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J. Perelman
"Entertaining physics". Book 1.
Chapter 2. Gravity and weight. A lever. Pressure

WHY POINTY OBJECTS PRICKLY?

Have you thought over the question: why the needle so easily penetrates the subject thoroughly? Why cloth or cardboard can easily be punctured with a fine needle and difficult to break blunt nail? In both cases, there is, seemingly, the same power.

The power is the same, but the pressure is still not the same. In the first case, all the power is concentrated at the tip; the second, the same force is distributed over a larger area of the end of the nail; therefore, the pressure of the needle is much greater than the pressure of a blunt rod with the same force our hands.

Everyone will say that harrow with 20 teeth deeper will loosen the soil, than that harrow the same weight, but with 60 teeth. Why? Because the load on each tooth in the first case, more than in the second.

When it comes to pressure, it is always necessary, but the power to take into account also the area to which this force acts. When we say that someone gets 1000 rubles salary, we don't know yet, a lot or a little: you need to know - a year or a month? Similarly, the force depends on whether it is distributed on a square centimeter or focuses on the hundredth fraction of a square millimeter.

Man skiing goes through soft snow, and without skis falls. Why? Because in the first case, the pressure of his body is spread over a much larger area than in the second. If the surface of the ski, for example, 20 times more surface soles of our feet, the skis we push the snow is 20 times weaker than standing on the snow in front of your feet. Loose snow survives the first pressure, but can not withstand the second.

For the same reason, the horses running in the swamp, tie up a special “shoes” to the hooves, to increase the bearing area of the foot and to reduce pressure on the marshy ground: the legs of horses not get stuck in the swamp. So do people in some swampy areas.

On thin ice, people move on all fours, to distribute the weight over a larger area.

Finally, a characteristic of tanks and tracked tractors not tie in loose soil, despite its considerable weight, is again explained by the distribution of weight on a large surface. Tracked vehicle weighing more than 8 tons has 1 square cm of the soil pressure is not more than 600, From this point of view interesting car on tracks for the carriage of goods in the swamps. This truck is loaded with 2 tons of cargo, has a ground pressure of just 160 g per 1 square cm; because of this, he goes well on peat bog and marshy or sandy areas.

In this case, a large area of support as best technically, as a small area in the event of a needle.

From the above it is clear that the tip pierces only due to the insignificance of the area over which the distributed force. Quite for the same reason, a sharp knife is better cuts than blunt: the force is concentrated on a smaller space.

So, pointed objects because good hack and cut on their edges and the blades focuses large pressure.

Entertaining physics J. Perelman

 




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