Why does wood crackle on fire? General repetition of program material in physics in high school in the form of solving illustrated qualitative problems

Good day, dear readers. As a child, each of us asked the question “Why?” We are growing up, and our children also often ask us this question. Some “whys” are quite difficult to answer. With that in mind, I thought it would be quite interesting and educational to answer some of the “whys.” And since there are a lot of questions, a whole series of articles will be devoted to these answers.

My article today will be devoted to answering the question why firewood crackles when burning.

Everything is quite simple and can be explained by simple physical phenomena.

Wood, like everything else organic matter, consists of tiny cells. All cells contain fluid. Like any other liquid, when exposed to high temperatures it evaporates and turns into gas. When heated, the gas expands, the pressure on the cell increases and, in the end, it bursts like a balloon.

Naturally, we won’t even notice such an “explosion” of one cell. But there are millions of cells, and it is impossible not to hear the “explosion” of so many cells.

As a result, we hear crackling and hissing noises when wood burns.

It’s interesting that no matter whether the firewood is dry or wet, we still hear a crackling sound. Why? No matter how well the firewood is dried, some moisture will still remain in it.

By the way, the drier the wood, the louder the crackling sound. And the strength of the crack also depends on the type of wood. Scientists still haven’t really figured out why this happens. Most likely, it depends on the structure of the wood itself and the presence of resins in it.

What associations do you have when you hear the words “burning wood”? A melted fireplace or stove, a blazing fire; pleasant warmth and heat from an open fire; flying sparks and, of course, crackling sounds. Few people think about why firewood crackles. Depending on the type of wood and the degree of its drying, logs can either make almost no acoustic noise, or continuously hiss, click, crackle... In silence, wood never burns.

Perhaps as a child you loved books and TV shows from the “Fun Physics for Kids” series. Simple phenomena that occur every day were popularly explained empirically. Try repeating a similar elementary experiment from childhood.

Open the burning stove or sit next to the fire, and then hold a metal ladle, ladle or spoon upside down to the flame. The surface of the metal will soon be covered with droplets. If you bring the dishes even closer to the fire, they will turn black with soot.

Dip a burning stick into a deep vessel at least twice. The flame will go out, and each time faster. The fact is that it accumulates in the container carbon dioxide. Water, coal and gas - this is what an armful of firewood in your stove will ultimately turn into.

It is believed that good firewood(producing a lot of heat) must be dried before use until the wood moisture content is approximately 15-16% (the permissible maximum is 25%). However, even the driest tree retains moisture: this is the so-called “capillary” water in the cavities of the cells, and “colloidal” water in the cell membranes, and a very small amount of chemically bound water.

Maybe you heard loud bangs during fires. These were episodic emissions of hot gases from pyrolysis - a process during which the thermal decomposition of wood into various chemical elements. Experts have even learned how to heat wood in a special way without access to air (or with limited access) for various production needs.

So, the burning logs crackle and, interestingly, each in its own way. If you put dried beech wood in the stove, you may hardly hear the familiar crackling sound. The aspen will crackle peacefully, while the pine in the flame will simply “shoot” - the accumulated resin will be released from its cavities and fill the room with a pleasant ether.

Burning wood not only “talks” in its own way, but also has different distinctive features. For example, the same fragrant pine should not be used in an open fireplace - sparks will fly. It’s better to burn cherry or apple tree branches there and enjoy the warmth and pleasant aroma. If you use a home fireplace or bathhouse with wood stove, be sure to consult with a specialist before preparing firewood.

It is well known that when burning logs (including logs of buildings during fires) hiss, crackle, click, “shoot” and even explode. There are many reasons for such acoustic phenomena, and it would be strange if the firewood was completely “silent” during combustion.

The burning logs crackle and, interestingly, each in its own way. If you put dried beech wood in the stove, you may hardly hear the familiar crackling sound. The aspen will crackle peacefully, while the pine in the flame will simply “shoot” - the accumulated resin will be released from its cavities and fill the room with a pleasant ether.

Burning wood not only “talks” in its own way, but also has other distinctive features. For example, the same fragrant pine should not be used in an open fireplace - sparks will fly. It is better to burn cherry or apple tree branches there and enjoy the warmth and pleasant aroma. If you use a home fireplace or sauna with a wood-burning stove, be sure to consult with a specialist before preparing firewood.

