Scrunch time: The peculiar physics of crumpled paper • Hacker News Board

Re: Scrunch time: The peculiar physics of crumpled paper

#11

Post by jerf » Mon, Jan 09, 2012, 2:37 AM UTC

javajosh wrote: post #5

I understand how guessing at the particular pattern of a crumple is a hard physics problem. What I do not understand is why understanding the macro-properties of crumpled paper is a hard physics problem. For example: "Do the balls absorb vibrations by trapping pressure waves or by dissipating them? Nobody knows, says Menon, 'but it means there are still plenty of beautiful problems to keep me interested'." This seems…

"What I do not understand is why [several things]..."

"... these research questions appear laughable."

How about before declaring these questions laughable, you try a little harder to understand why they are interesting? For instance, the fact that we don't have computers powerful enough to simply simulate the question being asked provides one clue of a possible reason why this is more interesting than you think.

Either one of "I don't understand this research" or "this research is laughable" I might accept from a given person, but the combination is unpleasant.

Re: Scrunch time: The peculiar physics of crumpled paper

#12

Post by javajosh » Mon, Jan 09, 2012, 5:11 PM UTC

jerf wrote: post #11

javajosh wrote: post #5

I understand how guessing at the particular pattern of a crumple is a hard physics problem. What I do not understand is why understanding the macro-properties of crumpled paper is a hard physics problem. For example: "Do the balls absorb vibrations by trapping pressure waves or by dissipating them? Nobody knows, says Menon, 'but it means there are still plenty of beautiful problems to keep me interested'." This seems…

"What I do not understand is why [several things]..." "... these research questions appear laughable." How about before declaring these questions laughable, you try a little harder to understand why they are interesting? For instance, the fact that we don't have computers powerful enough to simply simulate the question being asked provides one clue of a possible reason why this is more interesting than you think. Eit…

I have a degree in physics, so I guess it was a mistake to leave out the (two) punchlines and leave them implied:

1. "absorb vibrations by trapping pressure waves" is a meaningless concept in physics. Sponges, rubber, styrofoam, springs and crumpled paper all "absorb vibration" in the same way - converting mechanical energy into heat. Apart from a rather pedantic system in which somehow air pressure is used to add angular velocity to a flywheel, there is no sense in which vibrations are "stored". There is also the case of an irreversible deformation, as when a car crumples. And in that case, the object would become monotonically less capable of absorbing energy - a curve that would be easy to measure (place crumpled paper in a vice, repeatedly attempt to squeeze a bit, measure maximum and minimum circumference, repeat, graph)

2. Thermodynamically, any excess energy is going into heat. Period. This could be tested by very carefully measuring the temperature of the crumpled paper before and after being deformed several times, as with the vice above. Eventually (and with paper, sooner rather than later) there's going to be microscopic breakdown at the fiber and even molecular levels, and that increase in entropy must be factored in, too. (Now that is an interesting experimental challenge: how to measure the "looseness" of paper fiber in a crumpled sheet. There are some easy destructive ways but I cannot think of any non-destructive ones.)

So yes, I call shenanigans on this professor and, apart with the (already admittedly) interesting problem of describing the geometry of particular crumples, I think his research is laughable. Although I would enjoy being shown how I am in error, if that be the case.

Re: Scrunch time: The peculiar physics of crumpled paper

#13

Post by jerf » Mon, Jan 09, 2012, 10:18 PM UTC

javajosh wrote: post #12

jerf wrote: post #11

Earlier quoted context omitted.

"What I do not understand is why [several things]..." "... these research questions appear laughable." How about before declaring these questions laughable, you try a little harder to understand why they are interesting? For instance, the fact that we don't have computers powerful enough to simply simulate the question being asked provides one clue of a possible reason why this is more interesting than you think. Eit…

I have a degree in physics, so I guess it was a mistake to leave out the (two) punchlines and leave them implied: 1. "absorb vibrations by trapping pressure waves" is a meaningless concept in physics. Sponges, rubber, styrofoam, springs and crumpled paper all "absorb vibration" in the same way - converting mechanical energy into heat. Apart from a rather pedantic system in which somehow air pressure is used to add an…

"I have a degree in physics,"

... in which case you really ought to know better than to accept a popular press summation of somebody's research as accurate.

Re: Scrunch time: The peculiar physics of crumpled paper

#14

Post by noblethrasher » Mon, Jan 09, 2012, 10:29 PM UTC

srean wrote: post #10

noblethrasher wrote: post #7

Earlier quoted context omitted.

That's actually an illustration of the Contraction Mapping Principle. Another way of illustrating it is that if you're in city X and place a map of city X on the ground, then exactly one point on the map will be above the corresponding physical point.

Contraction mapping is not really required for this phenomenon. Contraction would mean that any two points are closer after the crumpling/mapping than before. In fact contraction will guarantee that there is only one such point which lies over the other. For the aforesaid property any form of crumpling that does not tear the paper would suffice. Provided no part of the crumpled paper extends beyond the boundary of th…

Thanks!