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


Why on a simple stool to sit rigidly, while on the chair, also made of wood, not tough? Why gently lay rope hammock, gossip from pretty solid laces? Why not hard to lie on a wire grid, arranged in beds instead of a spring mattress?

It is easy to guess. The simple seat stool flat; our body comes into contact with them only on a small surface and focuses the brunt of the trunk. At the same chair seat is concave; it is in contact with the body for the most surface; this surface and distributed the weight of the torso: per unit surface area have less load, less pressure.

So, it has to do more uniform pressure distribution. When we bask out of a soft bed, there are formed recesses corresponding to the irregularities of our body. Pressure is distributed here on the lower surface of the body pretty evenly, so that every square inch is only a few grams. It is not surprising that in these circumstances, we feel good.

It is easy to Express this difference in numbers. The surface of the body of an adult is about 2 square meters, or 20,000 square, see for example, that when we lie in bed, come in touch with it, relying on it, about 1/4 of the whole surface of our body, i.e. 0,5 sqm, or 5000 square see the Weight of our body is about 60 kg (average), or 60000, So every square inch is only 12, but When we're lying on the bare boards, in contact with the reference plane only in a few small areas, with a total area of some hundred square centimeters. On each square centimeter is, therefore, the pressure in the pound, and not a dozen grams. The difference is noticeable, and we immediately feel it on his body, saying that we “very hard”.

But even on the very hard bed we can be not at all hard, if the pressure is distributed evenly over a large surface. Imagine that you lay down on a soft clay and it etched the shape of your body. Leaving clay, leave it to dry (drying up, clay “sits” on 5-10%, but let's assume that this is not the case). When it will become hard as stone, retaining left your body a depression, lie down on her again, filling the stone form. You will feel as soft jacket without feeling rigidity, although literally lying on the stone. You become like the legendary Leviathan, about which we read in the poem Lomonosov:

On sharp stones is reclined
And the hardness thereof despise,
For the strength of the great powers,
Considering them for soft silt.

But the reason for our insensitivity rigidity of the bed will not “fortress of great strength, and the distribution of body weight on a very large bearing surface.

Entertaining physics J. Perelman


System Orphus


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