It is believed that good firewood (which produces a lot of heat) should be dried before use until the wood moisture content is approximately 15-16% (the permissible maximum is 25%). However, even the driest tree retains moisture: this is the so-called “capillary” water in the cavities of the cells, and “colloidal” water in the cell membranes, and a very small amount of chemically bound water.

When the fire flares up and the wood is covered with the first charred layer, all the water turns into steam. The continuous crackling of logs in the stove is nothing more than a series of real micro-explosions. The charcoal fibers break and water vapor escapes. Charred wood fragments bounce off each other, making loud clicks. All this indicates the beginning of pyrolysis - a process during which the thermal decomposition of wood into various chemical elements occurs.

Acoustic noise during combustion can be divided into continuous and one-time (episodic). Continuous noise manifests itself in the form of hissing (during the turbulent outflow of water vapor and gaseous pyrolysis products from the pores of wood) and in the form of a hum (from turbulent flame combustion). Conditionally continuous noise also includes the well-known crackling of wood during combustion, caused by brittle cracking of charcoal across the fibers, so that characteristic cracking occurs only after the appearance of a charred layer and clearly indicates that combustion has begun (and not just rapid drying).

Episodic clicks are caused by rebounds of fragments (scales) of the charring layer. In this case, any warping leads to brittle cracking of the carbon layer. So even burning paper begins to “rustle” only after charring and with the obligatory shrinking and bending of the hot carbon residue. And when massive logs warp, sharp splits are possible not only in the charred layer, but also in the wood, including due to the pressure of water vapor and pyrolysis gases deep in the wood. Emissions of hot gases are accompanied by pops and explosions during fires.

However, the reasons why wood cracks when burning is not completely clear. Thus, some believe that sanded birch cracks much less when burning than aspen, because birch has the smallest difference in tangential and radial shrinkage (and therefore does not warp), while aspen has the largest. At the same time, beech with a high shrinkage difference does not crack at all when burning. Therefore, others believe that the harder the wood (and the higher the tensile strength), the less it will crack. In any case, low-strength fir cracks very strongly when burning. It is difficult to understand and find the true connection between the crackling noise during combustion and cracking. Thus, species for artistic carving are divided into highly cracking (hornbeam, ash, maple, beech) and weakly cracking (spruce, pine, fir, cedar, larch).

Aspen is popularly considered an extremely controversial wood. If firewood from ash and birch burns even when damp, then freshly cut “aspen will not burn without kerosene.” Aspen is rarely harvested for firewood, since (like pine) it is used for construction (in particular, log houses for baths). It is believed that aspen is “uncookable”, produces “little heat”, burns quickly when dry and burns soot out of chimneys. Aspen smokes little; it has long been used to pluck a torch for lighting, and now it is used to make matches.

When burning, aspen crackles (“sparks”, “finely” cracks) in contrast to pine, for example, which rarely produces a crackling sound, but powerfully (“shoots”).

Perhaps as a child you loved books and TV shows in the "Fun for Kids" series. Simple phenomena that occur every day were popularly explained empirically. Try repeating a similar elementary experiment from childhood.

Dip a burning stick into a deep vessel at least twice. The flame will go out, and each time faster. The fact is that carbon dioxide accumulates in the container. Water, coal and gas - that’s what an armful of firewood in yours will ultimately turn into.

Maybe you heard loud bangs during fires. These were episodic emissions of hot pyrolysis gases, a process during which the thermal decomposition of wood into various chemical elements occurs. Experts have even learned how to heat wood in a special way without access to air (or with limited access) for various production needs.

So, burning logs and, interestingly, each in its own way. If you put beech firewood in dried wood, you may hardly hear the familiar crackling sound. The aspen will crackle peacefully, while the pine in the flame will simply “shoot” - the accumulated resin will be released from its cavities and fill the room with a pleasant ether.

Burning wood not only “talks” in its own way, but also has other distinctive features. For example, the same fragrant pine should not be used in a fireplace - sparks will fly. It is better to burn cherry or apple tree branches there and enjoy the warmth and pleasant aroma. If you use a home fireplace or sauna with a wood-burning stove, be sure to consult with a specialist before preparing firewood.

