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J. Perelman
"Entertaining physics". Book 1.
Chapter 5. Properties of liquids and gases

“BOTTOMLESS” GLASS

You poured water into the glass to the brim. It is full. Near the glass are pins. Maybe for one or two pins will be place in the glass? Try.

Start to throw pins and read them. Throw it with caution: carefully immerse the tip into the water and then gently release the pin from the hand, without shock or pressure to a concussion not to spill water. One, two, three pins fell to the bottom of the water level remained unchanged. Ten, twenty, thirty pins... the Liquid is poured. Fifty, sixty, seventy... a hundred pins lies at the bottom, and the water from the glass is still not translated.


An amazing experience with the pins in a glass of water.

Not only translates, but never went any appreciable way over the edges. Continue adding pins. The second, third, fourth hundred pins into the vessel and a single drop flowed over the edge; but now you can see how the surface of the water swelled, rising slightly over the edges of the glass. This swelling all the explanation of the strange phenomenon. Water little wets the glass, if it is a bit dirty fat; the edges of glass - like all used our dishes is inevitably covered with traces of fat from the touch of your fingers. Without moistening the edges, the water displaced by the pins from the glass forms a bulge. Swelling slightly at the eyes, but if you give yourself the trouble to calculate the volume of one of the pins and compare it with the displacement of the bulge, which is slightly swollen over the edges of the glass, you will see that the first volume is hundreds of times less than the second, and therefore in the “full” glass may be the place for several hundred pins. The wider the dishes, the more pins it can hold, because the more the amount of swelling.

Will make for clarity rough estimate. The length of the pins is approximately 25 mm, its thickness is half-millimetre. The volume of this cylinder is easy to calculate from the known formula geometry ( π*d2 *h/4), it is equal to 5 cubic mm with cylinder volume pins shall not exceed 5.5 cu. mm.

Now let's calculate the volume of the water layer, towering over the edges of the glass. The diameter of the glass 9 cm = 90 mm, the Area of this circle is equal to about 6400 sq. mm. Considering that the thickness of the raised layer of only 1 mm, for its volume 6400 cubic mm; this is more than the amount of pins 1200 times. In other words, “full” glass of water can take more than a thousand pins! Indeed, carefully lowering pins, you can immerse them in a thousand, so that to the eye they seem to take the whole vessel and will even act on its edges, and the water still will not leak.

Entertaining physics J. Perelman

 




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