The suggestion that two interleaved phone books cannot be pulled apart is more complex than it seems. It is an old barroom trick—making a next-to-impossible task seem easy. The force of friction and the weight of the pages combine, tightly binding the phone books together and making separation impossible for a single person.
The trick is set up by the laying of the pages of two telephone books on top of each other one by one—interleaving them. A volunteer then tries to pull the phone books apart by force alone, holding on to the books' spines. If they are set up properly no human, no matter how strong, is capable of pulling the phone books apart. Even attaching one phone book to a solid object like a wall will not provide enough force to separate the pages.
It might not be obvious, but interleaving the pages drastically increases the weight on each of them. The page at the top of the interleaved phone books has only the weight of gravity acting on it, but the second page has gravity as well as the weight of the top page holding it down. Carry this down to, for example, the thousandth page, and it has the weight of 999 pages pushing down on it in addition to gravity. The bottom pages have the weight of two phone books pushing down on them.
The force of friction acting on the pages is a major reason that pulling the books apart is so difficult. The fact that the pages of two phone books are interleaved means that friction as well as weight is preventing them from being separated. If there are 999 pages in total in contact with each other, the friction between the pages is equal to 500 times the friction between one book and a table.
The Discovery Channel's Mythbusters recently conducted an experiment to find out how much force is required to separate two interleaved phone books. They used vehicles to exert 8,000 pounds of force on the phone books before they separated. In theory, two cars could hang from interleaved phone books without their being pulled apart.
Try It Yourself
This experiment is easy to replicate if you have spare time. Simply lay the pages of two phone books on top of each other one by one before attempting to pull them apart. Just hope you don't need to look up a phone number in a hurry.
tug-of-war image by Pavel Losevsky from Fotolia.com