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Suppose I have a normal A4 paper sheet and I bend the paper in two. My question is:

Why the paper now have this line and why the paper didn't lose naturally this line?

My conjecture: I was thinking that the paper is a isotropic (relatively) material and then the atoms do not have a proper direction. So when I bend the paper I put work in to it so that some preference direction of the atoms is created. And one couldn't undo this new constraint, only if you insert more (an huge amount of) work, such as another process of creating a new paper or burning the paper.

Is this correct?

sammy gerbil
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    Please note that creating new paper or burning the paper is not undoing of the constraint. Other than that it's an interesting question. – Helen Apr 03 '17 at 18:05
  • https://www.reddit.com/r/askscience/comments/329nqi/what_happens_at_a_molecular_level_when_you_fold_a/ I am not vouching for the accuracy of this, but it seems very reasonable to me. The other explanation I could think of is that you're realigning the fibres in weird ways, but that doesn't explain why it can't be undone. – JMac Apr 03 '17 at 18:27
  • There is a nice discussion on it @JMac –  Apr 03 '17 at 18:47
  • @JMac Your link appears to be broken. – sammy gerbil Apr 03 '17 at 18:49
  • @sammygerbil I can click on it fine and it follows... odd. – JMac Apr 03 '17 at 18:53
  • @JMac I was getting "something broken at our end" but it is ok now. – sammy gerbil Apr 03 '17 at 18:57
  • See also https://www.reddit.com/r/askscience/comments/1278cn/why_does_paper_stay_folded_once_you_fold_or/ and https://www.reddit.com/r/askscience/comments/2dsfxq/what_happens_to_a_piece_of_paper_when_its_being/ and https://www.reddit.com/r/explainlikeimfive/comments/36uihr/eli5_why_is_it_easy_to_crease_paper_but_almost/ – sammy gerbil Apr 03 '17 at 18:59

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