Shulou(Shulou.com)11/24 Report--

Q1. I want to ask a question raised by a child that adults at home can't answer. Why don't people in the southern hemisphere walk head-down? by Li Ke

Answer:

If we look out into space and see the earth from the moon, how do we define "up" and "down"?

We will find that the earth is a sphere, there is no so-called up and down for the earth, from all directions are the same. For people on earth, due to the gravitational effect of the earth, the sky is above the head and the earth is below the feet, so there is a feeling of up and down, so the southern hemisphere and the northern hemisphere are stepping on the earth.

On the space station, due to weightlessness, astronauts do not have the concept of up and down, but in order to keep consistent with Earth, the spacecraft bottom is generally set to the same direction.

by Minke

Q2: Why do people walk on walls with their arms open? Is there any difference between this and using a long pole when walking a tightrope? by lover

Answer:

People open their arms on the fence to prevent the center of gravity from shifting and causing falls, which is basically the same as using long poles when walking on a tightrope.

When we walk on a narrow wall, our center of gravity is easy to deviate from the wall. When our center is outside the wall, we can see that the moment of the wall supporting the human body is 0, and the gravity will produce a net moment due to the deviation of the center of gravity from the wall. And when we open our arms, we can adjust the moment on our body by adjusting the position of our arms. For example, when we feel that the center of gravity of the body is too far to the right, we can spread the left arm, adjust the overall center of gravity position through the weight of the left arm, and make the center of gravity return to the wall range, as shown in the figure, so that the sum moment is zero, and the person will not lose balance and fall down.

by Single Young Men

Q3. Compared with the water surface at other positions, why is it not easy to freeze around the pier of the river bridge in winter? by Zhao Yue

Answer:

Flowing water does not freeze easily, not because the flow lowers the freezing point of water. In equilibrium state, matter has two trends of maximizing entropy and minimizing internal energy at the same time, and the solidification of liquid is due to the competition between energy and entropy: at high temperature, entropy occupies the dominant position, molecular arrangement is chaotic, so liquid phase with higher disorder degree is stable, while at low temperature, energy occupies the dominant position, molecular arrangement is orderly, so solid phase with lower internal energy is stable. Therefore, when the temperature decreases to the freezing point, the equilibrium phase will change from liquid to solid, that is, solidification will occur. It follows that freezing point depends on the microscopic composition and interaction of matter, and has little to do with macro-scale flow.

It is not because the kinetic energy of the liquid is converted into internal energy through internal friction that the flowing water surface is less likely to freeze. The microscopic thermal motion of matter is much faster than the macroscopic, so the macroscopic kinetic energy changes very little in general compared to the internal energy. It can be estimated that 1 kg of water flows at a high speed of 10 m/s, and its kinetic energy is completely converted into internal energy. If the heat released is completely absorbed by itself, it can only raise the temperature by less than 0.1℃; if the heat released is completely transferred to the ice surface, it can only melt less than 0.2 g of ice. Therefore, the kinetic energy of water to prevent freezing can be said to be a drop in the bucket.

However, in life, it is often observed that most of the river has been frozen, but there is still flowing water surface at piers and so on. This phenomenon is mainly caused by two reasons. First, the water depth at the pier is usually relatively large. When the ambient temperature is below freezing, the shallower water near the river bank will cool below freezing first, so the ice usually starts from the river bank. Secondly, even if it is also located in the center of the river, the position near the pier and other turbulent water flow is still more difficult to freeze, this is because in these positions, the heat exchange inside the water body in addition to conduction, there is strong forced convection, so that the heat exchange efficiency of the water surface and the bottom is greatly improved, and each depth has almost the same temperature. Therefore, freezing in turbulent water requires falling below freezing point from the surface to the bottom, which undoubtedly makes it more difficult to freeze.

Happy in your heart.

Q4. Why does the wall of the red wine glass surge upward into transparent liquid? by

Answer:

These are red wine tears!

