Interesting experiments in physics for children. In a deep mine
In school physics lessons, teachers always say that physical phenomena are everywhere in our lives. We just often forget about it. Meanwhile, the amazing is near! Do not think that you will need something supernatural to organize physical experiments at home. And here's some evidence for you ;)
magnetic pencil
What needs to be prepared?
- battery.
- Thick pencil.
- Copper insulated wire with a diameter of 0.2-0.3 mm and a length of several meters (the more the better).
- Scotch.
Conducting experience
Wind the wire tightly turn to turn on the pencil, not reaching its edges by 1 cm. One row is over - wind the other from above in the opposite direction. And so on, until all the wire is finished. Do not forget to leave two ends of the wire 8–10 cm each free. To prevent the turns from unwinding after winding, secure them with tape. Strip the free ends of the wire and connect them to the battery contacts.
What happened?
Got a magnet! Try to bring small iron objects to it - a paper clip, a hairpin. Are attracted!
Lord of the water
What needs to be prepared?
- A stick made of plexiglass (for example, a student's ruler or an ordinary plastic comb).
- A dry cloth made of silk or wool (for example, a wool sweater).
Conducting experience
Open the faucet so that a thin stream of water flows. Rub the stick or comb vigorously on the prepared cloth. Quickly bring the wand close to the stream of water without touching it.
What will happen?
A jet of water will be bent by an arc, being attracted to the stick. Try the same with two sticks and see what happens.
spinning top
What needs to be prepared?
- Paper, needle and eraser.
- A stick and a dry woolen cloth from a previous experience.
Conducting experience
You can manage not only water! Cut a strip of paper 1-2 cm wide and 10-15 cm long, bend along the edges and in the middle, as shown in the figure. Insert the needle with the pointed end into the eraser. Balance the workpiece-top on the needle. Prepare a “magic wand”, rub it on a dry cloth and bring it to one of the ends of the paper strip from the side or top, without touching it.
What will happen?
The strip will swing up and down like a swing, or it will spin like a carousel. And if you can cut a butterfly out of thin paper, then the experience will be even more interesting.
Ice and fire
(the experiment is carried out on a sunny day)
What needs to be prepared?
- A small cup with a round bottom.
- A piece of dry paper.
Conducting experience
Pour into a cup of water and place in the freezer. When the water turns to ice, remove the cup and place it in a bowl of hot water. After a while, the ice will separate from the cup. Now go out to the balcony, put a piece of paper on the stone floor of the balcony. With a piece of ice, focus the sun on a piece of paper.
What will happen?
The paper should be charred, because in the hands it is no longer just ice ... Did you guess that you made a magnifying glass?
Wrong mirror
What needs to be prepared?
- Transparent jar with a tight-fitting lid.
- Mirror.
Conducting experience
Pour excess water into a jar and close the lid to prevent air bubbles from getting inside. Place the jar upside down on a mirror. Now you can look in the mirror.
Zoom in on your face and look inside. There will be a thumbnail. Now start tilting the jar to the side without lifting it from the mirror.
What will happen?
The reflection of your head in the jar, of course, will also tilt until it is turned upside down, while the legs will not be visible. Pick up the jar and the reflection will flip again.
Bubble Cocktail
What needs to be prepared?
- A glass of strong salt solution.
- Battery from a flashlight.
- Two pieces of copper wire about 10 cm long.
- Fine sandpaper.
Conducting experience
Clean the ends of the wire with fine sandpaper. Connect one end of the wires to each pole of the battery. Dip the free ends of the wires into a glass of solution.
What happened?
Bubbles will rise near the lowered ends of the wire.
Lemon battery
What needs to be prepared?
- Lemon, thoroughly washed and wiped dry.
- Two pieces of insulated copper wire approximately 0.2–0.5 mm thick and 10 cm long.
- Steel paper clip.
- Bulb from a flashlight.
Conducting experience
Strip the opposite ends of both wires at a distance of 2-3 cm. Insert a paper clip into the lemon, screw the end of one of the wires to it. Insert the end of the second wire into the lemon 1-1.5 cm from the paper clip. To do this, first pierce the lemon in this place with a needle. Take the two free ends of the wires and attach the bulbs to the contacts.
What will happen?
The lamp will light up!
The high salinity of the Dead Sea determines one of its features: the water of this sea is much heavier than ordinary sea water. It is impossible to drown in such a heavy liquid: the human body is lighter than it.
The weight of our body is noticeably less than the weight of an equal volume of thickly salty water and, therefore, according to the law of swimming, a person cannot drown in the Dead Sea; it floats in it, as a chicken egg floats in salt water (which sinks in fresh water)
The humorist Mark Twain, who visited this lake-sea, describes with comical detail the extraordinary sensations that he and his companions experienced while swimming in the heavy waters of the Dead Sea:
“It was a fun swim! We couldn't drown. Here you can stretch out on the water at full length, lying on your back and folding your arms over your chest, with most of the body remaining above the water. At the same time, you can completely raise your head ... You can lie very comfortably on your back, raising the colonies to your chin and clasping them with your hands - but you will soon turn over, as your head outweighs. You can stand on your head - and from the middle of the chest to the end of the legs you will remain out of the water, but you will not be able to maintain this position for a long time. You cannot swim on your back, moving any noticeably, because your legs stick out of the water and you have to push off only with your heels. If you are swimming face down, then you are not moving forward, but backward. The horse is so unstable that it can neither swim nor stand in the Dead Sea - it immediately lies on its side.
On fig. 49 you see a man quite comfortably perched on the surface of the Dead Sea; the large specific gravity of the water allows him to read a book in this position, protecting himself with an umbrella from the burning rays of the sun.
The water of Kara-Bogaz-Gol (the bay of the Caspian Sea) and the no less salty water of Lake Elton, containing 27% salts, have the same extraordinary properties.
Something of this kind is experienced by those patients who take salt baths. If the salinity of the water is very high, as, for example, in the Staraya Russian mineral waters, then the patient has to make a lot of efforts to stay at the bottom of the bath. I heard a woman treated in Staraya Russa complain indignantly that the water "positively pushed her out of the bath." It seems that she was inclined to blame not the law of Archimedes, but the administration of the resort ...
Figure 49. A man on the surface of the Dead Sea (from a photograph).
Figure 50. Load line on board the ship. Brand designations are made at the waterline level. For clarity, they are also shown separately in an enlarged form. The meaning of the letters is explained in the text.
The degree of salinity of water in different seas varies somewhat, and, accordingly, ships do not sit equally deep in sea water. Perhaps some of the readers happened to see on board the ship near the waterline the so-called "Lloyd's mark" - a sign showing the level of limiting waterlines in water of various densities. For example, shown in Fig. 50 load line means the level of the limiting waterline:
in fresh water (Fresch Water) ............................... FW
in the Indian Ocean (India Summer) ....................... IS
in salt water in summer (Summer) .......................... S
in salt water in winter (Winter) ............................ W
all in. Atlant. ocean in winter (Winter North Atlantik) .. WNA
We have introduced these grades as mandatory since 1909. Let us note in conclusion that there is a variety of water, which in its pure form, without any impurities, is noticeably heavier than ordinary; its specific gravity is 1.1, i.e., 10% more than ordinary; consequently, in a pool of such water, a person who could not even swim could hardly drown. Such water was called "heavy" water; its chemical formula is D2O (the hydrogen in its composition consists of atoms, twice as heavy as ordinary hydrogen atoms, and is denoted by the letter D). "Heavy" water is dissolved in an insignificant amount in ordinary water: in a bucket of drinking water it contains about 8 g.
Heavy water of the D2O composition (there may be seventeen varieties of heavy water of different composition) is currently being extracted almost in its pure form; the admixture of ordinary water is about 0.05%.
How does an icebreaker work?
When taking a bath, do not miss the opportunity to do the following experiment. Before leaving the tub, open the outlet while still lying on the bottom. As more and more of your body begins to emerge above the water, you will feel a gradual weight on it. At the same time, you will be convinced in the most obvious way that the weight lost by the body in the water (remember how light you felt in the bath!), Reappears as soon as the body is out of the water.When a whale involuntarily makes such an experiment, finding itself aground at low tide, the consequences are fatal for the animal: it will be crushed by its own monstrous weight. No wonder whales live in the water element: the buoyant force of the liquid saves them from the disastrous effect of gravity.
The foregoing is closely related to the title of this article. The work of the icebreaker is based on the same physical phenomenon: the part of the ship taken out of the water ceases to be balanced by the buoyant action of the water and acquires its “land” weight. One should not think that the icebreaker cuts the ice on the move with the continuous pressure of its bow - the pressure of the stem. This is not how icebreakers work, but ice cutters. This mode of action is suitable only for relatively thin ice.
Genuine sea icebreakers, such as Krasin or Yermak, work differently. By the action of its powerful machines, the icebreaker pushes its bow onto the surface of the ice, which for this purpose is arranged strongly sloping under water. Once out of the water, the bow of the ship acquires its full weight, and this huge load (for the Yermak, this weight reached, for example, up to 800 tons) breaks the ice. To enhance the action, more water is often pumped into the bow tanks of the icebreaker - “liquid ballast”.
This is how the icebreaker operates until the thickness of the ice does not exceed half a meter. More powerful ice is defeated by the impact action of the vessel. The icebreaker steps back and hits the ice edge with its entire mass. In this case, it is no longer the weight that acts, but the kinetic energy of the moving ship; the ship turns, as if into an artillery shell of low speed, but of a huge mass, into a ram.
Ice hummocks several meters high are broken by the energy of repeated blows from the strong bow of the icebreaker.
