Pressure versus Force: Phone Book Tug-of-War
[Shopping List: an assortment of phone books, magazines and catalogs of various sizes and thicknesses]
- Using two thin magazines, or even two stacks of loose pages, interleave several pages between the two and demonstrate that it is very easy to slide them apart.
- Using two thicker books, interleave many more pages, then try to pull the two books apart. Now it should be much more difficult.
- Try increasing the number of interleaved pages until it is impossible to pull the books apart.
What's Happening: When you slide one page across another, there is a force of friction between the two pages which resists your pull force, but it is very small. Nonetheless, every pair of pages you interleave adds the same small amount to the total frictional force, so if you interleave enough pages (and it doesn't take as many as you might think), soon the total frictional force is much greater than any two people can provide to pull them apart. This also illustrates the difference between force and pressure, which is the force per unit area. In this case the force with which the two people are pulling may be very large, but the total area of all the pages is also very large, so the force per pair of pages is much smaller than the force of friction between each pair of pages. This is like saying that the pressure the two people are providing to pull the books apart is very small. The strength of Velcro can be explained the same way. This concept also explains why a person can easily lie on a bed of nails. He may weigh 200 pounds, but if there are 2000 nails on the bed, then each nail is only supporting 200/2000 pounds, or less than 2 ounces of weight, so even though it may be fairly sharp, 2 ounces of force is not enough to puncture his skin.
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