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J. Perelman
"Entertaining physics". Book 2.
Chapter 6. Properties of liquids and gases


In the autumn of 1912, the ocean steamer "Olympic" - then one of the greatest ships in the world - has been following the case. Olympic sailed in the open sea, and nearly parallel to it, at a distance of hundreds of meters, passed with great speed another ship, much less armored cruiser "Gauk". When both vessels have been in the position depicted in the figure, something unexpected happened: a smaller vessel rapidly turned from the path, as if obeying some invisible force, turned his nose to the big ship and not obeying steering, moved almost directly at him. The collision occurred. "Gauk ran nose to the side of "Olympic"; the blow was so strong that "Gauk" made in the Board of "Olympic" a big hole.

The position of the steamers "Olympic" and "Gauk" before the collision.

When this strange case was considered in the marine court, the guilty party was voted captain of the giant "Olympic"because, said the court, he did not give any instructions to give way to reaching across the "Gauck".

The court did not find here, therefore, nothing extraordinary: simple preparability captain, not more. Meanwhile, there was a completely unexpected: a case of mutual attraction of ships at sea.

Such cases happened more than once, possibly before with the parallel motion of the two ships. But not yet built a very large ships, it is not manifested with such force. When the waters of the oceans began to " surf "the floating city", the phenomenon of attraction vessels became much more noticeable; with him are the commanders of military courts while maneuvering.

Numerous accidents small vessels sailing in the vicinity of large passenger and military courts, was, probably for the same reason.

What accounts for this attraction? Of course, there can be no question of attraction the law of universal gravitation Newton; we have already seenthat this attraction is too little. The cause of the phenomena of a different kind and explained by the laws of the flow of fluids in pipes and channels. One can prove that if the liquid flows through the channel with contraction and expansion, in the narrow parts of the channel it flows faster and pushes against the walls of the channel is weaker than in the broad places where it runs quieter and pushes against the walls stronger (so-called "Bernoulli's principle").

In the narrow parts of the channel the water flows faster and pushes against the walls weaker than wide.

The same is true for gases. This phenomenon in the theory of gases is called the effect clément - Desarme (on behalf of open physicists) and often referred to as the "state of hydrostatic paradox". For the first time this phenomenon, as they say, was accidentally discovered in the following circumstances. In one of the French mines worker was ordered to close the shield hole of the outer tunnel, through which is supplied to the mine compressed air. Work has long struggled with a jet of air, but suddenly the shield itself slammed the tunnel with such force that if the shield is not big enough, would have drawn him in a ventilation hatch with a terrified worker.

Incidentally, this feature of the current of gases due to the action of the sprinklers. When we are involved in the knee andending of the contraction, the air passing into the narrowing reduces its pressure. Thus, over the tube b is provided with air at a reduced pressure, and because the atmospheric pressure pushes the liquid out of the Cup up the tube; the holes the liquid enters the stream of exhaust air and sprayed.

The sprinklers.

Now we will understand what is the reason of attraction of the courts. When two vessels are sailing parallel to one another between their flanges turns out as a water channel. In ordinary channel wall still and moving water; on the contrary: the motionless water, and moving walls. But the force of this does not change: in the narrow places of the rolling channel weaker water exerts on the walls than in the space around the ships. In other words, the sides of ships, facing each other, suffer from water less pressure than the outer parts of the vessels.

The flow of water between two floating vessels.

What should happen as a result? The court must under the pressure of the outside water to move toward each other, and of course, that the smaller the ship moves more noticeable, while more massive remains almost motionless. This is why attraction manifests itself with special force when a big ship quickly passes by little.

So, the attraction of ships due to the suction action of flowing water. This also explains the danger of the rapids for swimmers, the suction action of eddies. You can calculate that the water flow in the river at a moderate speed of 1 m per second draws the human body with the force of 30 kg! Against such forces are not easy to resist, especially in the water when the own weight of our body helps us to maintain stability. Finally, the retracting action quickly rushing trains due to the same Bernoulli's principle: the train at a speed of 50 km per hour draws near-human with a strength of around 8 kg

Phenomena associated with the "Bernoulli's principle", though very frequent, little known in the circle of specialists. It will be useful therefore to dwell on it more. Next , we present an excerpt from the article on this subject, placed in one of popular science magazine.

Entertaining physics J. Perelman


System Orphus


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