Why can’t you separate interleaved books?

The harder you pull, the greater the friction force resisting separation.

Friction has a long scientific pedigree. Leonardo da Vinci established that the friction force, or traction, 𝒯 applied when an object begins to slide is given by the simple formula 𝒯=μN, where N is the load, or normal force, and μ is the coefficient of friction. Leonardo had suggested that μ=¼ always, but Guillaume Amontons and Charles Augustin Coulomb later showed that μ is not universal; rather, it depends on the nature of the sliding object and the surface on which it slides. In total, the classic Amontons–Coulomb laws state that the friction force during sliding is independent of both the area of contact and the sliding velocity and that it is proportional to the load ...

Take two phone books, interleave their sheets, and attempt to separate the books by pulling on their spines. You cannot do it. The accumulated friction between the pages is so great that even the weight of a car cannot pull the phone books apart.

Less spectacular than lifting a car, but more instructive, is a pair of experiments you can try with two perfect-bound notebooks (that is, having rigid spines) whose sheets can be easily removed. Interleave the sheets and you will find that the traction force needed to separate the two books is immense. Next, remove every other sheet in each notebook and repeat the experiment. In that case you will have no trouble separating the notebooks ...

Excerpted from Physics Today.