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Name: Christina
Status: student
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Question:
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.



Replies:
Hi Christina,

You may be aware that with simple surface-on-surface static friction, there are just two parameters you need to know: the normal force (the force holding the two surfaces together) and the coefficient of static friction (which describes how much force it takes to get the two surfaces moving relative to each other).

In the case of two phone books together, you might imagine the total force to pull the books apart to be the sum of all the individual page-to-page static forces. With this model, the key is going to be how tightly you hold the two sets of pages together, as the force to pull them apart would related to the force pressing them together. You could measure the static coefficient of friction between two pages, and then calculate an *average* normal force based on the number of pages. It would be very difficult to accurately measure the force between each page, which is why I suggest the average method.

For example, if you inter-leaf the two books, but do not hold them together, you can gently and easily slide the two books apart. At some point, if you press the two sets of pages together tightly enough, the pages (or book bindings) will rip rather than the pages sliding. I did not see that show -- did the books tear, or did they pages slide apart without ripping? If they did not slide, then all you can say is that the static frictional force for sliding was larger than the strength of other components of the books -- you would not be able to estimate the actual force, and therefore not estimate the average normal force between the pages.

Hope this helps,

Burr Zimmerman


A simple calculation to begin with :
Work out the area of one page of the phone book and then multiply it by the number of interleaving pages to work out the total area of contact between the two books. ( Do not forget to take into account the fact that BOTH sides of the page are in contact - but you cannot count BOTH books that way. ) When you see the total area of contact between the two you probably will no longer be surprised at the figure of 8000lbs force. In fact when you finally work out the force per square inch you will be surprised at how small it is

Nigel Skelton
AUSTRALIA



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