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Friction between the sheets

Interleaved phone directories

Physics helps to explain why it is so hard to pull apart two interleaved phone directories. (CC  BY SA Mark Longair)

By Michael Banks

Ever tried – and duly failed – to pull apart two interleaved phone books? Well, a team of researchers from France and Canada, led by Héctor Alarcón of the University Paris-Sud, has now studied why this is such an impossible task.

The difficulty in seperating them is caused by the friction between the sheets, which is amplified by the huge number of pages in a directory. However, friction only occurs when two surfaces are pushed together and researchers were not sure how exactly this happens.

Gravity was one explanation, but this can be refuted by holding the directories vertically. By performing experiments and carrying out computer modelling, Alarcón and colleagues conclude that the pulling itself generates a force that brings the sheets together – so the harder you pull, the more tightly the pages bind (arXiv:1508.03290).

They also found that a relatively small increase in the number of pages has a dramatic effect on the pulling force necessary to separate them, so that a 10-fold increase in pages creates a four orders of magnitude increase in the force.

A late entry for an IgNobel, perhaps?

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  1. M. Asghar

    The angle θ that the paper-sheets of the interleaved telephone books make in the traction zone of the traction force, has to be “negative” relative to this zone to allow a projected component of this force to push the sheets down as an additional weight and to increse the resulting friction. If θ is positve, the resultant friction should decrease.


    I have wonderered about how tissue composed of numerous microlayers keeps together aside from vanderwalls effects it is possible this effect could cause much difference between jello n tissue


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