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Phone Book Swing

Two phonebooks can hold thousands of pounds. How is this possible?  A rigorous weight lifting regimen? Nope.

Steroids? Never.

Friction? YES!

Friction is the force created when objects move—or try to move—across each other. The force of friction can be small, like sliding a hockey puck across the ice, or it can be huge, like trying to drag a couch (complete with your napping Uncle Bob) over a shag carpet.

There are two types of friction: kinetic (moving) and non-moving (static). The phone books holding up the swing are demonstrating STATIC friction, because they are not moving.

Friction depends on a couple key factors:

  1. How rough or slippery the touching surfaces are (a slippery banana peel on a slick linoleum floor versus rusty barbells on top of some really rough sand paper).
  2. How much weight is on each of the surfaces (a very small and slippery banana peel on a slick linoleum floor versus several hundred pounds of rusty barbells on a really rough piece of sand paper).

But, isn’t the paper in a phonebook smooth and more slippery than rough? Not really. Look at a photo of a piece of paper through a microscope, and you will see that the surface of a piece of paper is actually bumpy and irregular. Put two pieces of phonebook paper together and this roughness will give you a small frictional force. Squeeze thousands of pages against each other and you get an enormous frictional force…capable of holding several thousand pounds.

 

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