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J. Perelman
"Entertaining physics". Book 2.
Chapter 2. Power. Work. Friction


It is hard to imagine how sailboats to go "against the wind" - or, in the words of the sailors, to go "to reach". However, the sailor will tell you that right against the wind to go sailing impossible, and can only move at an acute angle to the direction of the wind. But this angle is small - about a quarter of a right angle, and it seems, perhaps, equally incomprehensible to go right against the wind or at an angle thereto at 22°.

In fact it is not, however, indifferent, and we now explain how wind power to meet him at a slight angle. First consider how valid the wind on the sail, i.e., where he pushes the sail when the wind blows on it. You probably think that the wind pushes the sail in the direction in which he blows. But it is not so: wherever the wind blew, he pushes the sail perpendicular to the plane of the sail. Indeed: let the wind blows in the direction of the arrows in the figure below, line AB represents the sail.

The wind pushes the sail is always at right angles to its plane.

As the wind presses evenly on the entire surface of the sails, we replace the wind pressure force R applied to the middle of the sail. This force decompose into two: force Q, perpendicular to the sail, and a force P directed along it (see Fig. above, right). The last power anywhere but pushes the sail, as the friction of the wind on the canvas slightly. Remains a force Q, which pushes the sail at right angles to it.

Knowing this, we can easily understand, how can a sailing vessel going at a sharp angle towards the wind. Let the line CC represents the keel line of the vessel.

How can I go on the sails against the wind.

The wind is blowing at an acute angle to the line in the direction indicated next to the arrows. Line AB represents the sail; it is placed so that the plane he was divided in half the angle between the direction of the keel and wind direction. Make sure the figure for the decomposition of forces. The pressure of the wind on the sail we depicted the power of Q, which, we know, must be perpendicular to the sail. The power of this place in two: force R, perpendicular to the keel, and the force 'sdirected forward along the keel line of the vessel. As the boat moves in the direction R meets strong resistance to water (Kiel in sailing vessels is very deep), the force R is almost completely balanced by the resistance of the water. Only left with the power of S, which, as you can see, directed forward and, consequently, moves the vessel at an angle, as if against the wind. [You can prove that the power of S receives the greatest value when the plane sails divides in half the angle between the directions of the keel and the wind.]. Usually this movement is performed by zigzags, as shown in the figure below. In the language of sailors such movement of the ship is called "tacking" in the narrow sense of the word.

Tacking sailing ship.

Entertaining physics J. Perelman


System Orphus


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