Questions:

Why does the fire crackle?
Why does a freshly cut tree crack less in the fire than a dry one?
Where does brushwood go when thrown into a fire?
The smoke from the fire rises upward in a column, rather than spreading low. As luck would have it, this portends good weather for tomorrow. Why?
Why, wanting to light a fire faster, do they blow on it?
Why does water extinguish fire?
Why do objects on the opposite side of the fire seem to oscillate?
Why doesn't the flame go out on its own?
How long will it take to cook food in the mountains over a fire?
Why shouldn’t you worry that the stick on which the bucket of water is suspended will burn out?
Why does a spruce tree crack more than others in fire? tree species?
If chestnuts are placed on hot coals, they split with a loud crack. Why?
Even on a hot day, if you pick a leaf from a tree exposed to the sun and apply it to your cheek, you will feel that it cools your face. Why?
On a hot summer day, birds' feathers become ruffled. Explain why?
During the heat, the face becomes red. What causes this?
Why does a dog stick out its tongue in extreme heat?
Why does a person's skin become covered with sweat in hot weather?
Cracks on the surface of rocks or mountains most often form on a hot summer day. Why?
Everyone knows from experience that going uphill is difficult. And why?
Why do echoes appear in the mountains?
Why do we only hear echoes at great distances?
Why does a person, when entering a space where the pressure is lower than atmospheric pressure, for example, on high mountains, often experience pain in the ears and even throughout the body?
Why is the function of the joints disrupted on high mountains: the limbs do not obey well, and dislocations are easy?
What is the significance of atmospheric pressure for the articulation of bones in our body?
People in the mountains a short time get very sunburned. Why?
Are ultraviolet rays beneficial or harmful to the body?
Why do people's pupils narrow during the day and dilate at night?
Why does the pupil of our eye appear black?
Centuries of experience show that lightning most often strikes tall deciduous trees, mainly solitary ones. Consequently, such trees are a good conductor for atmospheric electricity. Why is a person caught in a thunderstorm warned not to hide under the trees? Why does a lightning rod divert lightning from a person, but a tree, on the contrary, attracts it to him?
Very often in the mountains you can see the tips of hair, metal parts of a camera, and the tips of a dog’s ears glowing. What kind of glow is this? What is it a harbinger of?
Why does water in a metal flask heat up quickly?
The water in the bucket is shaped like a bucket. Does a liquid have its own shape?
Why is a fatty sausage easier to chew and swallow than a lean one?
Most people like hot soup more than cold soup. Why is this so?
What are the most important chemical reactions occur in green leaves of trees under the influence of light?
Why are there no leaves on the bottom of the trees?
Can sound bounce off clouds?
Why do cameras usually use a short focal length lens?
What laws are the design and operation of a camera based on?
What image does the film produce?
Why a hat yellow color, and the backpack is green?
Why are backpack straps made wide?
Why is a sports jersey most often made from knitwear?
Why is a knife blade made sharp?
Do humans have fangs like dogs? What are they needed for?

Answers:

