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Name: Christina
Status: student
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I am attempting to do a Grade 12 physics project and chose the topic of Phone book Friction as shown in an episode of MythBusters. The situation is that the pages of 2 phone books are interwoven and produces an almost inseparable book. The MythBusters crew found that 8000 lbs of force was necessary to pull the books apart. I am trying to find an answer as to why the phone books are inseparable, focusing of course on friction. Could someone give me an answer as to why this is? Preferably with some calculations to back up the explanation.

I will give you the basis of an equation; you can work on the calculation from it. But you will see that the force to separate telephone books interlaced as you describe will be considerable.

A good empirical model for friction is that its magnitude is given by


Where (mu) is a dimensionless coefficient that is a property of the surfaces in contact, and N is the "normal" force pushing them together. In the case of your telephone books, the relevant quantities would be a mu for phone book paper against phone book paper, and the normal force would probably be best modeled as the weight of phone book above the page. So each page would have a different N, depending on what fraction of the phone books are on top of it.

That force (mu)N in particular is probably not very large. However, there are many pages in a phone book, and each one has two sides. So the force required to separate the phone books would not be the frictional drag on one side of a page, but the SUM of the drag forces on BOTH sides of ALL the pages of a book!

I believe you will see that for even small values of mu and N, the friction force to overcome is quite large when a book has hundreds of pages!

Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming

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