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Assuming you have a flat poster with no curvature, why is it that when you pin it to the wall (with thumbtacks) it gains curvature as seen in the picture below. When I put the poster up it was entirely flat to the wall with no curvature, but over time it somehow curls and develops this sort of curvature (it looks like positive curvature at the corner).

enter image description here

Is there a physics-based explanation for why this happens?

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Looks like it just stretched out under its weight (or the upper mounting sagged/tore), and the curve is only because it was constrained by the lower pins. – Kevin Reid Nov 12 '12 at 19:15
@KevinReid: I have observed this happening with posters my entire life and there is no problem to do with the pins coming loose or the poster tearing. Can you explain what you mean by "stretched out under its weight"? – Samuel Reid Nov 12 '12 at 19:17
up vote 4 down vote accepted

I don't think the weight of paper will be enough to significantly deform it, although the top pins cutting through the paper and leading to sagging seems a more likely explanation.

Another potential culprit is humidity. Paper is a very hygroscopic material, and this turns it into not very stable dimensionally. If your poster is coming from an industrial printing process, it probably has been treated to have a very low moisture content. Over time, it will absorb water and expand, and will do so in an anysothropical manner.

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+1: Humidity would be my guess. – John Rennie Nov 13 '12 at 9:39

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