Wood cells contain moisture. At the stake under the influence high temperature it quickly turns into steam, which tears the shell with force. You hear the sounds of small explosions throwing off pieces of heated wood.
Because its pores are filled with juice and contain less air.
When brushwood burns, it turns into water, coal and carbon dioxide. These combustion products are carried upward by streams of hot air. Only a little ash remains.
The air contains small amounts of tiny water particles. Therefore, coal dust particles flying away from the fire do not mix in the air with drops of water and do not become heavier, but rise very high.
During the combustion process, non-combustible substances are formed - carbon dioxide and water vapor, which are unable to support combustion. By blowing off these substances, we provide access to air that supports combustion.
Touching burning branches, water turns into steam, taking away a lot of heat from the burning body, and also the vapors formed when water evaporates surround the branches and push away the air, and without air, combustion is impossible.
The air from the fire heats up unevenly and becomes heterogeneous (has different densities). Air currents move all the time. Light rays in such a heterogeneous medium are refracted differently, the picture changes all the time. The image “floats”.
The gases produced during combustion expand when heated and become lighter. Only thanks to this, the heated combustion products do not remain at the place of their formation, in direct contact with the flame, but are immediately forced upward clean air. Archimedes' law applies to gases, so the flame does not go out by itself.
Air pressure decreases with increasing altitude above sea level. Consequently, the boiling point of a liquid decreases with increasing altitude, which means the time for cooking food increases.
Water has a greater heat capacity than wood (almost twice), so when heated, water will “take on” the main heat coming from the fire.
Because its pores are relatively large and contain a lot of air (see answer 12).
The air under the chestnut shell expands when heated and bursts it with a bang, causing compactions and rarefaction of the air, that is, sound waves.
Water rising through the capillaries of the tree to the leaves evaporates especially strongly through their stomata. As you know, the process of water evaporation occurs with the absorption of heat, which is why the surface of the leaf is cool.
Birds, unlike other warm-blooded organisms, do not have the process of evaporation from the body surface, which is important in hot weather, since they have dry skin and dense feather cover. But there is another adaptation that helps to withstand the heat: birds change the slope of their plumage depending on the degree of heating by the sun's rays. In hot weather, the bird's feathers become ruffled, which protects it from overheating.
During hot weather, intense heat transfer occurs from the human body through the skin into the surrounding air. Warmth from internal organs transported to the skin via blood streams. Obviously, a person will lose more heat the more it is transferred by blood to the skin. In hot weather, blood vessels dilate greatly, and much more blood passes through them than usual. The face has many superficial blood vessels. The increase in blood flow to the skin causes facial redness.
The evaporation of sweat from the animal's body promotes heat exchange, but the dog's sweat glands are located only on the pads of the fingers, therefore, in order to increase the cooling of the body on a hot day, the dog opens its mouth wide and sticks out its tongue. The evaporation of saliva from the surface of the mouth and tongue lowers her body temperature.
Sweating in the heat is the body's ability to fight overheating. The sweat secreted by the glands evaporates from the surface of the body and thus cools it.
On a hot day, the surface of the rocks quickly heats up, while the inner layers of large stones remain cold. As a result of thermal expansion, the surface layer of stones increases in size. Since at this time the inside of the stones has the same dimensions, enormous tensions arise between them. The expanded surface tends to stretch the interior or tear away from it. As a result, the stones may crack.
Moving on a flat road, we spend muscle strength mainly on overcoming friction and air resistance. When climbing, you have to overcome not only these forces, but also part of your own weight.
An echo is a sound wave reflected by some obstacle and returning to the place from where it began to propagate. For an echo to occur, there must be an obstacle from which the sound wave can be reflected. In mountainous areas, this obstacle is the mountains, which is why the echo occurs.
When you hear an echo, you hear sound waves reflected from distant objects and returned to you. In small rooms you cannot hear the echo because the walls are too close to you. The sounds come back to you very quickly, making it impossible to hear all the components of the echo separately.
This is explained by the fact that in the human body there are a number of cavities containing air, for example the intestines, middle ear, frontal and upper jaw bones. The air pressure in these planes is equal to atmospheric pressure. When the external pressure on the human body decreases rapidly, the air inside us begins to expand, putting pressure on various organs and causing pain.
