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


If I tell you: “Now you sit down on a chair so that you will not be able to stand up, though, and will not be bound”, you will accept it, of course, a joke.

Well. Sit the same way a person sits in the figure, i.e., keeping the torso upright and not pushing legs under the chair seat. Now try to stand up, without changing the position of the feet and not nagiba body forward.

In this position it is impossible to rise from his chair.

What cannot? Any effort of the muscles you will not be able to get out of the chair until you get legs under the seat or will not be served by the body forward.

To understand why this is so, we have to talk a little bit about the balance of pH in General and man in particular. Standing subject is not overturned only when the vertical line drawn from the center of gravity falls within the base things. Therefore, the tilting cylinder should tip over, but if he was so wide that the vertical line drawn from its center of gravity, would take place within its grounds, the cylinder is not overturned.

This cylinder should be removed, because the vertical line drawn from the center of gravity passes beyond the base.

The so-called “leaning tower in Pisa, Bologna, or at least leaning bell tower” in Arkhangelsk not fall, despite his inclination, because the vertical line from their center of gravity outside the base (another secondary reason is that they are recessed into the ground with their foundations).

The leaning bell tower in Arkhangelsk (old pictures).

Somebody does not fall only up until the vertical line of the center of gravity is inside the area bounded by the edges of his feet.

When a man stands, vertical line drawn from the center of gravity, passes inside the area bounded by the feet.

Therefore, it is difficult to stand on one leg; it is even more difficult to stand on the rope: the basis of very little vertical line can easily go beyond it. Have you noticed how strange gait differ old “sea wolves”? Spending your whole life on a rocking ship, where the vertical line from the center of gravity of their body every second can go beyond the space occupied by the feet, sailors develop a habit to walk to the base of their body (i.e., widely spaced legs) captured the largest possible space. This gives the sailors necessary stability on the vibrating deck; naturally, the same habit persists when walking on solid ground. You can give a counter example, when the need to maintain balance determines the beauty of the poses. Did you pay attention to what slender species has a man carrying load on the head? All known graceful sculptures of female figures with a pitcher on her head. Bearing in mind the load, by necessity, to keep the head and torso straight: the slightest deviation threatens to bring the center of gravity (raised in such cases, higher than the standard position) of the circuit substrate, and then the balance figures will be broken. Now back to the experience with the rise of the sitting person. The center of gravity of the torso of a seated person is inside the body, near the spine, 20 centimeters above the level of the navel. Draw vertical line from this point down: it will be held under the chair, behind the feet. And so that man could stand, this line should pass between the feet.

Then rising, we must either be submitted by breast forward, moving it to the center of gravity, or to push the legs back to bring support under the center of gravity. Usually we do it, when standing up from a chair. But if we are not allowed to do neither the one nor the other, to get wonder, as you see on the described experience.

Entertaining physics J. Perelman


System Orphus


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