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

THE BERNOULLI'S PRINCIPLE AND ITS CONSEQUENCES

The principle was first expressed by Daniel Bernoulli in 1726, says: in the stream of water or air pressure is high, if the speed is low, and the pressure is small, if the speed is high. There are known limitations to this principle, but here we will not dwell on them here.

Figure 1 illustrates this principle.


Fig. 1. Illustration of Bernoulli's principle. In the narrowed part (and) tube AB pressure less than wide (b).

Air is blown through the tube AB. If the cross-section of the tube is small, as in and, - the speed of the air is great; however, where the cross section is large, as in b, the velocity is small. Where speed is high, the pressure is small, and where the speed is low, the pressure is great. Because of the low pressures of air and liquid in the tube To rise; at the same time, strong air pressure in b causes lowered the liquid in the tube D.

In figure 2, the tube T mounted on a copper disk DD; the air is blown through the tube T and then by free disk dd*. The air between the two disks has great speed, but this speed decreases rapidly as it approaches the edges of the disks, because the cross-section of the air flow increases rapidly and overcome the inertia of the air flowing from the space between the discs. But the pressure of the surrounding disk of air is large, since the speed is low and the air pressure between the disks is low, because the speed is great. Therefore, the air surrounding the disk has a greater impact on the disks, seeking to draw them closer than the air flow between the discs, tending to move them apart; as a result, the disk dd sticks to the disk DD is stronger, the stronger the current of air in T.


Fig. 2. Experience with discs.

Fig.3 presents the analogy of Fig. 2, but only with water. Fast moving water on disk DD is at a low level, and she rises to a higher level of calm water in the pool when wrapping around the edge of the disc. Therefore, the water is calm under the drive has a higher pressure than moving water above the disk, causing the disk rises. Rod R. prevents lateral displacement of the disk.


Fig. 3. Disk DD is lifted by the rod Rwhen it pours a jet of water from the tank..

Fig. 4 depicts a light bulb floating in the air jet. The air jet hits the ball and not allow him to fall. When the ball jumps out of the jet and surrounding air returns it back into the stream as the ambient air pressure, low speed, large, and the air pressure in the jet, with more speed, a little.


Fig. 4. The ball is supported by the air jet.

Fig. 5 represents two ships, moving close in calm water, or, what amounts to the same, two of the ship, standing and flowing water. The flow is more restricted in the space between the ships and the water velocity in this space more than on both sides of the vessels. Therefore, the water pressure between vessels less than on both sides of the vessels; the higher the water pressure surrounding the court, brings them closer. Sailors know very well that the two ships coming near, are strongly attracted to each other.


Fig. 5. Two ships moving in parallel, as would attract each other.

A more serious case can occur when one vehicle follows another, as shown in Fig. 6.


Fig. 6. When the forward movement of vessels vessel turns its nose to the vessel A.

Two forces F and F, which bring together the ships will try to rotate them, and ship In turns to L with considerable force. The collision in this case is almost inevitable, because the wheel does not have time to change the direction of the ship.

The phenomenon described in connection with Fig. 5, it is possible to demonstrate, blowing air between the two light rubber balls, suspended, as indicated in Fig. 7.


Fig. 7. If two light bulbs blow out the air, they are closer to the ground.

If between them to blow out the air, they converge and clash each other.

* The same experiment can be done easier using coil and a paper circle. To the circle did not slide off to the side, his punch pin, passing in the channel coil.

Entertaining physics J. Perelman

 




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