Atmospheric pressure promotes a tighter fit of the joints to each other. With a decrease in pressure, when climbing high mountains, the connection between the bones in the joints, as a result, the limbs become difficult to obey, and dislocations easily occur.
Due to the absence of air between the mirror smooth surfaces atmospheric pressure firmly presses the joints together. In order to separate them, as in the experiment with the Magdeburg hemispheres, it is necessary to apply significant force.
Air strongly scatters ultraviolet rays. At high altitudes, where the air is thin, ultraviolet radiation is very intense. The body produces a protective pigment - tan.
The effect of ultraviolet rays on the retina of the eye is great and destructive. Not allowed in the mountains long time remain without clothes and without dark glasses. In small doses, ultraviolet rays have healing effect, promote growth and strengthening of the body, stimulate a number of vital functions in the body, and have a bactericidal effect.
During the day there is a lot of light, and it blinds the eyes. In order not to let too much light into the depths of the eye, the eye muscles constrict the pupil. In the evening, the illumination decreases, the pupil dilates, and light freely passes into the depths of the eye.
Multiple reflections of rays occur in the eye. The pupil in this case resembles a hole in a closed vessel.
Under no circumstances should you think that if you stand under a lightning rod during a thunderstorm, it will always protect you from a lightning strike. If you stand even a short distance from a lightning rod, an induced charge will form in your body at the moment of a lightning strike. A discharge in the form of a spark can easily occur between it and the lightning rod charge. All these considerations apply to tall, solitary trees. If you are standing in the steppe at a distance of tens of meters from a lonely standing tree, then you are better protected from a lightning strike than if there were no tree. If a person is near a tree, then it may happen that lightning in some cases chooses its path through the human body, since it is the same conductor as the tree.
The described phenomenon is called “St. Elmo’s Fire.” This is a very rare natural phenomenon. A bluish light appears on the tips, on the ends of the ears of animals, and sometimes even on the heads of people. This is a silent discharge - the movement of electrical charges in the air at atmospheric pressure and high voltage. This phenomenon is a harbinger of a thunderstorm.
The thermal conductivity of the metal is quite high.
Natural shape any liquid is a sphere. Typically, gravity prevents a liquid from taking the shape of a ball, so it either spreads out in a thin layer if it is poured without a container, or takes the shape of a container if it is poured into one.
Fat acts as a lubricant and reduces friction.
The “physical” advantage of hot soup over cold soup is its better mixing with gastric juice (with high temperature molecules move faster and faster, mixing due to diffusion)
Leaves absorb carbon dioxide CO 2 from the air and break down its molecules into their component parts: carbon and oxygen. This happens in chlorophyll molecules under the influence of red rays of the solar spectrum. This is the process of photosynthesis. By attaching atoms of other elements extracted from the earth by roots to the carbon chain, plants build molecules of proteins, fats and carbohydrates.
The main condition for leaf growth is photosynthesis. And photosynthesis occurs only in the presence of light. Little light falls on the lower part of the trees due to the shadow cast by the upper branches.
Sound can be reflected not only from solid obstacles, but also from such delicate formations as clouds. Moreover, even completely transparent air can, under certain conditions, reflect sound waves, precisely in the case when, in its ability to conduct sound, it differs in some way from the rest of the air mass. A phenomenon similar to what is called “total reflection” in optics occurs here. The sound is reflected from an invisible obstacle, and we hear a mysterious echo coming from nowhere.
Because the image must be located close to the lens, since the distance between the lens and the film is limited by the size of the camera.
Laws of geometric optics.
Real inverted, diminished.
Each tissue reflects light of a certain wavelength, absorbing others. The hat reflects yellow from all the rays falling on it, the Backpack reflects green, and all other rays are absorbed.
The weight of the backpack is distributed over large area, and the pressure on the shoulders will decrease.
Due to its structure, knitwear stretches well, so a knitted T-shirt fits tightly to the human body. And since heat exchange increases during physical exercise and the athlete is exposed to sharp temperature fluctuations, knitwear, being hygroscopic, absorbs sweat and, as it were, regulates evaporation, preventing hypothermia and overheating of the body.
It is easier to cut with such a knife, since the blade has a small area, therefore, with the help of even a small force, it creates a lot of pressure, and such a tool is easy to work with.
There are fangs, only a little smaller, since a person eats mostly boiled, which means soft food. Fangs are needed for tearing tough and hard food, since with a small effort of the chewing muscles, with the help of sharp teeth, you can create a lot of pressure on the object.