A participant in the famous Sibiryakov crossing in 1932, polar explorer N. Markov, describes the operation of this icebreaker as follows:
“Among hundreds of ice rocks, among the continuous cover of ice, the Sibiryakov began the battle. For fifty-two hours in a row, the needle of the machine telegraph jumped from “full back” to “full forward”. Thirteen four-hour sea watches "Sibiryakov" crashed into the ice from acceleration, crushed it with its nose, climbed onto the ice, broke it and again retreated. The ice, three-quarters of a meter thick, gave way with difficulty. With each blow they made their way to a third of the corps.
The USSR has the largest and most powerful icebreakers in the world.
Where are the sunken ships?
It is widely believed, even among sailors, that ships sunk in the ocean do not reach the seabed, but hang motionless at a certain depth, where the water is "correspondingly compacted by the pressure of the overlying layers."This opinion was apparently shared even by the author of 20,000 Leagues Under the Sea; in one of the chapters of this novel, Jules Verne describes a sunken ship hanging motionless in the water, and in another he mentions ships "rotting, hanging freely in the water."
Is such a statement correct?
There seems to be some basis for it, since the water pressure in the depths of the ocean really reaches enormous degrees. At a depth of 10 m, water presses with a force of 1 kg per 1 cm2 of a submerged body. At a depth of 20 m, this pressure is already 2 kg, at a depth of 100 m - 10 kg, 1000 m - 100 kg. The ocean, in many places, has a depth of several kilometers, reaching more than 11 km in the deepest parts of the Great Ocean (the Mariana Trench). It is easy to calculate what enormous pressure the water and objects immersed in it must experience at these enormous depths.
If an empty corked bottle is lowered to a considerable depth and then removed again, it will be found that the pressure of the water has driven the cork into the bottle and the whole vessel is full of water. The famous oceanographer John Murray, in his book The Ocean, says that such an experiment was carried out: three glass tubes of various sizes, sealed at both ends, were wrapped in canvas and placed in a copper cylinder with holes for the free passage of water. The cylinder was lowered to a depth of 5 km. When it was removed from there, it turned out that the canvas was filled with a snow-like mass: it was shattered glass. Pieces of wood, lowered to a similar depth, after being removed, sank in water like a brick - they were so squeezed.
It would seem natural to expect that such a monstrous pressure should so condense the water at great depths that even heavy objects will not sink in it, just as an iron weight does not sink in mercury.
However, this opinion is completely unfounded. Experience shows that water, like all liquids in general, is not very compressible. Compressed with a force of 1 kg per 1 cm2, water is compressed by only 1/22,000 of its volume and is compressed in approximately the same way with a further increase in pressure per kilogram. If we wanted to bring water to such a density that iron could float in it, it would be necessary to condense it 8 times. Meanwhile, for compaction only by half, i.e., to reduce the volume by half, a pressure of 11,000 kg per 1 cm2 is necessary (if only the mentioned measure of compression took place for such enormous pressures). This corresponds to a depth of 110 km below sea level!
From this it is clear that there is absolutely no need to speak of any noticeable compaction of water in the depths of the oceans. In their deepest place, the water is only 1100/22000 thick, that is, 1/20 of its normal density, only 5%. This almost cannot affect the conditions for floating various bodies in it, especially since solid objects immersed in such water are also subjected to this pressure and, therefore, also become denser.
Therefore, there cannot be the slightest doubt that sunken ships rest on the bottom of the ocean. “Anything that sinks in a glass of water,” says Murray, “should go to the bottom and into the deepest ocean.”
I have heard such an objection to this. If a glass is carefully immersed upside down in water, it may remain in that position, as it will displace a volume of water that weighs as much as the glass. A heavier metal glass can be held in a similar position and below the water level without sinking to the bottom. In the same way, as if, a cruiser or other ship capsized with a keel can stop halfway. If in some rooms of the ship the air is tightly locked, then the ship will sink to a certain depth and stop there.
After all, quite a few ships sink upside down - and it is possible that some of them never reach the bottom, remaining hanging in the dark depths of the ocean. A slight push would be enough to unbalance such a ship, turn it over, fill it with water and make it fall to the bottom - how can there be shocks in the depths of the ocean, where silence and calm reign forever and where even the echoes of storms do not penetrate?
All these arguments are based on a physical error. An overturned glass does not submerge itself in water - it must be submerged by an external force in water, like a piece of wood or an empty corked bottle. In the same way, a ship overturned with a keel up will not begin to sink at all, but will remain on the surface of the water. He cannot find himself halfway between the level of the ocean and its bottom.
How the dreams of Jules Verne and Wells came true
The real submarines of our time in some respects not only caught up with Jules Verpe's fantastic Nautilus, but even surpassed it. True, the speed of current submarine cruisers is half that of the Nautilus: 24 knots versus 50 for Jules Verne (a knot is about 1.8 km per hour). The longest passage of a modern submarine is a round-the-world trip, while Captain Nemo made a trip twice as long. On the other hand, the Nautilus had a displacement of only 1,500 tons, had a crew of only two or three dozen people on board, and was able to stay under water without a break for no more than forty-eight hours. The submarine cruiser "Surkuf", built in 1929 and owned by the French fleet, had a displacement of 3200 tons, was controlled by a team of one hundred and fifty people and was able to stay under water, without surfacing, up to one hundred and twenty hours.This submarine could make the transition from the ports of France to the island of Madagascar without entering any port along the way. In terms of the comfort of living quarters, the Surkuf, perhaps, was not inferior to the Nautilus. Further, Surkuf had the undoubted advantage over Captain Nemo's ship that a waterproof hangar for a reconnaissance seaplane was arranged on the upper deck of the cruiser. We also note that Jules Verne did not equip the Nautilus with a periscope, giving the boat the opportunity to view the horizon from under the water.
In only one respect, real submarines will still lag far behind the creation of the French novelist's fantasy: in the depth of submersion. However, it must be noted that at this point the fantasy of Jules Verne crossed the boundaries of plausibility. “Captain Nemo,” we read in one place in the novel, “reached depths of three, four, five, seven, nine, and ten thousand meters below the surface of the ocean.” And once the Nautilus sank even to an unprecedented depth - 16 thousand meters! “I felt,” says the hero of the novel, “how the fasteners of the iron plating of the submarine shudder, how its braces bend, how they move inside the windows, yielding to water pressure. If our ship did not have the strength of a solid cast body, it would be instantly flattened into a cake.”
The fear is quite appropriate, because at a depth of 16 km (if there were such a depth in the ocean), the water pressure would have to reach 16,000: 10 = 1600 kg per 1 cm2 , or 1600 technical atmospheres ; such an effort does not crush the iron, but would certainly crush the structure. However, modern oceanography does not know such a depth. The exaggerated ideas about the depths of the ocean that dominated the era of Jules Verne (the novel was written in 1869) are explained by the imperfection of methods for measuring depth. In those days, not wire was used for lin-lot, but hemp rope; such a lot was held back by friction against the water the stronger, the deeper it sank; at a considerable depth, friction increased to the point that the lot ceased to fall at all, no matter how much the line was poisoned: the hemp rope only tangled, creating the impression of great depth.
Submarines of our time are capable of withstanding a pressure of no more than 25 atmospheres; this determines the greatest depth of their immersion: 250 m. Much greater depth was achieved in a special apparatus called the "bathysphere" (Fig. 51) and designed specifically for studying the fauna of the ocean depths. This apparatus, however, does not resemble Jules Verne's Nautilus, but the fantastic creation of another novelist - Wells' deep-sea ball, described in the story "In the Deep of the Sea." The hero of this story descended to the bottom of the ocean to a depth of 9 km in a thick-walled steel ball; the device was immersed without a cable, but with a removable load; having reached the bottom of the ocean, the ball was freed here from the load that carried it away and swiftly flew up to the surface of the water.
In the bathysphere, scientists have reached a depth of more than 900 m. The bathysphere descends on a cable from a ship, with which those sitting in the ball maintain a telephone connection.
Figure 51. Steel spherical apparatus "bathysphere" for descent into the deep layers of the ocean. In this apparatus, William Beebe reached a depth of 923 m in 1934. The thickness of the walls of the ball is about 4 cm, the diameter is 1.5 m, and the weight is 2.5 tons.
How was Sadko raised?
In the wide expanse of the ocean, thousands of large and small ships perish every year, especially in wartime. The most valuable and accessible of the sunken ships began to be recovered from the bottom of the sea. The Soviet engineers and divers who are part of EPRON (i.e., the Special Purpose Underwater Expedition) became famous throughout the world by successfully lifting more than 150 large vessels. Among them, one of the largest is the Sadko icebreaker, which sank in the White Sea in 1916 due to the negligence of the captain. After lying on the seabed for 17 years, this excellent icebreaker was raised by EPRON workers and put back into operation.The lifting technique was entirely based on the application of the law of Archimedes. Under the hull of the sunken ship in the soil of the seabed, divers dug 12 tunnels and pulled a strong steel towel through each of them. The ends of the towels were attached to pontoons deliberately sunk near the icebreaker. All this work was carried out at a depth of 25 m below sea level.
The pontoons (Fig. 52) were hollow impenetrable iron cylinders 11 m long and 5.5 m in diameter. The empty pontoon weighed 50 tons. According to the rules of geometry, it is easy to calculate its volume: about 250 cubic meters. It is clear that such a cylinder should float empty on water: it displaces 250 tons of water, while itself weighs only 50; its carrying capacity is equal to the difference between 250 and 50, i.e. 200 tons. To make the pontoon sink to the bottom, it is filled with water.
