Pulling apart phone directories

A recent paper has analysed the causes of the high frictional force that results from the interleaving of two phone books. It is reported that attempts to separate two interleaved phone books using trucks and military tanks have failed. The difficulty of pulling the books apart arises because the force exerted to separate the pages amplifies the small frictional force arising from the interaction of the pages. The relationship between the size of the frictional force and the number of pages is non-linear. A roughly tenfold increase in the number of pages (from N=12 to N=100) resulted in an increase in the traction force required by four orders of magnitude. The authors of the study conclude that the high magnitude of frictional force arises because the pages of the books do not lie exactly parallel to each other, resulting in loads perpendicular to the paper-paper interfaces.