We have all marveled at the astonishing feat of being able to hold up massive amounts of weight with only phonebooks. Mythbusters even went so far as to use tanks to try to pull apar the phone books. Since then, however, the origin of this phenomenon has remained very elusive and escaped full description.

It was first thought that the mechanism is due to the friction between interleaving pages. A new paper by Hector Alacron and colleagues¹ finds that this explaination is incorrect and offer a more intuitive and more predictive explaination. Alacron and his colleagues posited that the extreme amounts of traction force come from an orthogonal component of force that is due to the non-zero angles that the phonebook pages make as the sheets are pushed to the outside. That is, the strength of this system is then completely due to the operator that is pulling the pages apart - as they exert more force, the pages on the outside push in and thus make it hard to pull out. It is, most simply, a chinese finger trap.

It’s important to correct ourselves when we mispeak and to re-interpret theories when new evidence presents itself. The previous explaination of the phonebooks ammends itself to this. The previous assertion and accepted answer was that the increase in the number of pages increased the frictional force. While this is true, the researchers noticed that friction cannot explain the strength because the gravitational forces that can apply orthogonal forces do not change when the books are flipped to their side (where the friction is changed).

Enthusiasts of physics should care, as now their is better theory and experiment to support a better hypothesis about the interleaved phonebooks. Popular culture enthusiasts might care too, to learn that Mythbusters maybe got it wrong.

References

1. Alarcon, Hector, et al. “The enigma of the two interleaved phonebooks.” arXiv preprint arXiv:1508.03290 (2015).