Questions:

How can you make various figures from sand?
Why does the shape of the figures remain the same when the sand dries?
Why does the fisherman have an “unsuccessful” catch?
Why do people on the beach sunbathe and even get sunburned?
Why do you need to stay on the beach under an umbrella or light clothing?
Why is it harder to walk on sand than on a hard surface?
Why is it advisable to wear light-colored clothes in summer?
Why does a person come out of the water, even in hot weather, feel cold?
Why is it better to wear clothes colored yellow and yellow in summer? orange colors, and you shouldn’t wear bright blue or purple clothes?
Why is it easier to walk on sand with solid soles than with heels?
A poorly inflated ball, after lying in the sun, “swelled up” as if it had been pumped up. Now boys can play football. Explain the reason for the increase in the volume of the ball.
In the picture, the ball is thrown vertically upward. What energy transformations occur during the movement of the ball?
Why is the frame of a sun lounger made of wood or plastic, and not metal?
Explain why albatross birds that live in coastal areas, can they stay at the same height for a long time without flapping their wings?
What type of deformation is experienced by: a) fishing rod; b) fishing line on a fishing rod; c) the leg of a chaise lounge; d) chaise lounge seat.
Why is the sea water temperature on a hot day in summer lower than the ambient air temperature?
How is the shadow of a sun lounger formed?
A person puts a can of Coca-Cola to his mouth and “sucks” its contents into himself. How does he do this?
Why did circles form in the water around the fishing line?
Why do ridges bend? sea waves, running ashore?
Why are children not allowed to stay in the water and in the sun for a long time?
Does all radio emission from the sun pass through the atmosphere?
Where are objects heavier: in water or on land?
Water is lighter than sand. Why can the wind raise clouds of sand, but very little water spray?
How does a radio wave travel from a radio receiver when a radio transmission passes near the sea surface?
What is the “voice of the sea”?
Why does a cup or a large sink make noise when you put it to your ear?
Why doesn’t the sound that occurs under water when one stone hits another reach a person in the air?
Why is the sky blue?
How does a person fall, when does he trip and when does he slip?
In which position is a person more stable: when he sits or when he stands? Why?
In what case does a person do a lot of work: when he moves in small steps or when he takes large steps?
Can the center of gravity of a human body lie outside the body, and if so, under what conditions?
Explain why a person can lie on the water with his hands under his head.
Why does a person lying calmly on the water have his legs sink deeper into the water when he inhales?
Why is it much easier to float on the surface of the water in the sea than in a river?
Why is our eye not adapted to perceive ultraviolet rays?
Who gets hotter in the sun: a well-tanned person or not a tanned person at all?
Why is an oil film on the surface of the water harmful to birds?