When (see Fig. 52) the ends of the steel straps were firmly attached to the sunken pontoons, compressed air was injected into the cylinders using hoses. At a depth of 25 m, water presses with a force of 25/10 + 1, i.e. 3.5 atmospheres. Air was supplied to the cylinders under a pressure of about 4 atmospheres and, therefore, had to displace water from the pontoons. Lightweight cylinders with great force were pushed by the surrounding water to the surface of the sea. They floated in the water like a balloon in the air. Their joint lifting force with the complete displacement of water from them would be 200 x 12, i.e. 2400 tons. This exceeds the weight of the sunken Sadko, so for the sake of a smoother rise, the pontoons were only partially freed from water.
Figure 52. Scheme of lifting "Sadko"; shows a section of the icebreaker, pontoons and slings.
Nevertheless, the rise was carried out only after several unsuccessful attempts. “The rescue party suffered four accidents on it until it succeeded,” writes T. I. Bobritsky, chief ship engineer of EPRON, who led the work. “Three times, tensely waiting for the ship, we saw, instead of the rising icebreaker, spontaneously escaping upwards, in the chaos of waves and foam, pontoons and torn, snake-writhing hoses. Twice the icebreaker appeared and disappeared again in the abyss of the sea before it surfaced and finally stayed on the surface.
"Eternal" water engine
Among the many projects of the "perpetual motion machine" there were many that are based on the floating of bodies in the water. A tall tower 20 meters high is filled with water. Pulleys are installed at the top and bottom of the tower, through which a strong rope is thrown in the form of an endless belt. Attached to the rope are 14 hollow cubic boxes a meter high, riveted from iron sheets so that water cannot penetrate inside the boxes. Our pic. 53 and 54 depict the appearance of such a tower and its longitudinal section.How does this setting work? Everyone familiar with the law of Archimedes will realize that the boxes, being in the water, will tend to float up. They are pulled upwards by a force equal to the weight of the water displaced by the boxes, that is, the weight of one cubic meter of water, repeated as many times as the boxes are immersed in water. It can be seen from the drawings that there are always six boxes in the water. This means that the force that carries the loaded boxes up is equal to the weight of 6 m3 of water, i.e. 6 tons. They are pulled down by the own weight of the boxes, which, however, is balanced by a load of six boxes hanging freely on the outside of the rope.
So, a rope thrown in this way will always be subject to a pull of 6 tons applied to one side of it and directed upwards. It is clear that this force will cause the rope to rotate non-stop, sliding along the pulleys, and with each revolution to do work of 6000 * 20 = 120,000 kgm.
Now it is clear that if we dot the country with such towers, then we will be able to receive from them an unlimited amount of work, sufficient to cover all the needs of the national economy. The towers will rotate the anchors of the dynamos and provide electrical energy in any quantity.
However, if you look closely at this project, it is easy to see that the expected movement of the rope should not occur at all.
In order for the endless rope to rotate, the boxes must enter the tower's water basin from below and leave it from above. But after all, entering the pool, the box must overcome the pressure of a column of water 20 m high! This pressure per square meter of the area of the box is equal to neither more nor less than twenty tons (the weight of 20 m3 of water). The upward thrust is only 6 tons, that is, it is clearly insufficient to drag the box into the pool.
Among the many examples of water "perpetual" motion machines, hundreds of which were invented by failed inventors, one can find very simple and ingenious options.
Figure 53. The project of an imaginary "perpetual" water engine.
Figure 54. The device of the tower of the previous figure.
Take a look at fig. 55. Part of a wooden drum, mounted on an axle, is immersed in water all the time. If the law of Archimedes is true, then the part immersed in water should float up and, as soon as the buoyancy force is greater than the friction force on the axis of the drum, the rotation will never stop ...
Figure 55. Another project of a "perpetual" water engine.
Do not rush to build this "perpetual" engine! You will certainly fail: the drum will not budge. What is the matter, what is the error in our reasoning? It turns out that we did not take into account the direction of the acting forces. And they will always be directed along the perpendicular to the surface of the drum, that is, along the radius to the axis. Everyone knows from everyday experience that it is impossible to make a wheel turn by applying force along the radius of the wheel. To cause rotation, it is necessary to apply force perpendicular to the radius, i.e., tangent to the circumference of the wheel. Now it is not difficult to understand why the attempt to implement "perpetual" motion will also end in failure in this case.
The law of Archimedes provided seductive food for the minds of the seekers of the "perpetual" motion machine and encouraged them to come up with ingenious devices for using apparent weight loss in order to obtain an eternal source of mechanical energy.
Who coined the words "gas" and "atmosphere"?
The word "gas" belongs to the number of words invented by scientists along with such words as "thermometer", "electricity", "galvanometer", "telephone" and above all "atmosphere". Of all the invented words, “gas” is by far the shortest. The ancient Dutch chemist and physician Helmont, who lived from 1577 to 1644 (a contemporary of Galileo), produced "gas" from the Greek word for "chaos". Having discovered that air consists of two parts, one of which supports combustion and burns out, while the rest does not have these properties, Helmont wrote:“I called such steam gas, because it almost does not differ from the chaos of the ancients”(the original meaning of the word "chaos" is a radiant space).
However, the new word was not used for a long time after that and was revived only by the famous Lavoisier in 1789. It became widespread when everyone started talking about the flights of the Montgolfier brothers in the first balloons.
Lomonosov in his writings used another name for gaseous bodies - "elastic liquids" (which remained in use even when I was at school). We note, by the way, that Lomonosov is credited with introducing a number of names into Russian speech, which have now become standard words of the scientific language:
atmosphere
manometer
barometer
micrometer
air pump
optics, optical
viscosity
uh (e) electric
crystallization
e(e)fir
matter
and etc.
The ingenious ancestor of Russian natural science wrote about this: “I was forced to look for words to name some physical instruments, actions and natural things, which (i.e. words) although at first seem somewhat strange, but I hope that they will become more familiar with time through use will."
As we know, Lomonosov's hopes were fully justified.
On the contrary, the words subsequently proposed by V.I. Dahl (the well-known compiler of the Explanatory Dictionary) to replace the “atmosphere” - the clumsy “myrocolitsa” or “colosseum” - did not take root at all, just as his “heavenly earth” did not take root instead of the horizon and other new words .
Like a simple task
A samovar containing 30 glasses is full of water. You put a glass under his faucet and, with a watch in your hand, follow the second hand to see what time the glass is filled to the brim. Let's say that in half a minute. Now let's ask the question: at what time will the entire samovar be emptied if the tap is left open?It would seem that this is a childishly simple arithmetic problem: one glass flows out in 0.5 minutes, which means that 30 glasses will pour out in 15 minutes.
But do the experience. It turns out that the samovar is empty not at a quarter of an hour, as you expected, but at half an hour.
What's the matter? After all, the calculation is so simple!
Simple, but wrong. It cannot be thought that the speed of the outflow remains the same from beginning to end. When the first glass has flowed out of the samovar, the jet is already flowing under less pressure, since the water level in the samovar has dropped; it is clear that the second glass will be filled in a longer time than half a minute; the third will flow even more lazily, and so on.
The rate of flow of any liquid from a hole in an open vessel is directly dependent on the height of the liquid column above the hole. The brilliant Toricelli, a student of Galileo, was the first to point out this dependence and expressed it with a simple formula:
Where v is the outflow velocity, g is the acceleration of gravity, and h is the height of the liquid level above the hole. It follows from this formula that the speed of the outflowing jet is completely independent of the density of the liquid: light alcohol and heavy mercury at the same level flow out of the hole equally quickly (Fig. 56). It can be seen from the formula that on the Moon, where gravity is 6 times less than on Earth, it would take about 2.5 times more time to fill a glass than on Earth.
But let's get back to our task. If after the expiration of 20 glasses from the samovar, the level of water in it (counting from the tap opening) has dropped four times, then the 21st glass will fill up twice as slowly as the 1st. And if in the future the water level drops 9 times, then it will take three times more time to fill the last glasses than to fill the first. Everyone knows how sluggishly water flows from the tap of the samovar, which is already almost empty. By solving this problem using the methods of higher mathematics, it can be proved that the time required for the complete emptying of the vessel is twice as long as the time during which the same volume of liquid would pour out at a constant initial level.
Figure 56. Which is more likely to pour out: mercury or alcohol? The liquid level in the vessels is the same.
Pool problem
From what has been said, one step to the notorious problems about the pool, without which not a single arithmetic and algebraic problem book can do. Everyone remembers classically boring, scholastic problems like the following:“There are two pipes in the pool. After one first empty pool can be filled at 5 o'clock; in one second the full pool can be emptied at 10 o'clock. At what time will the empty pool be filled if both pipes are opened at once?
Problems of this kind have a respectable prescription - almost 20 centuries, going back to Heron of Alexandria. Here is one of Heron's tasks - not as intricate, however, as her descendants:
For two thousand years, swimming pool problems have been solved, and such is the power of routine! – two thousand years are solved incorrectly. Why it is wrong - you will understand for yourself after what has just been said about the outflow of water. How are they taught to solve swimming pool problems? The first problem, for example, is solved in the following way. At 1 hour, the first pipe pours 0.2 pools, the second pours 0.1 pools; this means that under the action of both pipes, 0.2 - 0.1 = 0.1 enters the pool every hour, from which the time for filling the pool is 10 hours. This reasoning is incorrect: if the inflow of water can be considered to occur under constant pressure and, therefore, uniform, then its outflow occurs at a changing level and, therefore, unevenly. From the fact that the pool is emptied by the second pipe at 10 o'clock, it does not at all follow that 0.1 part of the pool flows out every hour; school decision, as we see, is erroneous. It is impossible to solve the problem correctly by means of elementary mathematics, and therefore problems about a pool (with flowing water) have no place at all in arithmetic problem books.
