# Mythbusters phone book friction worksheet

### MythBusters

Leave the bottom half on the table. Now look at the card and set the deck back together. For the trick, push the side of the top half of the deck. I set up a simple experiment to test the question if there are more forces at work than static friction. The experiment was one notebook, the clamping notebook, sat along the edge of a table and another notebook, hanging notebook, hung from it by a sheet sandwiched near the bottom of the clamp.

## Phone Book Friction - MythBusters | Discovery

The clamp was secured by its spiral using some pens and tape. I can go over the physics, if you like, but I had calculated it would take the whole weight of the clamping notebook to hold the hanging notebook in place. There appeared to be a force equal to that of static friction assuming a standard coefficient of static friction for paper that was unaccounted for.

I think what best explains this is whatever explains the anecdote of the card trick.

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There must be some kind of air pressure missing from the air being squeezed out between the pages. I noticed it a great deal in the experiment, the first time the hanging notebook slipped out there was 16 pages pressing down on it. It stood stationary a few seconds before falling. Lower amounts of pages also held but only for a few seconds. It had a consistent delay, even when only one sheet and the cover clamped on it, very little weight, after pressing down there was a several second delay while air was drawn in before the notebook slipped outright.

There seems to be some claim that 1. By looking at several sources online I found the coefficient of friction seemed to be close to 0. This is important because I calculated a coefficient of friction from my tests of 0. Here is one of the sources: This seemed to agree with a search, for example Fuji printers says it is between 0. An average of 0.

Again, in my experiment there is no "binding", both literally and figuratively, so there is an additional force, other than static friction, that nearly doubles the holding power. Static friction and the normal force most certainly do play key roles here. The thickness of the doubled-up phone books where the pages interleave is twice as thick as the individual phone books at the binding. Tension results in a compression at the middle that results in a good-sized normal force, and hence a good-sized static friction force. Another and perhaps more important factor that comes into play is that paper is a somewhat unique substance.

Apply tension to a rubber band or steel rod and they stretch and get thinner.

## Phone Book Friction

Most substances work this way; they have a positive Poisson's ratio. Paper doesn't act like this. Apply tension to paper and it gets shorter and fatter. Paper is one of those rare auxetic materials with a negative Poisson's ratio. This is why this trick wouldn't work so well with phone books made of rubber or steel. The doubled-up layer will get thinner and thinner with increasing tension, and eventually those rubber or steel phone book pages will simply slip apart.

With paper, the doubled-up layer gets thicker and thicker with increasing tension. This makes the normal force and hence friction increase with increased tension. Finally, phone book paper is amongst the cheapest of papers. It is not made to last long, and it is not particularly well-made.

Phone book paper is not uniform, from sheet to sheet, and even within a sheet. These non-uniformities result in different responses to tension from sheet to sheet and across a single sheet. The result of this non-uniformity is a huge number of dovetail-like joints that make the grip exceedingly strong. Friction over a massive surface.

Usually, when you're dealing with friction over a large surface, the normal force is distributed over that surface. Here, the normal forces are only distributed over the surface of a single page, but you have hundreds of pages in parallel each subject to the same normal force. Thank you for your interest in this question.

Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site the association bonus does not count. Would you like to answer one of these unanswered questions instead? Home Questions Tags Users Unanswered. Pulling apart two interleaved phone books Ask Question.

They ended up needing two tanks to pull the phone books apart against the resistance of friction: I found the following online discussion from , the same year in which the episode aired: Can anyone give a good analysis?

Ben Crowell Ben Crowell I would guess it's partly because the paper is bound normal to the spine so there is a bending force created when you interleave the pages. Also because the pages are at an angle, when you pull on the spines there is a component of the force you apply normal to the page surfaces.

Perhaps the large normal force results from a vacuum being created between the pages? Quick order of magnitude calculation: This is too large by 2 orders of magnitude, but that's for a perfect vacuum. Even if the actual vacuum between the pages is a small fraction of this, the result wouldn't be surprising? Where for a block of stacked papers, individually they can be easily torn apart, but here we don't have any special shape that gives us an advantage in distributing the pressure, but maybe instead here a strong cohesion lies between the surfaces of each pair of paper. But I think John's approach is more sound here.

Perhaps it's related to the kind of paper phonebooks are made of? Perhaps it's related to how thin phonebook pages are? At HowStuffWorks, Josh and Chuck use a company-wide time capsule initiative to settle an old bet on time travel, getting physicist Michio Kaku tangled in the scheme. In the podcast, the guys cover time paradoxes and causal loops. Trust No One.

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## Physicists Solve the Mystery of Interleaved Phone Books

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