

Phone Book Friction
Name: Christina
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
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 surfaceonsurface 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
pagetopage 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 interleaf 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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Update: June 2012