Answers:

To make sand figures, you need to moisten the sand with water. Water molecules interact with the molecules of sand grains, causing the wet sand to take the shape of a vessel.
When water molecules evaporate, the sand molecules come closer to the distance at which the attractive forces between the molecules of the sand grains themselves act.
There is a lot of noise on the seashore. Sound waves travel first in the air, then reaching the surface of the water, causing vibrations of water particles to which fish react.
The air on the coast is clean. Ultraviolet rays are weakly absorbed by the atmosphere. It is the action of these rays that causes tanning. Prolonged exposure to these rays can cause burns.
Umbrellas and clothing reflect and absorb light rays, which prevents burns.
The sand particles interact weakly with each other, so the feet sink deep into the sand, and with each step it becomes difficult to move quite a lot of sand and crush it with your feet. This additionally wastes energy, so walking on sand is more difficult.
Dark surfaces absorb more visible radiation but reflect less, while light colors absorb much less light energy but reflect more.
Because water from the surface of the skin quickly evaporates, which leads to cooling of the skin.
Because colors close to violet in the spectrum fade faster than colors close to red, as the chemical activity is greater, the shorter the wavelength.
How smaller area the soles of the shoes, that is, the contact area, the greater the pressure on the sand, therefore, the deeper the foot sinks into the sand.
As the temperature rises (under the influence of sunlight), the speed of movement of the gas molecules inside the ball increases, as a result of which they more often hit the walls of the ball's container. The gas pressure inside the ball increases, and the ball increases in volume.
At the moment of throwing the ball has kinetic energy. As the ball moves upward, its kinetic energy decreases, and its potential energy increases as the height of the ball increases.
The frame of the sun lounger is not made of metal due to the fact that in thirty-degree heat it will feel hotter to the touch than wood, since metal has better thermal conductivity.
A bird “soaring” at a height is supported by rising convection currents of warm air.
Bend; stretching; compression; bend
When a liquid evaporates, the fastest moving particles leave it, and the temperature of the evaporating liquid decreases. Thus, evaporation occurs due to a decrease internal energy liquids.
A shadow is formed due to the fact that the sun's rays propagate rectilinearly in a homogeneous medium, and from an obstacle whose dimensions longer light waves are reflected.
When drinking, we expand our chest and thereby thin out the air in our mouth; under the pressure of the outside air, the liquid rushes into the space where the pressure is less, and thus penetrates into our mouth.
Circles formed in the place where the fishing line comes into contact with the surface of the water, that is, in this place the particles began to oscillate, these vibrations began to spread from particle to particle in all directions, i.e. waves appeared.
The bending of wave crests running onto a flat shore is explained by the fact that the speed of wave propagation over the surface of a shallow reservoir depends on the depth of this reservoir. When a wave runs over a shallow part of the sea, its crest rises above the bottom more than the valley of the wave; therefore, the ridge must move faster than the valley ahead of it, and, overtaking it, bend forward.
Children's bodies are less resistant to adverse influences external environment: the human (child) body has good heat transfer, due to which it heats the water around it, and because Since the water near the body is in constant motion, it is constantly cold and takes a lot of heat from the body, which can lead to hypothermia.
Radio emission does not completely pass through, since the earth's atmosphere in the radio range is transparent to waves with lengths ranging from several mm to several m.
Objects are heavier on land, since a body in a liquid is subject to a buoyancy force from the water (Archimedes' force).
Water molecules are attracted to each other. Therefore, to tear off a molecule or group of molecules (a drop) from the surface of water, noticeable energy must be expended. Sand molecules are also attracted to each other. However, due to irregular shape grains of sand, the number of points at which they come into contact with each other is small. Therefore, a relatively small force can separate grains of sand from each other. A drop of water comes into contact with the thickness at all points of contact. Therefore, it is much more difficult to tear it off the surface of the water.
Sea water contains dissolved salts, i.e. is an excellent current conductor. Therefore, it “holds” the radio wave due to repeated reflection from the surface of the water, forcing it to move along the surface of the sea.
“Voice of the Sea” - infrasonic vibrations generated by wind turbulence on the crests of sea waves. Lightning discharges are also a source of infrasonic vibrations.
The noise that we hear when we put a cup or a large shell to our ear occurs due to the fact that the shell is a resonator, amplifying numerous noises in the environment around us, which we usually do not notice due to their weakness. This mixed sound resembles the roar of the sea - which gave rise to various legends that have developed around the noise of the shell.
When sound passes from water to air, 99.9% of the energy is reflected and the sound pressure sharply decreases. Because of this, the sound that occurs under water when one stone hits another does not reach a person in the air.
Reason blue color the sky is scattered sunlight in the earth's atmosphere. Since light waves with higher particles scatter more intensely, the spectrum of scattered light is shifted towards higher frequencies, hence the blue color of the sky.
When a person stumbles, the legs stop, but the body continues to move, so the person falls face down. When a person slips, he falls mainly on his back.
When a person sits, his center of gravity is lower than when he stands. As you know, a more stable position is when the center of gravity of the body occupies a lower position.
When a person takes small steps, the work of lifting his own body will be less, since the center of gravity rises a small distance.
The position of the center of gravity in the human body changes depending on the position of the torso and limbs. If a person bends over, then his center of gravity may be located outside his body.
The weight of the human body, if the cavities of the lungs are filled with air, although not much, is still less than the water displaced by it, so a person can lie freely on the water with his hands under his head. But as soon as you stick at least one hand out of the water and thereby reduce the volume of the submerged part of the body, the buoyancy force decreases and the head is completely immersed in the water. A person who does not know how to swim randomly kicks the water with his hands, which is not necessary; sticks his hands out of the water, trying to grab onto something, and at the same time his head goes under the water.
During entry, the volume of the chest increases, therefore, according to Archimedes' law, it begins to be pushed out of the water with greater force, while turning the entire human body.
The density of sea water is slightly greater than the density of river water, therefore, according to Archimedes’ law sea water pushes the body out with greater force.
The retina of the eye is very sensitive to the perception of ultraviolet rays, but they are absorbed by the lens and therefore do not reach the retina.
The formation of a brown tan is the body's self-defense from excessive exposure to ultraviolet and violet rays: high-frequency rays are strongly absorbed by this pigment, causing only harmless heating. Therefore, a well-tanned person heats up in the sun more than a non-tanned person, but he does not experience the harmful chemical effects of light rays.
The plumage of birds creates a light and durable movable surface of the wings, and also serves to insulate the body. The feather cover is a layer filled with air bubbles, has a streamlined shape and is water-repellent on the outside. Although the role of the secretion of the coccygeal gland in water repellence is great, the most important has a tight adhesion of the microstructures of the contoured wings. Unlike most aquatic animals, their outer covering is homophilous, i.e. oil penetrates very easily into the bird's body. A small amount of oil is enough to cause damage to the structure of the wing: water fills the spaces that usually contain air, disrupting thermal insulation and fluidity. The bird becomes heavier and its swimming movements are hidden; it cannot fly. A spot of oil on a bird's chest a few centimeters in diameter is enough to cause death, especially in cold waters. If the birds do not die, then they begin to continuously clean themselves, while the structure of their wings is disrupted, they partially swallow oil, which leads to their illness.

Questions:

