# Phone book friction

The Mythbusters look at how hard it is to separate two phone books whose pages have been interwoven together so that it is just paper-on-paper friction that binds them.

1. wtfwhateverd00d says

What’s a phone book old-timer?

2. trucreep says

Seems like it’s the same sort of strength that comes from a rope or something. Individual strands are weak, but weave them together and they’re stronger than steel.

3. rq says

They should try a smaller phonebook.

4. wtfwhateverd00d says

While I am not disputing this, I think it maybe more of how the test was set up then is explained by friction alone.

Friction is proportional to the normal force, but here that normal force is not the gravity of a two sheet of paper, or if the phonebooks are each n pages long, not the 2n x friction of two sheets of paper.

The normal force for any page x is the weight of the phone book above x, or (n – x). So to find the total friction you have to integrate over the force for any one sheet depending on where it is in the stack.

5. invivoMark says

Nope, it works even if the phone books are held vertically.

6. invivoMark says

I tried this once. I took two small-ish phone books, set them on a table in my apartment, and every time I walked by I would flip a page from each. I had three major observations from this:

1) It is difficult to keep the book spines parallel to each other. Just a little bit of skew can add up to make a really awkward arrangement of the books and their pages.

2) Even with a small number of pages (~1/4 of a small-sized phone book), it quickly becomes too difficult to pull apart with human strength without shaking or another method of loosening the pages.

3) It is really boring and tedious work to interleave two phone books, and takes too long for anyone but a perversely dedicated individual to do. I gave up before I was halfway through and chucked the books (seriously, who uses phone books these days?).

Part of me wondered if one could make a chain out of phone books, interleaving each half with a different book on either side. But short of a phone-book-interleaving robot, I’m certain that nobody will ever try it.

7. Rob Grigjanis says

Works with ordinary paperbacks, ten or so pages from each at a time, as long as the overlap is at least a couple of inches. Try it!

8. Acolyte of Sagan says

I’ve just tried it with two paperback books of only 60 pages each* and found it impossible to pull them apart.

*A pair from a set of Miller’s A Collector’s Guides which have glossy pages (to enhance the photographic images), so the friction will be considerably less than a phone book or a novel.

9. wtfwhateverd00d says

Again though, phone books held vertically are NOT the same as two sheets of paper since the phonebooks slump and through the slump gravity of a complex volume of the phone book is applied to the sheets of paper.

10. lochaber says

I don’t think gravity has anything to do with it- as trucreep mentioned up above, it’s the same principle that lets us make a strong rope hundreds of feet long out of individual fibers that are only a couple inches long each.

I think much more force would come from a ‘clamping’ type pressure (as opposed to gravity) as the phone books are pulled in opposite directions.

Even without any pulling, there is going to be a fair amount of compression on the pages, just due to the sheer amount of individual sheets packed into such a small area.

Anyways, kinda cool to watch the video and all the various rigs. I do pity whatever intern had to sit there and interleave a couple pairs of phonebooks. 🙁