planet language... pulp friction
Name a few smooth surfaces. Paper is smooth, isn't it? It is not uneven like a carpet or cobblestone pavement.
But how smooth is paper really? And can two books be stronger than two people?
what you need:
Two large, thick paperback text books - the closer they are in size the better
A strong volunteer
what you do:
1. Lock the books together. You can do this as if you were riffle-shuffling a pack of cards (but much slower!). Alternatively flick through about 20 pages of one book and flick about 20 pages of the second book on top.
2. The deeper the books are inserted into each other the better the grip.
3. Now ask your strong volunteer to pull in the spine of one of the books while you pull on the spine of the other.
4. To take the books apart, you'll need to give them a shake and ease them apart.
what's going on:
Paper isn't quite as smooth as it seems. We can write on it because there is friction between a pencil and the paper - the friction rubs off some of the pencil's graphite.
If you hold two pieces of paper together with flat palms and try to rub them together you get an idea of the amount of friction between just two pieces of paper. The dips and grooves in the surface of one piece of paper lock into the dips and grooves of the other.
Each time a page from one of the books comes into contact with a page from the other book there will be friction when they are pulled in opposite directions. Multiply this friction by the number of pages in contact, and the amount of friction increases enormously.
The spine, and the fact that you are squeezing the books to get a good grip, just pushes the dips and grooves in the interleaved pages of the books harder into each other. The result is books that can't be pulled apart.
The only way to release them is the lift the pages apart - and you can do this by giving the books a bit of a shake.
special safety advice:
Don't pull so hard that either one of you falls back. Also be careful if the books are important: the friction can hold them together very strongly and you could rip the spine off before getting them apart.
pdf version
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