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J. Perelman
"Entertaining physics". Book 2.
Chapter 2. Power. Work. Friction


On index fingers outstretched arms lay smooth stick, as shown in the figure.

Experience with a ruler. Right-to - end experience.

Now move the fingers toward each other until they converge closely. A strange thing! Turns out that in this final position the stick is not tipped, and retains the balance. You do experience a lot of times, changing the initial position of the fingers, but the result is invariably the same: the stick is balanced. Replacing the stick drawing with a ruler, a stick with a knob, billiard cue, sex brush, you will notice the same feature.

The clue to the unexpected finale?

First of all, clear the following: once the stick is balanced on the fixed fingers, it is clear that the fingers come together under the center of gravity of the stick (the body remains in equilibrium, if a vertical line drawn from the center of gravity is inside the boundaries of the support).

When the fingers are moved apart, a large load is placed on the finger, which is closer to the center of gravity of the stick. With pressure growing and friction: the finger closer to the center of gravity feels more friction than the remote. So close to the center of gravity of the finger does not slide under the stick; always moves only one finger, which is farther from this point. As soon as the moving finger will be closer to the center of gravity than the other, the fingers change their roles; this exchange is performed several times until the fingers will not agree closely. And as it moves each time only one of the fingers, the one that is farther from the center of gravity, it is natural that in the end position, the two fingers meet at the center of gravity of the stick.

Before this experience to end, repeat it with sexual brush and put ourselves this question; if you cut the brush in the place where it is built with your fingers, and put both parts on different weighing pan, which Cup will pull with a stick or with a brush?

The same experience with sexual brush. Why the scale is not in balance?

Seemingly once both parts of the brush they complemented one another on the fingers, they should be balanced and cups of scales. In fact, the Cup with a brush drag and drop. About the cause guess what, if you take into account that, when the brush was balanced on the fingers, the weight of both parts were applied to unequal arms of the lever; if the same weights, the same forces applied to the ends of the equal armed lever.

For the "Pavilion of entertaining science" in Leningrad Park of culture I have ordered a set of sticks with different position of the center of gravity; the stick was raznilis on usually two unequal parts just in that place where he was the center of gravity. Putting these parts on the scales, visitors were surprised to find that the short part of the heavier long.

Entertaining physics J. Perelman


System Orphus


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