves through the material. Yes?

mattbrowne's avatar

Yes, @cazzie, that makes sense, but what about the details. Sound is a mechanical wave and the oscillating material is air. How exactly does the repulsion from the stiff bag molecules push the nitrogen molecules?

Some weeks ago I saw a documentary in which scientists were investigating the mechanism behind the sound of ocean waves. It’s considered a non-trivial problem and scientists had been struggling for years obviously. Somehow it all boils down to the creation and destruction of small air bubbles. Maybe the “bag sound” problem is similar.

cazzie's avatar

@mattbrowne the dynamics of a liquid, like water, in motion is a different kettle of fish from a stiff plastic sheet.

mattbrowne's avatar

@cazzie – Yes, but it’s the combination of the liquid (ocean) and air. The dynamics of liquids alone cannot explain the typical ocean sound.

Stinley's avatar

If you change a baby’s disposable nappy/diaper in the dark, as you rip off the sticky tabs at the side there is a noise, of course, but also little green flashes of light. Maybe bags do give off light when crushed, has anyone looked?

cazzie's avatar

@mattbrowne my point is that the water dynamics with the air is different from the sheet of plastic and the air, that’s all.

RealEyesRealizeRealLies's avatar

Your eardrum is transducing the vibrational wave into what your brain perceives as sound. The sound wouldn’t exist without your eardrum to convert the energy into one. Don’t blame the bag. Blame your brain and the sensory input equipment it controls.

krrazypassions's avatar

@RealEyesRealizeRealLies lol! by that approach, we shall be describing everything that we experience in terms of how our brain interprets it. Who knows?- we might be merely floating brains in empty space receiving electric impulses of a very high voltage from somewhere and we are simply interpreting those electric impulses as the world we live in and the laws according to which it runs.
However, if we consider our normal approach of looking at the world as a system outside our brains and we ourselves as one of the ingredients of the system, it seems to help us as we have habituated (evolved) to live and think like that :)

cazzie's avatar

I’m almost sure that the plastic bag sound comes from it’s structural resistance to being folded. There is a resistance to the kenetic energy being forced upon it and when it does fold, the built up energy is released with a quick motion that results in a snapping sound… much like breaking a piece of dried spaghetti noodle. It’s the transfer of kinetic energy.

RealEyesRealizeRealLies's avatar

I love this question @krrazypassions, so don’t think I’m just arguing for arguing’s sake. It’s questions like these that force us to reconsider what reality actually is.

Now I can’t comment on brain in a vat scenarios based on maybes. But as a person with a brain inside the system, I can vouch that earplugs have a direct affect upon the energy which my eardrums transduce into sound. That’s why I can hear the music of the snake charmer, but the snake is actually charmed by his movements rather than the tune he plays.

suzanna28's avatar

ha intereesting question.. I like deep thinkers :)

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