When I try to tear the newspaper from top to bottom (or bottom to top), it's torn pretty neatly and like a line, but when I try to tear it from side to side, it goes all over the place and tries to lead the tear towards the top or bottom.

Why does this happen?

  • $\begingroup$ Hint: Paper is anisotropic. $\endgroup$
    – Qmechanic
    Jun 12 '17 at 8:44
  • $\begingroup$ similar question on Quora $\endgroup$
    – valerio
    Jun 12 '17 at 8:44
  • $\begingroup$ @Qmechanic Yeah, that's my line of reasoning too. As in we can easily cut wood from top to bottom as opposed to side to side. I'm just not sure of the details so I decided I'd ask here. Should I have included that in the question? $\endgroup$ Jun 12 '17 at 8:46

Newspaper is made out of cellulose fibres (linear unbranched ones) bonded entangled together.

The fibre structure is anisotropic. The orientation of most of the fibres is along the direction of the movement of the machine.

In the direction of this orientation, it is relatively easier to tear a newspaper because it's just a matter of prying two fibres apart. (without significant tearing of fibres)

In the direction perpendicular to this, fibres have to be broken to tear the paper, and this requires a greater force.

The same asymmetry account for why tears are neat/messy depending on the direction you take.

If a piece of paper was isotropic, with random orientation of fibres, tearing would roughly take the same effort in any direction.

  • 3
    $\begingroup$ I wouldn't say that the fibers are "bonded" together. They are rather entangled together. Also, the fibers are absolutely not oriented in one single direction, but rather they have a preferred orientation. $\endgroup$
    – valerio
    Jun 12 '17 at 8:47
  • 1
    $\begingroup$ Yes, I agree. Bonded sends a wrong image. Thanks, added the necessary changes. $\endgroup$ Jun 12 '17 at 8:52
  • 2
    $\begingroup$ Newsprint is produced in large rolls. Other production techniques for paper may make matted fiber, like felt, with only one (the thickness direction) anisotropy. $\endgroup$
    – Whit3rd
    Jun 13 '17 at 5:46

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