Four fountains are given. An extensive reservoir is given.
In a day, the first fountain fills it to the brim.
Two days and two nights the second one should work on the same.
The third is three times the first, weaker.
In four days, the last one keeps up with him.
Tell me how soon it will be full
If during one time all of them open?
Figure 57. The pool problem.
Amazing Vessel
Is it possible to arrange such a vessel from which water would flow out all the time in a uniform stream, without slowing down its flow, despite the fact that the level of the liquid is lowering? After what you have learned from the previous articles, you are probably ready to consider such a problem unsolvable.Meanwhile, it is quite feasible. The bank shown in fig. 58, is just such an amazing vessel. This is an ordinary jar with a narrow neck, through the cork of which a glass tube is pushed. If you open tap C below the end of the tube, liquid will flow from it in an unremitting stream until the level of water in the vessel drops to the bottom end of the tube. By pushing the tube almost to the level of the faucet, you can make all the liquid above the level of the hole flow out in a uniform, albeit very weak stream.
Figure 58. The device of the Mariotte vessel. From hole C, water flows evenly.
Why is this happening? Follow mentally what happens in the vessel when tap C is opened (Fig. 58). First of all, water is poured out of a glass tube; the liquid level inside it drops to the end of the tube. With further outflow, the water level in the vessel already drops and outside air enters through the glass tube; it bubbles through the water and collects above it at the top of the vessel. Now, at all level B, the pressure is equal to atmospheric. This means that water from tap C flows out only under the pressure of the water layer BC, because the pressure of the atmosphere inside and outside the vessel is balanced. And since the thickness of the BC layer remains constant, it is not surprising that the jet flows at the same speed all the time.
Try now to answer the question: how fast will the water flow out if you remove the cork B at the level of the end of the tube?
It turns out that it will not flow out at all (of course, if the hole is so small that its width can be neglected; otherwise, water will flow out under the pressure of a thin layer of water, as thick as the width of the hole). In fact, here inside and out the pressure is equal to atmospheric, and nothing induces water to flow out.
And if you took out plug A above the lower end of the tube, then not only would water not flow out of the vessel, but outside air would also enter it. Why? For a very simple reason: inside this part of the vessel, the air pressure is less than atmospheric pressure outside.
This vessel with such extraordinary properties was invented by the famous physicist Mariotte and named after the scientist "the vessel of Mariotte."
Load from the air
In the middle of the 17th century, the inhabitants of the city of Rogensburg and the sovereign princes of Germany, headed by the emperor, who had gathered there, witnessed an amazing spectacle: 16 horses tried their best to separate two copper hemispheres attached to each other. What connected them? "Nothing" - air. And yet, eight horses pulling in one direction and eight pulling in the other, were unable to separate them. So the burgomaster Otto von Guericke showed with his own eyes to everyone that air is not “nothing” at all, that it has weight and presses with considerable force on all earthly objects.This experiment was carried out on May 8, 1654, in a very solemn atmosphere. The learned burgomaster managed to interest everyone with his scientific research, despite the fact that the matter took place in the midst of political turmoil and devastating wars.
A description of the famous experiment with the "Magdeburg hemispheres" is available in physics textbooks. Nevertheless, I am sure that the reader will listen with interest to this story from the lips of Guericke himself, that “German Galileo,” as the remarkable physicist is sometimes called. A voluminous book describing a long series of his experiments appeared in Latin in Amsterdam in 1672 and, like all books of this era, bore a lengthy title. Here it is:
OTTO von GUERICKE
The so-called new Magdeburg experiments
over AIRLESS SPACE,
originally described by a mathematics professorat the University of Würzburg by Kaspar Schott.
Author's own edition
more detailed and supplemented by various
new experiences.
Chapter XXIII of this book is devoted to the experiment that interests us. Here is a literal translation of it.
“An experiment proving that air pressure connects the two hemispheres so firmly that they cannot be separated by the efforts of 16 horses.
I ordered two copper hemispheres three-quarters of a Magdeburg cubit in diameter. But in reality, their diameter was only 67/100, since the craftsmen, as usual, could not make exactly what was required. Both hemispheres fully responded to each other. A crane was attached to one hemisphere; With this valve, you can remove air from the inside and prevent air from entering from the outside. In addition, 4 rings were attached to the hemispheres, through which ropes tied to the harness of horses were threaded. I also ordered a leather ring to be sewn; it was saturated with a mixture of wax in turpentine; sandwiched between the hemispheres, it did not let air through them. An air pump tube was inserted into the faucet, and the air inside the ball was removed. Then it was discovered with what force both hemispheres were pressed against each other through a leather ring. The pressure of the outside air pressed them so tightly that 16 horses (with a jerk) could not separate them at all, or achieved this only with difficulty. When the hemispheres, yielding to the tension of all the strength of the horses, were separated, a roar was heard, as from a shot.
But it was enough to open free access to air by turning the tap - and it was easy to separate the hemispheres with your hands.
A simple calculation can explain to us why such a significant force (8 horses on each side) is needed to separate the parts of an empty ball. Air presses with a force of about 1 kg per sq.cm; the area of a circle with a diameter of 0.67 cubits (37 cm) is 1060 cm2. This means that the pressure of the atmosphere on each hemisphere must exceed 1000 kg (1 ton). Each eight horses, therefore, had to pull with the force of a ton to counteract the pressure of the outside air.
It would seem that for eight horses (on each side) this is not a very large load. Do not forget, however, that when moving, for example, a load of 1 ton, horses overcome a force not of 1 ton, but much smaller, namely, the friction of the wheels on the axle and on the pavement. And this force is - on the highway, for example - only five percent, that is, with a one-ton load - 50 kg. (Not to mention the fact that when the efforts of eight horses are combined, as practice shows, 50% of traction is lost.) Therefore, traction of 1 ton corresponds to a cart load of 20 tons with eight horses. Such is the air load that the horses of the Magdeburg burgomaster were supposed to carry! It was as if they were supposed to move a small steam locomotive, which, moreover, was not put on rails.
It is measured that a strong draft horse pulls a cart with a force of only 80 kg. Consequently, to break the Magdeburg hemispheres, with a uniform thrust, 1000/80 \u003d 13 horses on each side would be required.
The reader will probably be amazed to learn that some of the articulations of our skeleton do not fall apart for the same reason as the Magdeburg hemispheres. Our hip joint is just such Magdeburg hemispheres. It is possible to expose this joint from muscular and cartilaginous connections, and yet the thigh does not fall out: atmospheric pressure presses it, since there is no air in the interarticular space.
New Heron Fountains
The usual form of the fountain, attributed to the ancient mechanic Heron, is probably known to my readers. Let me remind you here of its device, before passing on to a description of the latest modifications of this curious device. Heron's Fountain (Fig. 60) consists of three vessels: the upper open a and two spherical b and c, hermetically closed. The vessels are connected by three tubes, the location of which is shown in the figure. When there is some water in a, ball b is filled with water, and ball c is filled with air, the fountain begins to operate: water flows through the tube from a to c. displacing air from there into ball b; under the pressure of the incoming air, water from b rushes up the tube and beats like a fountain over vessel a. When ball b is empty, the fountain stops beating.
Figure 59. The bones of our hip joints do not disintegrate due to atmospheric pressure, just as the Magdeburg hemispheres are held back.
Figure 60. Ancient Heron Fountain.
Figure 61. Modern modification of the Heron Fountain. Above - a variant of the plate device.
This is the ancient form of the Heron fountain. Already in our time, a schoolteacher in Italy, impelled to ingenuity by the meager furnishings of his physical study, has simplified the arrangement of the Heron fountain and devised such modifications of it that anyone can arrange with the help of the simplest means (Fig. 61). Instead of balls, he used pharmacy bottles; instead of glass or metal tubes, I took rubber ones. The upper vessel does not need to be perforated: one can simply insert the ends of the tubes into it, as shown in fig. 61 above.
In this modification, the device is much more convenient to use: when all the water from jar b overflows through vessel a into jar c, you can simply rearrange jars b and c, and the fountain again operates; we must not forget, of course, to also transplant the tip onto another tube.
Another convenience of the modified fountain is that it makes it possible to arbitrarily change the location of the vessels and study how the distance of the levels of the vessels affects the height of the jet.
If you want to increase the height of the jet many times over, you can achieve this by replacing water with mercury in the lower flasks of the described device, and air with water (Fig. 62). The operation of the device is clear: mercury, pouring from jar c into jar b, displaces water from it, causing it to spurt like a fountain. Knowing that mercury is 13.5 times heavier than water, we can calculate how high the fountain jet should rise. Let us denote the level difference as h1, h2, h3, respectively. Now let's look at the forces under which mercury flows from vessel c (Fig. 62) into b. The mercury in the connecting tube is subject to pressure from both sides. On the right, it is affected by the pressure of the difference h2 of mercury columns (which is equivalent to the pressure of 13.5 times the higher water column, 13.5 h2) plus the pressure of the water column h1. The water column h3 presses on the left. As a result, mercury is carried away by force
13.5h2 + h1 - h3.
But h3 – h1 = h2; therefore, we replace h1 - h3 with minus h2 and get:
13.5h2 - h2 i.e. 12.5h2.
Thus, mercury enters vessel b under the pressure of the weight of a water column with a height of 12.5 h2. Theoretically, the fountain should therefore beat to a height equal to the difference in mercury levels in the flasks, multiplied by 12.5. Friction lowers this theoretical height somewhat.
