phonebooksNote:  This is the part of a series of posts about science activities to be done in the bathroom – the lav-oratory if you will.  How you ask?  Well I’m sure most of you have a couple of magazines laying around in the bathroom. Skip to the end for something to do while you’re in there for a while…

I just started a new job doing STEM activities with kids at a roller rink. What could be more fun?!

Well, I’ll tell ya – friction! One of the sessions focuses on different types of friction. Friction plays a crucial role in the development of roller skates as well as the means with which you choose to stop. In the process I revived the conundrum of phone book friction.

According to legend, if you interweave the pages of two phone books it is nearly impossible to pull them apart. MythBusters discovered that it takes at least 8,000 pounds of force to separate the books – not an insubstantial amount!

So what is happening here?

It turns out that for the longest time, physicists just weren’t really sure. I found several discussion boards about the problem here and here but no consensus on how this trick works.

First, let’s look at the simplest case of friction – a box on the floor. To push the box you first need to use enough force to overcome static friction and then keep using enough force to overcome kinetic or sliding friction. The force you need to start something moving is usually more than that to keep it moving. In any case, each of these depend on just two things:

  • The force between the two objects. In this case, the weight of the box. Heavier boxes will have more friction.
  • The properties of the two materials in contact. In general (but not always) smoother surfaces have less friction than rougher surfaces. It will be easier to slide the box on a smooth wood floor than on a rougher shag carpet.

In the phone book conundrum, physicists couldn’t figure out how the force between the two objects (called the normal force) could cause such an insurmountable friction force since this could be no more than the weight of the two books. There was some speculation about the increased surface area with all those pages trying to slide against each other but no one could agree how. Some even guessed that a vacuum was formed inside the books.

Last month, a paper was released by French physicists, that takes on the problem by creating a mathematical model to identify where all of this friction is coming from.

You can read the paper for yourself but a simple explanation is that the pages don’t lay exactly flat. This slight bending of the sheets causes a huge force on the middle sheets that increases exponentially as you add more pages. In other words, the angle of the pages amplifies the amount of total friction between the two books.

 When you weave the pages of the book together they spread out at an angle from the spine but when you pull on the spine of the book they squeeze back together on the pages of the other book and dramatically increase the friction.   This is similar to the way a knot tightens when you pull the ends of the rope or the Chinese finger trap that tightens when you pull your fingers outward.

But here is the best part! To remove this amplification of friction is simple! Just tear out every other page of the book before weaving them together. This gives room for the pages to lay flat and you can easily pull them apart!

TRY IT!

You can do this a bit more easily (and quickly) with two notebooks or magazines instead of phonebooks.

First interweave the pages of the notebook. How many is the minimum number of pages for the books to stick together? If you don’t want to experiment, I found that an 85-page magazine is more than sufficient.

Pull with all your might and you will not be able to pull them apart!

Flip open the pages to separate the two books.

Now tear out every other page of both books. This might take a while. You might want to watch an episode of MythBusters while you do this…

Again, interweave the pages of the notebook. You will notice that this time the pages lay flat in the spots where you tore out pages.

Pull on the two notebooks and they will easily slide apart!

Eureka! Fiction Friction
Tagged on: