Phone Books Impossible to Separate

In the past, the series “MythBusters” from Discovery Channel addressed very interesting topics. In the episode called Phone Book Friction, a couple of followers sent Jamie and Adam a video showing how difficult it can be to separate two phone books whose pages have been neatly interleaved. Can they withstand the combined effort of two war tanks? An experiment you can also do at home (minus the war tanks, of course).

Myth or reality? That is the question that, at the beginning of each new show, Jamie Hyneman and Adam Savage, the hosts of “MythBusters”, the interesting Discovery Channel program, asked themselves. The wildest ideas, the strangest myths, were attacked with a rigorous method that mixed equal parts intelligence and brute force. In the episode Phone Book Friction, the myth was posed that says it is impossible to separate the pages of two phone books after interleaving their pages one with another.

Imagine placing two phone books of about 800 or 1000 pages face to face and patiently turning their pages one by one, first the right and then the left, so that the pages become interleaved with each other. It can take a while to finish the job but, at the end, you will have the two books strongly joined. One might think that if you pull on the spines of both directories at the same time, a bit of effort would be enough to separate them. However, the myth says that this task is practically impossible to achieve.

What keeps both pieces together is, undoubtedly, friction. As any student knows, the friction force between two surfaces in contact is the one that opposes the movement of one over the other (friction dynamic) or the force that opposes the start of movement (force of friction static). This phenomenon occurs due to the microscopic imperfections that the surfaces in contact possess.

In the case of our phone books, the effect is multiplied by the number of pages, which, being so large, acquires truly high values. Furthermore, friction depends to a large extent on the force with which one surface presses on the other, and given the particular arrangement that the interleaved pages adopt, this force increases when pulling on them. In some way, they behave like those knots that tighten when you try to pull on the rope that forms them.

The idea is to measure how much force is needed to overcome the friction originated between both directories. To do that, Jamie and Adam drill both spines and attach wooden handles that allow a rope to be passed through to pull comfortably. The first attempt, with both characters pulling as if it were a tug-of-war, with the arrangement of directories in the middle of both, fails spectacularly. Apparently, a force of more than 100 kilograms is not enough to overcome the resistance originated by the microscopic imperfections existing in the interleaved pages.

Going a step further, other team members join the challenge. But even 20 people pulling cannot separate them. At this point, one can appreciate the true magnitude of the forces at play. When 20 people pull on the pages, the applied force is much more than 1000 kilograms. However, the paper arrangement resists stoically.

How much more force can they withstand? For the next attempt, the Mythbusters use two rental cars to -by brute force- separate the books again. But they discover that pulling with two cars -adding between them a force of 2160 kilograms- they only manage to burn the tires without the paper yielding a millimeter.

At this point, more than one amateur investigator would give up. But Hyneman and Savage are true professionals, so they decide to go a step further. After making a couple of phone calls, they get a collector of war tanks (yes, there are people who collect things of that price and size without breaking a sweat) to lend them a pair of robust, strong and ugly armored vehicles. As the effort is going to be -at least- significant, they replace the plywood handles with a pair of solid metal plates. Once screwed to the phone books, they are joined by chains to the rear of each tank. The last test is about to begin.

The tanks can exert enormous force on the paper. However, for a few seconds the books remain united. It takes the dynamometer placed on one of the chains to indicate more than 3600 kilograms of force before the friction force between the pages of the books is overcome.

Something apparently as weak as paper can transform into something very different when a large quantity is used. The almost negligible friction force existing between two simple and modest cellulose pages becomes a powerful bond capable of stopping two cars. Do you dare to do these tests at home? We assure you that your friends will be impressed.