Nevertheless, the described device provides a convenient opportunity to get a high-up jet. To force, for example, a fountain to beat to a height of 10 m, it is enough to raise one can above the other by about one meter. It is curious that, as can be seen from our calculation, the elevation of the plate a above the flasks with mercury does not in the least affect the height of the jet.
Figure 62. Mercury pressure fountain. The jet beats ten times higher than the difference in mercury levels.
Deceptive Vessels
In the old days - in the 17th and 18th centuries - the nobles amused themselves with the following instructive toy: they made a mug (or jug), in the upper part of which there were large patterned cutouts (Fig. 63). Such a mug, poured with wine, was offered to an ignorant guest, over whom one could laugh with impunity. How to drink from it? You can’t tilt it: wine will pour out of many through holes, and not a drop will reach your mouth. It will happen like in a fairy tale:
Figure 63. Deceptive jug of the end of the 18th century and the secret of its construction.
Honey, drinking beer,
Yes, he just wet his mustache.
But who knew the secret of the arrangement of such mugs, the secret shown in fig. 63 on the right, - he plugged hole B with his finger, took the spout into his mouth and drew the liquid into himself without tilting the vessel: the wine rose through the hole E along the channel inside the handle, then along its continuation C inside the upper edge of the mug and reached the spout.
Not so long ago, similar mugs were made by our potters. It happened to me in one house to see an example of their work, rather skillfully concealing the secret of the construction of the vessel; on the mug was the inscription: "Drink, but do not pour over."
How much does water weigh in an overturned glass?
“Of course, it doesn’t weigh anything: water doesn’t hold in such a glass, it pours out,” you say.- And if it does not pour out? I will ask. – What then?
In fact, it is possible to keep water in an overturned glass so that it does not spill out. This case is shown in Fig. 64. An overturned glass goblet, tied at the bottom to one scale pan, is filled with water, which does not pour out, since the edges of the goblet are immersed in a vessel with water. An exactly the same empty glass is placed on the other pan of the scales.
Which pan of the scales will outweigh?
Figure 64. Which cup will win over?
The one to which the overturned glass of water is tied will pull. This glass experiences full atmospheric pressure from above, and atmospheric pressure from below, weakened by the weight of the water contained in the glass. To balance the cups, it would be necessary to fill a glass placed on top of another cup with water.
Under these conditions, therefore, the water in an overturned glass weighs the same as in a glass placed on the bottom.
Why are ships attracted?
In the autumn of 1912, the ocean steamer Olympic, then one of the greatest ships in the world, had the following incident. The Olympic sailed in the open sea, and almost parallel to it, at a distance of hundreds of meters, another ship, a much smaller armored cruiser Gauk, passed at high speed. When both ships took the position shown in fig. 65, something unexpected happened: the smaller ship quickly turned off the track, as if obeying some invisible force, turned its bow to the large steamer and, not obeying the helm, moved almost directly towards it. There was a collision. The Gauk slammed its nose into the Olmpik's side; the blow was so strong that the "Gauk" made a large hole in the side of the "Olympic".
Figure 65. The position of the steamers "Olympic" and "Gauk" before the collision.
When this strange case was considered in the maritime court, the captain of the giant "Olympic" was found guilty, because, - the court's ruling read, - he did not give any orders to give way to the "Gauk" going across.
The court did not see here, therefore, anything extraordinary: the captain's simple carelessness, nothing more. Meanwhile, a completely unforeseen circumstance took place: a case of mutual attraction of ships on the sea.
Such cases have occurred more than once, probably before, with the parallel movement of two ships. But until very large ships were built, this phenomenon did not manifest itself with such force. When the waters of the oceans began to plow the "floating cities", the phenomenon of the attraction of ships became much more noticeable; commanders of warships reckon with him when maneuvering.
Numerous accidents of small ships sailing in the vicinity of large passenger and military ships probably occurred for the same reason.
What explains this attraction? Of course, there can be no question of attraction according to Newton's law of universal gravitation; we have already seen (in Chapter IV) that this attraction is too negligible. The reason for the phenomenon is of a completely different kind and is explained by the laws of the flow of liquids in tubes and channels. It can be proved that if a liquid flows through a channel that has constrictions and expansions, then in narrow parts of the channel it flows faster and puts less pressure on the channel walls than in wide places, where it flows more calmly and puts more pressure on the walls (the so-called "Bernoulli principle"). ").
The same is true for gases. This phenomenon in the doctrine of gases is called the Clément-Desorme effect (after the physicists who discovered it) and is often referred to as the "aerostatic paradox". For the first time this phenomenon, as they say, was discovered by accident under the following circumstances. In one of the French mines, a worker was ordered to close the opening of the outer adit with a shield, through which compressed air was supplied to the mine. The worker struggled for a long time with a stream of air, but suddenly the shield slammed the adit by itself with such force that, if the shield were not large enough, he would have been drawn into the ventilation hatch along with the frightened worker.
Incidentally, this feature of the flow of gases explains the action of the atomizer. When we blow (Fig. 67) into knee a, ending in a constriction, the air, passing into the constriction, reduces its pressure. Thus, there is air with reduced pressure above the tube b, and therefore the pressure of the atmosphere drives the liquid from the glass up the tube; at the hole, the liquid enters the jet of blown air and is sprayed in it.
Now we will understand what is the reason for the attraction of ships. When two steamships sail parallel to one another, a kind of water channel is obtained between their sides. In an ordinary channel, the walls are stationary, and the water moves; here it is the other way around: the water is stationary, but the walls are moving. But the action of the forces does not change at all: in the narrow places of the moving drip, the water presses on the walls less than in the space around the steamers. In other words, the sides of the steamers facing each other experience less pressure from the water side than the outer parts of the ships. What should happen as a result of this? The ships must, under the pressure of the outer water, move towards each other, and it is natural that the smaller ship moves more noticeably, while the more massive one remains almost motionless. That is why attraction is especially strong when a large ship quickly passes a small one.
Figure 66. In narrow parts of the canal, water flows faster and presses on the walls less than in wide ones.
Figure 67. Spray gun.
Figure 68. The flow of water between two sailing ships.
So, the attraction of ships is due to the suction action of flowing water. This also explains the danger of rapids for bathers, the suction effect of whirlpools. It can be calculated that the flow of water in a river at a moderate speed of 1 m per second draws in a human body with a force of 30 kg! Such a force is not easy to resist, especially in the water, when our own body weight does not help us to maintain stability. Finally, the retracting action of a fast-moving train is explained by the same Bernoulli principle: a train at a speed of 50 km per hour drags a nearby person with a force of about 8 kg.
The phenomena associated with the "Bernoulli principle", although quite common, are little known among non-specialists. It will therefore be useful to dwell on it in more detail. The following is an excerpt from an article on this topic published in a popular science journal.
Bernoulli's principle and its consequences
The principle, first stated by Daniel Bernoulli in 1726, says: in a jet of water or air, the pressure is high if the speed is low, and the pressure is low if the speed is high. There are known limitations to this principle, but we will not dwell on them here.Rice. 69 illustrates this principle.
Air is blown through the tube AB. If the cross section of the tube is small, as in a, the air velocity is high; where the cross section is large, as in b, the air velocity is low. Where the speed is high, the pressure is low, and where the speed is low, the pressure is high. Due to the low air pressure in a, the liquid in tube C rises; at the same time, strong air pressure in b causes the liquid in tube D to sink.
Figure 69. Illustration of the Bernoulli principle. In the narrowed part (a) of the tube AB, the pressure is less than in the wide part (b).
On fig. 70 tube T is mounted on a copper disk DD; air is blown through the tube T and further past the free disk dd. The air between the two disks has a high speed, but this speed decreases rapidly as it approaches the edges of the disks, as the cross section of the air flow rapidly increases and the inertia of the air flowing out of the space between the disks is overcome. But the pressure of the air surrounding the disk is large, since the speed is low, and the air pressure between the disks is small, since the speed is high. Therefore, the air surrounding the disk has a greater effect on the disks, tending to bring them closer than the air flow between the disks, tending to push them apart; as a result, the disk dd sticks to the disk DD the stronger, the stronger the air current in T.
Rice. 71 represents the analogy of fig. 70, but only with water. The fast moving water on the DD disc is at a low level and rises to a higher still water level in the basin as it circles around the edges of the disc. Therefore, the still water below the disk has a higher pressure than the moving water above the disk, causing the disk to rise. Rod P does not allow lateral displacement of the disk.
Figure 70. Experience with disks.
Figure 71. Disk DD rises on rod P when a jet of water from the tank is poured onto it.
Rice. 72 depicts a light ball floating in a jet of air. The air jet hits the ball and prevents it from falling. When the ball pops out of the jet, the surrounding air pushes it back into the jet because the pressure of the low velocity ambient air is high and the pressure of the high velocity air in the jet is low.
Rice. 73 represents two ships moving side by side in calm water, or, what amounts to the same thing, two ships standing side by side and flowing around the water. The flow is more constrained in the space between the vessels, and the water velocity in this space is greater than on both sides of the vessels. Therefore, the water pressure between ships is less than on both sides of the ships; the higher pressure of the water surrounding the ships brings them closer together. Sailors know very well that two ships sailing side by side are strongly attracted to each other.
Figure 72. A ball supported by a jet of air.
Figure 73. Two ships moving in parallel seem to attract each other.
Figure 74. When ships move forward, ship B turns her bow towards ship A.
Figure 75. If air is blown between two light balls, they approach each other until they touch.
A more serious case may occur when one ship follows another, as shown in fig. 74. The two forces F and F, which bring the ships together, tend to turn them, and the ship B turns towards L with considerable force. A collision in this case is almost inevitable, since the rudder does not have time to change the direction of the ship.
