Before I knew it, 97% of 2020 had been loaded, and beautiful lanterns were hung up in many places to welcome the new year.
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Christmas is a festival that Western children are full of expectations for, and it is also a period for giving gifts. Although I know that Santa Claus is a fictional character like Sun Wukong, I am still curious about how Santa Claus distributes gifts?
If Santa Claus only delivers gifts to children aged 18 and under, according to the sixth census data of the National Bureau of Statistics, the population aged 0-18 is about 5.3 billion , and my country’s land area is about 9.6 million square kilometers . Assume that Santa Claus Working only in China 24 hours , and 300 million friends are evenly distributed on the land of our country, then Santa Claus needs to run from one child to the next in 1/3472 seconds, and his average speed is estimated to be :
is about two thousandths of the speed of light and two thousand times of the speed of sound . For Santa Claus to move at such a high speed, in addition to requiring a very powerful electronic reindeer, it will also produce many interesting physical phenomena, such as " sonic boom ". You may feel that it has considerable power just from the name of this physical phenomenon.
《Simpsons》
In fact, sonic boom phenomenon is also very common in our daily life. You must have seen whipping, right? Take a slightly longer rope with a softer tail and swing it down hard. After each whip is thrown out, you can hear a crisp and loud "pop" sound. In fact, this is the sonic boom phenomenon . Under normal circumstances, the vibration of the whip does not make such a loud sound. It can make such a loud sound because the tail of the whip moves very fast, reaching supersonic speed , and every time the whip is whipped, the tail end of the whip A small "sonic boom" produced when the speed of sound is exceeded. It's awesome, don't underestimate the little whip in your hand, we can use it to create supersonic effects at any time.
What is a sonic boom
So what exactly is a sonic boom?
We know that no matter what object is moving in the air, it will always encounter resistance . This resistance will cause the object to be disturbed, especially when the speed is close to the speed of sound (the speed of sound in the air is about 1 standard atmosphere and 15°C) 340 m/s), this resistance will increase sharply, causing the object to vibrate strongly and the speed to decay, like an invisible barrier. We call it " sound barrier ". However, if we apply a strong enough propulsion force to this object and let it reach supersonic speed and break through the sound barrier, a powerful shock wave will be generated, and our ears will also hear a "pop" explosion. This It is the formation of sonic boom .
Aircraft breaks the sound barrier | Source: Imgur.com
Therefore, to put it simply, sonic boom is the sound caused by the huge shock wave generated when an object moves in the air exceeding the speed of sound. Usually the powerful sonic boom phenomenon is produced by supersonic fighter or other supersonic flight aircraft flying at transonic speeds. In fact, the first time humans broke the sound barrier and produced a sonic boom was also accomplished by an airplane.
On October 14, 1947, American Charles E. Yeager (Charles E. Yeager) piloted a Bell XS-1 rocket engine aircraft "Glamorous Glennis" (Glamorous Glennis) to 1.015 Flying over the Edwards Air Force Base in California at a speed of Mach (1078.23 kilometers/hour), accompanied by a cloud of fog and a loud noise in the sky, this was the first time that humans broke the sound barrier, and this pilot was also the first The one who left his voice behind.
Charles E. Yeager
The principle of sonic boom
So how is sonic boom produced?
Just like throwing a stone into a lake and creating circles of ripples on the water surface, the movement of objects in the air will squeeze the air, causing disturbance and forming waves.
Source: xuite.com
In fact, the propagation of sound in the air is realized by waves. We can imagine sound waves as circles emanating from a small dot (sound source) in all directions. Each circle represents Sound "waves" in the air.
Sonic Boom | Source: wiki
When the sound source is stationary, the sound waves generated are concentric circles. When the sound source starts to move, we will find that the distance between the sound wave circles in front of the direction of movement will become closer. , the frequency of the sound we receive will become higher, this is the Doppler effect , which is why the roar of the car approaching you on the road will become sharp.
Doppler effect | Source: wiki
When the speed of an aircraft is close to the speed of sound, it can catch up with the sound waves it emits - the sound wave circle emitted at the previous moment spreads to a position, and the aircraft has almost moved to the same position. This position, so these sonic circles will be squeezed together. The closer the speed is to the speed of sound, the closer these sound wave circles will be squeezed, forming a shock wave surface . When the aircraft reaches and exceeds the speed of sound, the compression of the air cannot be spread out in time, and gradually compresses and accumulates in front of the aircraft, forming a line Invisible wall, when the aircraft breaks through this wall, the energy of the sound wave is highly concentrated at the head of the aircraft. When this energy is transmitted to the human ear, we can hear a short and strong explosion, which produces sonic boom .