This phenomenon was first noticed by the British physicist James Thomson, who published a paper in 1855 entitled "Some Strange Motions Observed on the Surface of Wine and Other Liquor," which initially determined that it was caused by surface tension. Carlo Marangoni, an Italian physicist, explained the phenomenon further and called it the Marangoni effect.

The Marangoni effect is the mass transfer phenomenon that occurs when there is a difference in surface tension between the two phases. A liquid with high surface tension has a strong pull on a liquid with low surface tension around it, causing the liquid to flow from a place with low tension to a place with high tension.

Wine climbs on the glass wall under the action of surface tension, but alcohol is more volatile, so when alcohol volatilizes, the surface tension of water is greater than alcohol, and the liquid in the glass transfers to the glass wall, but falls under the action of gravity, thus forming wine tears, also known as hanging walls.

Because wine still has residual sugar and glycerin, these sticky substances will also affect the density and speed of wine tears, so wine tears can not be used to judge the quality of wine. Wine quality is judged by richness and complexity of aroma, elegance of color and persistence of finish.

References:

Understanding the Marangoni Effect

Li Jiming. Correctly understand the phenomenon of hanging cup [J]. Chinese Wine, 2012 (5):2.

by Minke

Q5. When riding high-speed rail, I found that some rails have no middle sleepers. Why is this? Well, since there can be no sleepers, what is the meaning of sleepers? by Wuchang Founding Duke

Answer:

It should be ballastless track.

Ballastless track refers to the track structure that adopts concrete, asphalt mixture and other integral foundations to replace granular ballast bed, also known as ballastless track. The ballast here is the small stone on the traditional railway.

Traditional track, i.e. ballasted track, the rails are placed on the sleepers mentioned in the title, and the sleepers are laid on the ballast. Sleepers and ballast have certain elasticity, which mainly increases the bearing area of rails, bears and disperses the pressure from rails, and can also reduce noise and shock absorption. However, under repeated train loads, residual deformation of ballast-bearing track accumulates rapidly and is distributed unevenly along the track longitudinal direction, which will lead to track unevenness, affect passenger comfort and increase track maintenance workload.

Ballastless track is usually prepared for high-speed railway, which can be said to be a more advanced railway technology. However, it should be noted that the operation experience and experimental research of foreign high-speed railways show that when the train speed reaches 300km / h, the ballasted track can still ensure the safe operation of the train. Sleeper of ballastless track is made of reinforced concrete. Rail is directly laid on concrete sleeper, which is damped and decompressed by fastener and other mechanisms.

Compared with ballasted track, ballastless track has good stability, reduced maintenance workload by more than 50%, good durability, long service period, small excavation area in tunnel subway, low structural height and light weight, which can reduce tunnel clearance. However, ballastless track has a large initial investment. Once the foundation is deformed and sunk, it is more difficult to repair than ballastless track. It cannot be laid in deep clay cutting, soft soil embankment or earthquake area. In any case, anti-freezing layer should be laid. Therefore, the laying of railway tracks should be adapted to local conditions.

References:

[Railway Knowledge·Transportation] High-speed Railway Track Structure (II)-Ballastless Track

Dry goods! Structure, classification, application, advantages and disadvantages analysis of ballastless track

by White Frost

Q6. How does a radio receiver receive electromagnetic waves of different frequencies? by Name

Answer:

The antenna! You can take a look at your own wifi router. The vertical "sticks" are antennas.

Look at the picture below

An antenna is essentially a transducer that captures electromagnetic waves in nearby space and converts them into guided waves. For electromagnetic waves, we should not be unfamiliar with it, it is generated by electric and magnetic fields oscillating in space alternating electromagnetic field, in the form of waves spread. The electric field and magnetic field are perpendicular to each other, and the propagation direction is perpendicular to the plane composed of electric field and magnetic field.

Electromagnetic wave schematic diagram| Wikipedia Although you may not know much about guided waves, we can understand them as: a transmission mode of electromagnetic waves, in guided wave systems (systems capable of propagating electromagnetic waves) there are special structural forms of electromagnetic fields. We can classify guided waves according to the presence or absence of longitudinal field components: TEM waves (transverse electromagnetic waves), TE waves (transverse electric waves), TM waves (transverse magnetic waves). If you want to continue to learn about this, you can read the reference materials and read the books on electrodynamics, and I won't go into it here.