The phenomenon described in connection with fig. 73 can be demonstrated by blowing air between two light rubber balls suspended as shown in fig. 75. If air is blown between them, they approach and hit each other.
Purpose of the fish bladder
About what role the swim bladder of fishes plays, they usually say and write - it would seem quite plausible - the following. In order to emerge from the depths to the surface layers of the water, the fish inflates its swim bladder; then the volume of its body increases, the weight of the displaced water becomes greater than its own weight - and, according to the law of swimming, the fish rises. To stop the rise or go down, she, on the contrary, compresses her swim bladder. The volume of the body, and with it the weight of the displaced water, decrease, and the fish sinks to the bottom according to the law of Archimedes.Such a simplified idea of the purpose of the swimming bladder of fish dates back to the time of the scientists of the Florentine Academy (XVII century) and was expressed by Professor Borelli in 1685. For more than 200 years it was accepted without objection, managed to take root in school textbooks, and only by the works of new researchers (Moreau, Charbonel) the complete inconsistency of this theory was discovered,
The bubble undoubtedly has a very close connection with the swimming of fish, since the fish, in which the bubble was artificially removed during the experiments, could stay in the water only by working hard with their fins, and when this work was stopped, they fell to the bottom. What is its true role? Very limited: it only helps the fish to stay at a certain depth - exactly at the one where the weight of the water displaced by the fish is equal to the weight of the fish itself. When the fish, by the work of its fins, falls below this level, its body, experiencing great external pressure from the water, contracts, squeezing the bubble; the weight of the displaced volume of water decreases, becomes less than the weight of the fish, and the fish falls uncontrollably down. The lower it falls, the stronger the water pressure becomes (by 1 atmosphere when lowering for every 10 m), the more the body of the fish is compressed and the more rapidly it continues to fall.
The same thing, only in the opposite direction, occurs when the fish, having left the layer where it was in balance, is moved by the work of its fins to higher layers. Her body, freed from part of the external pressure and still bursting from the inside with a swim bladder (in which the gas pressure was up to this point in equilibrium with the pressure of the surrounding water), increases in volume and, as a result, floats higher. The higher the fish rises, the more its body swells and, consequently, the faster its further rise. The fish is not able to prevent this by “squeezing the bladder”, since the walls of its swim bladder are devoid of muscle fibers that could actively change its volume.
That such a passive expansion of the volume of the body actually takes place in fish is confirmed by the following experiment (Fig. 76). The bleak in the chloroformed state is placed in a closed vessel with water, in which an increased pressure is maintained, close to that prevailing at a certain depth in a natural reservoir. on the surface of the water, the fish lies inactive, belly up. Submerged a little deeper, it rises to the surface again. Placed closer to the bottom, it sinks to the bottom. But in the interval between both levels there is a layer of water in which the fish remains in balance - it does not sink and does not float. All this becomes clear if we recall what has just been said about the passive expansion and contraction of the swim bladder.
So, contrary to popular belief, a fish cannot voluntarily inflate and contract its swim bladder. Changes in its volume occur passively, under the influence of increased or weakened external pressure (according to the Boyle-Mariotte law). These changes in volume are not only not useful for the fish, but, on the contrary, are harmful to it, since they cause either an unstoppable, ever accelerating fall to the bottom, or an equally unstoppable and accelerating rise to the surface. In other words, the bubble helps the fish to keep its balance in a stationary position, but this balance is unstable.
This is the true role of the swim bladder of fish, as far as its relation to swimming is concerned; whether it also performs other functions in the body of the fish and what exactly is unknown, so this organ is still mysterious. And only its hydrostatic role can now be considered as being fully elucidated.
Observations of fishermen confirm what has been said.
Figure 76. Experience with bleak.
When catching fish from great depths, it happens that other fish are released halfway through; but, contrary to expectation, it does not descend again into the depth from which it was extracted, but, on the contrary, rapidly rises to the surface. In such and such fish, it is sometimes noticed that the bladder protrudes through the mouth.
Waves and whirlwinds
Many of the everyday physical phenomena cannot be explained on the basis of the elementary laws of physics. Even such a frequently observed phenomenon as sea waves on a windy day cannot be fully explained within the framework of a school physics course. And what causes the waves that scatter in calm water from the bow of a moving steamer? Why do flags wave in windy weather? Why is the sand on the seashore undulating? Why is there smoke coming out of a factory chimney?
Figure 77. Calm (“laminar”) flow of fluid in a pipe.
Figure 78. Vortex ("turbulent") flow of fluid in a pipe.
To explain these and other similar phenomena, one must know the features of the so-called vortex motion of liquids and gases. We will try to tell here a little about vortex phenomena and note their main features, since vortices are hardly mentioned in school textbooks.
Imagine a liquid flowing in a pipe. If all the particles of the fluid move along the pipe along parallel lines, then we have the simplest form of fluid movement - a calm, or, as physicists say, a "laminar" flow. However, this is by no means the most common case. On the contrary, much more often liquids flow restlessly in pipes; vortices go from the walls of the pipe to its axis. This is a whirlwind or turbulent motion. This is how, for example, water flows in the pipes of the water supply network (if we do not mean thin pipes, where the flow is laminar). A vortex flow is observed whenever the flow rate of a given fluid in a pipe (of a given diameter) reaches a certain value, the so-called critical speed.
Whirlwinds of liquid flowing in a pipe can be made visible to the eye if a little light powder, such as lycopodium, is introduced into a transparent liquid flowing in a glass tube. Then the vortices going from the walls of the tube to its axis are clearly distinguished.
This feature of the vortex flow is used in technology for the construction of refrigerators and coolers. A fluid flowing turbulently in a tube with cooled walls brings all its particles into contact with the cold walls much faster than when moving without vortices; it must be remembered that liquids themselves are poor conductors of heat and, in the absence of mixing, cool or warm up very slowly. A lively thermal and material exchange of blood with the tissues washed by it is also possible only because its flow in the blood vessels is not laminar, but vortex.
What has been said about pipes applies equally to open canals and riverbeds: in canals and rivers, water flows turbulently. When accurately measuring the speed of a river, the instrument detects ripples, especially near the bottom: ripples indicate a constantly changing direction of flow, i.e. eddies River water particles move not only along the river channel, as is usually imagined, but also from the banks to the middle . That is why the statement is incorrect that in the depths of the river the water has the same temperature all year round, namely + 4 ° C: due to mixing, the temperature of the flowing water near the bottom of the river (but not the lake) is the same as on the surface. Whirlwinds that form at the bottom of the river carry light sand with them and give rise to sandy "waves" here. The same can be seen on the sandy seashore, washed by the oncoming wave (Fig. 79). If the flow of water near the bottom were calm, the sand at the bottom would have a flat surface.
Figure 79. Formation of sand waves on the sea coast by the action of water eddies.
Figure 80. The undulating motion of a rope in flowing water is due to the formation of vortices.
So, near the surface of a body washed by water, vortices are formed. Their existence is told to us, for example, by a serpentine coiling rope stretched along the water current (when one end of the rope is tied and the other is free). What's going on here? The section of the rope near which the whirlwind has formed is carried away by it; but in the next moment this section moves already by another vortex in the opposite direction - a serpentine meander is obtained (Fig. 80).
From liquids to gases, from water to air.
Who has not seen how air whirlwinds carry away dust, straw, etc. from the earth? This is a manifestation of the vortex flow of air along the surface of the earth. And when the air flows along the water surface, then in places where vortices are formed, as a result of a decrease in air pressure here, the water rises like a hump - excitement is generated. The same cause generates sand waves in the desert and on the slopes of the dunes (Fig. 82).
Figure 81. Flying flag in the wind...
Figure 82. Wavy surface of sand in the desert.
It is easy to understand now why the flag is agitated in the wind: the same thing happens to it as to a rope in flowing water. The hard plate of the weather vane does not maintain a constant direction in the wind, but, obeying the whirlwinds, oscillates all the time. Of the same vortex origin and puffs of smoke coming out of the factory chimney; flue gases flow through the pipe in a vortex motion, which continues for some time by inertia outside the pipe (Fig. 83).
The importance of turbulent air movement for aviation is great. The wings of the aircraft are given such a shape in which the place of rarefaction of air under the wing is filled with the substance of the wing, and the vortex effect above the wing, on the contrary, is enhanced. As a result, the wing is supported from below, and sucked from above (Fig. 84). Similar phenomena take place when a bird soars with outstretched wings.
Figure 83. Puffs of smoke coming out of a factory chimney.
How does the wind blowing over the roof work? Whirlwinds create rarefaction of air above the roof; trying to equalize the pressure, the air from under the roof, being carried upwards, presses on it. As a result, something happens that, unfortunately, one often has to observe: a light, loosely attached roof is blown away by the wind. For the same reason, large window panes are squeezed out from the inside by the wind (and not broken by pressure from the outside). However, these phenomena are more easily explained by a decrease in pressure in moving air (see Bernoulli's principle above, p. 125).
When two streams of air of different temperature and humidity flow one along the other, vortices appear in each. The various forms of clouds are largely due to this reason.
We see what a wide range of phenomena is associated with vortex flows.
Figure 84. What forces are subject to the wing of an aircraft.