Through the following figure, we can more intuitively understand the principle of sonic boom . At the head of the aircraft, the pressure of the air surges. After passing through the shock wave surface, the pressure decreases and quickly rises to normal pressure at the tail of the aircraft. , the change in pressure presents an N shape, so it is also called N-shaped wave . When the pressure of the air changes drastically, a sonic boom will occur. Therefore, when the aircraft is flying at supersonic speed, we should feel two sonic booms. The first time is when the air in front of the nose is violently compressed and the pressure surges. , the second time was caused by the air pressure at the tail of the aircraft suddenly rising back to normal. The intensity of the two sonic booms was basically the same.
The generation of sonic boom clouds
As mentioned in the previous article, when an aircraft moves at high speed and breaks the sound barrier, not only a sonic boom phenomenon may occur, but also a cone-shaped cloud and mist may appear at the tail of the aircraft, which looks like the aircraft is dressed in white. cloak, but it will disappear without a trace immediately. This is the so-called sonic boom cloud (scientific name is Prandtl-Glauert condensation clouds, Prandtl-Glauert condensation clouds).
So how did the sonic boom cloud come about? ? In fact, when the aircraft moves at supersonic speed, the principle of forming a shock wave at the head of the aircraft is exactly opposite. At the tail of the aircraft, the air flow speed will temporarily fail to keep up with the movement of the aircraft, resulting in a sparse effect . As described in the previous pressure distribution diagram, the pressure near the tail will be lower than the surrounding pressure.
This rarefaction effect will cause the air to cool down briefly. If the humidity and temperature of the atmosphere are suitable, the water vapor in the air may condense into small water droplets due to this short cooling, forming a condensation cloud at the tail of the aircraft. However, the appearance time of this kind of condensation cloud is relatively short-lived, usually only a few seconds. When the pressure returns to normal, it will disappear without a trace.
It is worth noting that although we call this kind of condensation cloud a sonic boom cloud, its appearance does not require supersonic speed . When the humidity of the atmosphere is very high and the atmospheric temperature is near the condensation point of water, the aircraft cannot If you need to reach supersonic speed, you may also put on this white cloak.
Source: Quora
Will a sonic boom deafen a reindeer’s ears?
The strength of the "sonic boom" and its impact on the ground are directly related to the flight height of the aircraft, because the intensity of the sonic boom caused by the shock wave will weaken as the propagation distance becomes farther.When an aircraft takes off, lands, or flies at low-altitude supersonic speeds, people on the ground will hear a deafening loud noise, which affects their lives and work. In severe cases, it can even shatter glass, damage weak buildings, and cause direct economic losses. Precisely because sonic booms produce powerful energy, many regions have set strict restrictions on supersonic aircraft flying across the ground.
However, the pilot inside the aircraft will not hear the sonic boom. After flies at supersonic speed, the pilot will feel that the surroundings are much quieter, because the speed of the aircraft has exceeded the speed of sound , and all external sounds are left behind the aircraft. The pilot can only hear the sound of the plane itself and nothing else. So if a reindeer breaks the sound barrier inside a special barrier, it won't feel anything special.
Ordinary reindeer | Source: Baidu Encyclopedia
It seems that even if Santa Claus really exists, this position is not easy to fill. Even if there is a powerful enough electronic reindeer to ensure that the gifts can be delivered on time, you have to calmly put yourself in every time you set off. The mount has risen to an altitude of 10,000 meters. If you are in a hurry and take off and land at supersonic speeds at low altitude, your neighbors will suffer. Classical physics may not work for Santa Claus.
Reference:
[1] Japanese physicists believe that Santa Claus flying across the sky will produce a sonic boom
[2] Sixth Census Data
[3] Baidu Encyclopedia entry: Sonic Boom
[4] Wikipedia: Condensation clouds
[5] Wikipedia: Sonic boom
[6] Wikipedia: Rarefaction
[7] Zhihu: How is a sonic boom formed?
Editor: Kobayashi Midoriko