So the antenna picks up all the electromagnetic waves, so how does the radio receiver distinguish these different frequencies? This requires circuit knowledge.

wireless receiving system| The reference circuit receives the electromagnetic wave and selects the frequency, then converts the frequency, amplifies, and finally demodulates the baseband signal. For frequency selection, the simple principle is that the capacitance and inductance in the circuit will form resonance (resonance) phenomenon in the process of charging and discharging, and the frequency of this resonance phenomenon will be different due to the "size" of the capacitance and inductance in the circuit. We can change the capacitance and inductance in the circuit to select different frequencies. Have you ever seen a TV with a manual button to select different TV signals? This manual process of turning the button is the process of selecting the frequency ~ Now everyone should understand the working principle of the radio receiver!

References:

Zang Yiping. Research on Filtered Antenna and Its Frequency Reconfiguration Technology [D]. Xidian University of Science and Technology, 2021.

by just iu

Q7: What is light pressure? What does light pressure do to comets? by thousand

Answer:

The concept of light pressure had been proposed by Galileo as early as when he explained the formation of comet tails: the dust and gas molecules of comets pointed in the opposite direction from the sun under the influence of solar radiation. Light pressure refers to the pressure produced by light. Maxwell pointed out the existence of light pressure on the basis of classical electromagnetic theory and proved that light incident on a perfect absorber produces half the light pressure incident on a perfect reflector.

Lebedev's experimental device for measuring light pressure| Light pressure theory was first measured by Lebedev. The principal part of his apparatus consisted of a very light suspension R suspended by thin thread, to which were fixed winglets a and b, one black and the other bright. the suspension R is placed in a vacuum vessel G as shown in figure 2. Light emitted by arc lamp B is directed to one of the winglets by means of a lens and mirror system. The suspension R rotates due to the light pressure acting on the winglet. The size of the rotation can be observed with the aid of telescopes and small mirrors fixed on the axis. Moving the mirrors causes light to fall on the blackened winglet. Comparing the magnitude of the rotation of the suspension in the two cases, Lebedev measured that the light pressure on the blackened surface was half that on the reflective surface, in perfect agreement with the theory.

According to quantum mechanics, light has wave-particle duality, which has both wave and particle properties, so light can produce pressure like gas molecules, and the existence of light pressure is also a reasonable thing.

by Minke

Q8: Can humans fly to the edge of the universe? by Anonymous

Answer:

No, in fact, we don't even know where the real edge of the universe is.

Modern cosmology generally believes that our universe originated in a big bang about 13.7 billion years ago. Is the radius of our universe 13.7 billion years old? We can observe the universe radius of about 46 billion light-years, this is because we found that after the big bang, the universe experienced a rapid expansion phase, early expansion speed even faster than the speed of light, this is the inflationary theory of the universe. The faster-than-light inflation of the universe does not violate relativity, because cosmic expansion is an expansion of spacetime itself and is not subject to relativity. So light from the early days of the Big Bang is now far more distant than 13.7 billion light-years.

What about 46 billion light-years away? In fact, we'll never get there. For us, the universe at the edge of the observable universe is expanding at exactly the speed of light, and even if we went there at the speed of light, we'd never get there. It would be a desperate journey. What's out there, is there a universe? The answer is no. There are many hypotheses or hypotheses about cosmology, such as the multiverse theory, but none of these hypotheses are supported by astronomical evidence and cannot be tested.

Is the universe still expanding? Yes, we introduced Hubble's Law in the previous Q & A (343 & 344), and the expansion rate of the universe is called Hubble's constant, which is a quantity proportional to distance. That is, the farther apart two objects are, the faster they expand, so we don't have to worry about getting farther away from each other.

by White Frost

This article comes from Weixin Official Accounts: Institute of Physics, Chinese Academy of Sciences (ID: cas-iop), Author: Frions

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