Distribution of pressures (+) and rarefaction (-) of air over the wing based on experiments. As a result of all the applied efforts, supporting and sucking, the wing is carried upwards. (Solid lines show pressure distribution; dotted lines show the same with a sharp increase in flight speed)
Journey to the bowels of the Earth
Not a single person has yet descended into the Earth deeper than 3.3 km - and yet the radius of the globe is 6400 km. There is still a very long way to the center of the Earth. Nevertheless, the inventive Jules Verne sent his heroes deep into the bowels of the Earth - the eccentric professor Lidenbrock and his nephew Axel. In Journey to the Center of the Earth, he described the amazing adventures of these underground travelers. Among the surprises they met under the Earth, among other things, was an increase in air density. As it rises, the air is rarefied very quickly: its density decreases exponentially, while the height of the rise increases in an arithmetic progression. On the contrary, when lowering down, below the level of the ocean, the air under the pressure of the overlying layers should become ever denser. Underground travelers, of course, could not fail to notice this.Here is a conversation between a scientist uncle and his nephew at a depth of 12 leagues (48 km) in the bowels of the Earth.
“Look what the manometer shows? Uncle asked.
- Very strong pressure.
“Now you see that, as we descend little by little, we gradually become accustomed to the condensed air and do not suffer from it at all.
“Except for the pain in my ears.
- Rubbish!
“Very well,” I answered, deciding not to contradict my uncle. “It’s even nice to be in the condensed air. Have you noticed how loud sounds are heard in it?
- Certainly. In this atmosphere, even the deaf could hear.
“But the air will keep getting denser. Will it eventually acquire the density of water?
- Of course: under a pressure of 770 atmospheres.
- And even lower?
– Density will increase even more.
How are we going to get down then?
We'll fill our pockets with stones.
- Well, uncle, you have an answer for everything!
I did not go further into the realm of conjectures, because, perhaps, I would again come up with some kind of obstacle that would annoy my uncle. It was, however, obvious that under a pressure of several thousand atmospheres, the air could pass into a solid state, and then, even assuming that we could endure such pressure, we would still have to stop. No arguments will help here.”
Fantasy and mathematics
This is how the novelist narrates; but but it turns out, if we check the facts, which are spoken of in this passage. We do not have to go down into the bowels of the Earth for this; for a small excursion into the field of physics, it is enough to stock up on a pencil and paper.First of all, we will try to determine to what depth we need to go down so that the pressure of the atmosphere increases by a 1000th part. The normal pressure of the atmosphere is equal to the weight of a 760 mm column of mercury. If we were immersed not in air, but in mercury, we would have to go down only 760/1000 = 0.76 mm in order for the pressure to increase by 1000th. In the air, of course, we must descend much deeper for this, and exactly as many times as the air is lighter than mercury - 10,500 times. This means that in order for the pressure to increase by a 1000th part of normal, we will have to go down not by 0.76 mm, as in mercury, but by 0.76x10500, i.e., by almost 8 m. When will we go down another 8 m, then the increased pressure will increase by another 1000 of its magnitude, and so on ... At whatever level we are - at the very "ceiling of the world" (22 km), on top of Mount Everest (9 km) or near the surface of the ocean, - we need to go down 8 m so that the pressure of the atmosphere increases by 1000th of the original value. It turns out, therefore, such a table of increasing air pressure with depth:
pressure at ground level
760 mm = normal
"depth 8 m" \u003d 1.001 normal
"depth 2x8" \u003d (1.001) 2
"depth 3x8" \u003d (1.001) 3
"depth 4x8" \u003d (1.001) 4
And in general, at a depth of nx8 m, the pressure of the atmosphere is (1.001) n times greater than normal; and while the pressure is not very high, the air density will increase by the same amount (Mariotte's law).
Note that in this case we are talking, as can be seen from the novel, about deepening into the Earth by only 48 km, and therefore the weakening of gravity and the associated decrease in the weight of air can be ignored.
Now you can calculate how big it was, approximately. the pressure that Jules Verne's underground travelers experienced at a depth of 48 km (48,000 m). In our formula, n equals 48000/8 = 6000. We have to calculate 1.0016000. Since multiplying 1.001 by itself 6000 times is rather boring and time consuming, we will turn to the help of logarithms. about which Laplace rightly said that by reducing labor, they double the life of calculators. Taking the logarithm, we have: the logarithm of the unknown is equal to
6000 * log 1.001 = 6000 * 0.00043 = 2.6.
By the logarithm of 2.6 we find the desired number; it is equal to 400.
So, at a depth of 48 km, the pressure of the atmosphere is 400 times stronger than normal; The density of air under such pressure will increase, as experiments have shown, by 315 times. Therefore, it is doubtful that our underground travelers would not suffer at all, experiencing only “pain in the ears” ... In the novel by Jules Verpe, however, it is said that people have reached even greater underground depths, namely 120 and even 325 km. The pressure of the air must have reached monstrous degrees there; a person is able to endure harmlessly air pressure no more than three or four atmospheres.
If, using the same formula, we began to calculate at what depth the air becomes as dense as water, that is, it becomes 770 times denser, then we would get a figure: 53 km. But this result is incorrect, since at high pressures the density of the gas is no longer proportional to the pressure. Mariotte's law is quite true only for not too significant pressures, not exceeding hundreds of atmospheres. Here are the data on air density obtained by experience:
Pressure Density
200 atmospheres... 190
400" .............. 315
600" .............. 387
1500" ............. 513
1800" ............. 540
2100" ............. 564
The increase in density, as we see, noticeably lags behind the increase in pressure. In vain did the Jules Verne scientist expect that he would reach a depth where air is denser than water - he would not have had to wait for this, since air reaches the density of water only at a pressure of 3000 atmospheres, and then it almost does not compress. There can be no question of turning air into a solid state by one pressure, without strong cooling (below minus 146 °).
It is fair to say, however, that the Jules Verne novel in question was published long before the facts now cited became known. This justifies the author, although it does not correct the narrative.
We will use the formula given earlier to calculate the greatest depth of the mine, at the bottom of which a person can remain without harm to his health. The highest air pressure that our body can still endure is 3 atmospheres. Denoting the desired depth of the mine through x, we have the equation (1.001) x / 8 \u003d 3, from which (logarithmically) we calculate x. We get x = 8.9 km.
So, a person could be without harm at a depth of almost 9 km. If the Pacific Ocean suddenly dried up, people could almost everywhere live on its bottom.
In a deep mine
Who moved closest to the center of the Earth - not in the novelist's fantasy, but in reality? Of course, miners. We already know (see Chapter IV) that the deepest mine in the world has been dug in South Africa. It goes deeper than 3 km. Here we mean not the depth of penetration of the drill bit, which reaches 7.5 km, but the deepening of the people themselves. Here is what, for example, the French writer Dr. Luc Durten, who personally visited it, tells about the mine at the Morro Velho mine (depth of about 2300 m):“The famous gold mines of Morro Velho are located 400 km from Rio de Janeiro. After 16 hours of railroad riding in rocky terrain, you descend into a deep valley surrounded by jungle. Here, an English company is mining gold-bearing veins at depths never before seen by man.
The vein goes into the depths obliquely. The mine follows it with six ledges. Vertical shafts - wells, horizontal - tunnels. It is extremely characteristic of modern society that the deepest shaft dug in the crust of the globe - the most daring attempt of man to penetrate the bowels of the planet - is made in search of gold.
Wear canvas overalls and a leather jacket. Be careful: the smallest pebble falling into the well can hurt you. We will be accompanied by one of the "captains" of the mine. You enter the first tunnel, well lit. You are shivering from a chilling 4° wind: this is ventilation to cool the depths of the mine.
Having passed the first well 700 m deep in a narrow metal cage, you find yourself in the second tunnel. You go down to the second well; the air is getting warmer. You are already below sea level.
Starting from the next well, the air burns the face. Drenched in sweat, hunched under the low arch, you move towards the roar of the drilling machines. Naked people work in thick dust; Sweat drips from them, hands pass a bottle of water nonstop. Do not touch the fragments of ore, now broken off: their temperature is 57 °.
What is the result of this terrible, disgusting reality? “About 10 kilograms of gold a day…”.
Describing the physical conditions at the bottom of the mine and the degree of extreme exploitation of the workers, the French writer notes the high temperature, but does not mention the increased air pressure. Let us calculate what it is like at a depth of 2300 m. If the temperature remained the same as on the surface of the Earth, then, according to the formula already familiar to us, the air density would increase by
Raz.
In reality, the temperature does not remain constant, but rises. Therefore, the air density increases not so significantly, but less. Ultimately, the air at the bottom of the mine differs in density from the air on the surface of the Earth a little more than the air of a hot summer day from the frosty air of winter. It is clear now why this circumstance did not attract the attention of the visitor to the mine.
But of great importance is the significant humidity of the air in such deep mines, which makes staying in them unbearable at high temperatures. In one of the South African mines (Johansburg), at a depth of 2553 m, the humidity reaches 100% at 50°C; the so-called "artificial climate" is now being arranged here, and the cooling effect of the installation is equivalent to 2000 tons of ice.
Up with the stratostats
In previous articles, we mentally traveled into the bowels of the earth, and the formula for the dependence of air pressure on depth helped us. Let us now venture upward and, using the same formula, see how the air pressure changes at high altitudes. The formula for this case takes the following form:p = 0.999h/8,
where p is the pressure in atmospheres, h is the height in meters. The fraction 0.999 replaced the number 1.001 here, because when moving up 8 m, the pressure does not increase by 0.001, but decreases by 0.001.
Let's start by solving the problem: how high do you need to rise so that the air pressure is halved?
To do this, we equate the pressure p = 0.5 in our formula and start looking for the height h. We get the equation 0.5 \u003d 0.999h / 8, which will not be difficult to solve for readers who know how to handle logarithms. The answer h = 5.6 km determines the height at which the air pressure must be halved.