How can we explain that a dragonfly floats in the air?
Why do fish have a streamlined shape?
How can we explain that the color is various items different under sunlight?
Why does water with mineral salts dissolved in it get to the stems and leaves from the roots located deep in the soil?
Why do trees bend?
Why do insects crawling along the stems of aquatic plants, encountering an air bubble on their way and penetrating inside it, become invisible?
Why do air bubbles shine?
Why does a bug climb up an almost vertical tree trunk and not fall from it?
Why is it necessary to wear dark sunglasses in summer?
Why is it cooler in the shade of trees than outside? open place?
Why do sand and stones heat up much more than a beach covered with grass?
Why does water in clean, transparent reservoirs have more low temperature than in muddy bodies of water?
What color would the sky be if there was no atmosphere surrounding the earth?
Why is the frog always cold to the touch?
Optimal temperature butterfly body32.5 0 -35.5 0 N.V sunny weather the butterfly maintains it regardless of the temperature environment. How does she do it?
Why, if you dive underwater, do all objects seem blurry, with unclear outlines, and very small objects are not visible at all?
Why do trees seem further away from us in wet weather than they really are?
Why are most animals of the Far North white, and those whose color is different, for example, a squirrel, a hare, change it to white in winter?
Why are insects living in polar regions and high mountain areas predominantly dark in color?
Why can't turtles turn over on their backs on their own?
Why does the bottom of a pond appear raised to the eye?
Why is extreme heat worse in swampy areas?

Answers:

This is explained by the fact that by spreading its wings, the dragonfly increases the area of support for the air, so it is held in the air.
With this shape, the drag force from the liquid is less than with other shapes, so they can move through water at high speed, although the drag force in liquid is much greater than in air.
This is explained by the fact that white light is complex, and objects are painted in different colors because of all the colors contained in white light, they reflect only certain colors, and absorb the rest.
In the stems of plants, roots, leaves there are a lot of thin channels - capillaries through which they rise nutrients. The thinner the capillary, the higher the level of liquid rising in it.
This is a deformation that has occurred in the tree trunk due to constant wind, the direction of which does not change, and a residual deformation has occurred in the trunk.
The air bubble formed on the stem represents the interface between two media: water-air, the refractive index of water is greater than the refractive index of air, therefore, at this boundary the rays pass from a more optically dense medium to an optically less dense one. The phenomenon of total internal reflection occurs.
The interface between two media, water-air, is transparent, but as rays move from an optically denser medium to a less dense medium, the phenomenon of total internal reflection occurs.
The beetle's legs are rough and so is the tree trunk, so the frictional force is great, and it is held by this force.
Glasses have lenses, but the glasses block ultra-violet rays, destructive to the retina of the eye. Dark glasses also protect from bright light; dark glasses also absorb light rays, weakening their effect on the eyes.
Because tree leaves and trunks protect us from direct exposure to rays, that is, from radiation.
The thermal conductivity of sand and stones is better than the thermal conductivity of green leaves. In addition, between the leaves and blades of grass there is a lot of air with poor thermal conductivity. And green leaves are more reflective than brown, dark sand.
Rays through clear water penetrate to great depths, being weakly absorbed. In muddy reservoirs there is a lot of clay, silt, and many particles with a dark surface that absorb visible light and emit infrared light, which is retained and heats the water.
Even in daytime the sky would be black color, and the stars and the sun would be visible on it.
The frog's body is covered with a mucous film that reflects heat rays and... evaporating, cools the frog's body.
Butterfly supports constant temperature your body; using wings. The wings receive the most heat if the sun's rays fall perpendicular to them. The greater the angle of incidence, the weaker the heating. As soon as the body temperature reaches 35C, the butterfly changes the position of its wings until it finds a position in which the amount of heat received will steadily maintain the desired temperature
In the air, the outer cornea of the eye collects light rays, creates an image on the retina, and the lens only helps a little in this. However, under water, the effect of the cornea is reduced to zero due to the fact that the refractive indices of water and the liquid inside our eye are almost the same and the rays, without being refracted, pass directly through the cornea. Under water we become farsighted.
The fog scatters some of the light reflected from the trees. Since the trees appear to be weakly consecrated, it seems that they are further from us than they actually are.
A white animal emits less heat into the surrounding space, which is especially important in the conditions of the Far North.
Dark color absorbs heat rays well. This allows insects to have a body temperature in sunny weather significantly higher than the ambient temperature.
An inverted turtle is like a heavy spherical segment lying on a convex surface. Such a segment is very stable, and in order to turn it over, you need to raise its center of gravity high enough.
At the water-air boundary, light is refracted and this leads to the feeling that everything is immersed in water above its true position. The bottom of a pond, river, or reservoir appears to the eye to be raised to almost a third of its depth. The reason is the refraction of light rays.
In swampy places, even in warm weather, there is high air humidity, so sweat evaporates slowly. When sweat evaporates, the human body cools down a little and the heat is easier to bear.

Why does wood crackle on fire? General repetition of program material in physics in high school in the form of solving illustrated qualitative problems

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