Astonishingly, it turns out to be practically impossible to separate two interleaved phone books by pulling on their spines, however much force is applied. It is even possible to suspend a car from them.
Using a model that reproduces the traction and friction forces involved, researchers at the Laboratoire de Physique des Solides (CNRS/Université Paris-Sud), Laboratoire Gulliver (CNRS/ESPCI ParisTech), Laboratoire de Génie des Procédés Papetiers (CNRS/Grenoble INP) and McMaster University in Canada have shown that when the spines of the interleaved phonebooks are pulled on vertically, part of the vertical force is converted into a horizontal force that presses on the sheets. The pages then remain stuck together due to friction. The work, which began as a result of a challenge on the program On n’est pas que des cobayes on the France 5 TV channel, will be published in 7 January 2016 in the journal Physical Review Letters, and is already available on ArXiv.