Let us now head even higher, following the brave Soviet aeronauts, who have reached a height of 19 and 22 km. These high regions of the atmosphere are already in the so-called "stratosphere". Therefore, the balls on which such ascents are made are given the name not of balloons, but of "stratospheric balloons". I don’t think that among the people of the older generation there was at least one who would not have heard the names of the Soviet stratospheric balloons “USSR” and “OAH-1”, which set world altitude records in 1933 and 1934: the first - 19 km, the second - 22 km.
Let's try to calculate what is the pressure of the atmosphere at these heights.
For a height of 19 km, we find that the air pressure should be
0.99919000/8 = 0.095 atm = 72 mm.
For a height of 22 km
0.99922000/8 = 0.066 atm = 50 mm.
However, looking at the records of the stratonauts, we find that other pressures were noted at the indicated altitudes: at an altitude of 19 km - 50 mm, at an altitude of 22 km - 45 mm.
Why is the calculation not confirmed? What is our mistake?
Mariotte's law for gases at such a low pressure is quite applicable, but this time we made another omission: we considered the air temperature to be the same throughout the entire 20-kilometer thickness, while it drops noticeably with height. On average they accept; that the temperature drops by 6.5° for every kilometer raised; this happens up to a height of 11 km, where the temperature is minus 56 ° and then remains unchanged for a considerable distance. If this circumstance is taken into account (for which the means of elementary mathematics are no longer sufficient), results will be obtained that are much more consistent with reality. For the same reason, the results of our previous calculations relating to the air pressure in the depths must also be regarded as approximate.
On this page I will collect books on entertaining physics known to me: books that I have at home, links to stories and reviews about such books.
Please add in the comments what entertaining scientific books you know.
N.M. Zubkov "Tasty Science" Experiences and experiments in the kitchen for children from 5 to 9 years old. A simple little book. I would lower the age, too simple and well-known experiments, such as swimming an egg in salt water and wrapping ice cream in a fur coat. Mostly answers to children's "why?". Although, maybe I'm overly demanding) So, in principle, everything is nice and understandable)
L. Gendenstein and others "Mechanics" is a book from my childhood. In it, in the form of comics, friends get acquainted with the laws of mechanics. This acquaintance takes place in the game, in conversation, in general, in between times. I really liked her then, and still do. Maybe it was with her that my passion for physics began?
"Children's Encyclopedia". This Talmud is also from my childhood. It contains 5 volumes. There is also about art, and about geography, biology, history. And this one is natural. How many times I open it, I am so convinced that the old encyclopedias are not like the current ones. The drawings are true black and white (mostly), but there is much more information.
A. V. Lukyanova "Real physics for boys and girls". The first book on physics that I bought myself. What to say? Not immediately impressed. The book is large format, the drawings are beautiful, the paper is thick, the price is high. And in fact, not much. But, in principle, you can read, look at pictures with your child. 
A. Dmitriev "Grandfather's Chest". This little pamphlet is my favorite. Almost self-published in design, but all experiments, scientific toys are described in a very accessible and simple way. 
Tom Tit "Science Fun". Everywhere this book is very praised, but I also did not really like it. Experiments are interesting. But there is no explanation. And without an explanation, it somehow turns out poorly. 
Y. Perelman "Entertaining mechanics", "Physics at every step", "Entertaining physics". Perelman, of course, is a classic of the genre. However, his books are not for the little ones.
Bruno Donath "Physics in games". It looks like Tom Tit, only somehow easier on my perception and explanations of all experiments and games are given. 
L.A. Sikoruk "Physics for kids". It looks something like my "Mechanics" Gendenstein from childhood. No, there are no comics here, but acquaintance with the physical laws of nature goes on in conversation and casually. I did not find this book for sale, because I only have it in print. 
Well, my last hobby is cards with scientific experiments.
© 2009, RIMIS Publishing House, edition, design
The text and figures have been restored according to the book “Entertaining Physics” by Ya. I. Perelman, published by P. P. Soikin (St. Petersburg) in 1913.
All rights reserved. No part of the electronic version of this book may be reproduced in any form or by any means, including posting on the Internet and corporate networks, for private and public use, without the written permission of the copyright owner.
© Electronic version of the book prepared by Litres (www.litres.ru)
"Entertaining Physics" - 85!
I confess: with excitement I recently leafed through the first edition of the book - the ancestor of a new literary genre. "Entertaining physics" - so called his "first-born", born in St. Petersburg 85 years ago, its author, then little-known Yakov Isidorovich Perelman.
Why do bibliographers, critics, and popularizers unequivocally link the beginning of scientific interest with the appearance of this book? Wasn't there anything like it before? And why was Russia destined to become the birthplace of a new genre?
Of course, popular science books on various sciences were published before. If we confine ourselves to physics, we can recall that already in the 19th century, good books by Beuys, Tisandier, Titus and other authors were published abroad and in Russia. However, they were collections of experiments in physics, often quite amusing, but, as a rule, without explaining the essence of the physical phenomena illustrated by these experiments.
"Entertaining Physics" is, first of all, a huge selection (from all sections of elementary physics) of entertaining problems, intricate questions, amazing paradoxes. But the main thing is that all of the above is certainly accompanied in it by fascinating discussions, or unexpected comments, or spectacular experiments that serve the purposes of intellectual entertainment and familiarizing the reader with a serious study of science.
For several years the author worked on the contents of "Entertaining Physics", after which the publisher P. Soikin kept the manuscript in the editorial "portfolio" for two and a half years, not daring to publish a book with that title. Still: such a fundamental science and suddenly ... entertaining physics!
But the genie was nevertheless released from the jug and began its victorious march, first in Russia (in 1913-1914), and then in other countries. During the life of the author, the book went through 13 editions, and each subsequent edition differed from the previous one: additions were made, shortcomings were eliminated, and the text was re-edited.
How was the book received by contemporaries? Here are some reviews of her from the leading magazines of the time.
“Among various attempts to interest physics with a selection of the most “entertaining” things from it and with a more or less playful presentation, Mr. Perelman’s book stands out for its thoughtfulness and seriousness. It provides good material for observation and reflection from all departments of elementary physics, neatly published and beautifully illustrated” (N. Drenteln, Pedagogical Collection).
“A very instructive and entertaining book, introducing the basic laws of physics in the most ordinary and seemingly simple questions and answers ...” (“New Time”).
“The book is supplied with many drawings and is so interesting that it is difficult to put it down without reading it to the end. I think that when teaching natural science, a teacher can benefit from a lot of instructive things from this wonderful book” (Professor A. Pogodin, “Morning”).
“Mr. Perelman is not limited only to describing various experiments that can be performed at home ... The author of Entertaining Physics analyzes many issues that are not amenable to experiment at home, but are nevertheless interesting both in essence and in the form that he knows how to give to his storytelling" ("Amateur Physicist").
“The internal content, the abundance of illustrations, the beautiful appearance of the book and the very low price - all this is the key to its wide distribution ...” (N. Kamenshchikov, “Bulletin of Experimental Physics”).
And indeed, "Entertaining Physics" has received not just wide, but the widest distribution. So, in our country in Russian it was published about thirty times and in mass editions. This amazing book has been translated into English, Arabic, Bulgarian, Spanish, Kannada, Malayalam, Marathi, German, Persian, Polish, Portuguese, Romanian, Tamil, Telugu, Finnish, French, Hindi, Czech, Japanese.
Down and Out trouble started! Inspired by the success of readers and critics, Y. Perelman prepares and publishes in 1916 the second (not a continuation of the first, but the second) book on entertaining physics. Further more. His entertaining geometry, arithmetic, mathematics, astronomy, mechanics, algebra are published one after another - forty (!) scientifically entertaining books in all.
"Entertaining Physics" has been read by several generations of readers. Of course, not everyone who read it became a scientist, but there is hardly a physicist, at least in Russia, who is not familiar with it.
Now in the Russian card index of entertaining books there are more than 150 branches of science. No country has such wealth, and the place of honor among these publications belongs, without a doubt, to Entertaining Physics.
Yuri Morozov
Source of information - the website of the journal "Knowledge is Power" www.znanie-sila.ru
Foreword
This book is a stand-alone collection that is not a continuation of the first book of Entertaining Physics; it is called "second" only because it was written later than the first. The success of the first collection prompted the author to process the rest of the material he had accumulated, and thus this second - or rather, another - book was compiled, covering the same departments of school physics.
This book of Entertaining Physics, like the first, is meant to be read, not studied. Its goal is not so much to inform the reader of new knowledge, but rather to help him “learn what he knows”, i.e., to deepen and revive the basic information he already has in physics, teach him to consciously manage them and encourage him to use them in many ways. This is achieved, as in the first collection, by considering a motley series of puzzles, intricate questions, entertaining problems, amusing paradoxes, unexpected comparisons from the field of physics, related to the circle of everyday phenomena or drawn from popular works of general and science fiction fiction. The compiler used the material of the latter type especially widely, considering it to be the most appropriate for the purposes of the collection: excerpts from the well-known novels of Jules Verne, Wells, Kurd Lasswitz, and others are involved. Fantastic experiments, in addition to their temptation, can play an important role in teaching as living illustrations; they found a place for themselves even in school textbooks. “Their goal,” writes our famous teacher V. L. Rozenberg, “is to free the mind from the shackles of habit and to clarify one of the aspects of the phenomenon, the understanding of which is obscured by ordinary conditions that invade the mind of the student regardless of his will, due to habit.”
The compiler tried, as far as he could, to give the presentation an outwardly interesting form, to impart attractiveness to the subject, without sometimes stopping before drawing interest from outside. He was guided by the psychological axiom that interest to the subject increases attention, attention facilitates understanding and therefore contributes to a more conscious assimilation.
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