Why can't humans be so powerful that they can't shoot the flies with their bare hands?| NO.316

Author:Institute of Physics of the Ch Time:2022.07.15

Everyone has a spirit except flies. Humans deal with flies can be said to be rich, but why can't we kill the flies with bare hands? Q1 Why does the sound -proof wall be made into uneven honeycomb shape? What is the principle of weakening the reflection of sound waves? By 海海 Answer in Haidian:

This uneven design should be to reflect the direction of the sound waves, that is, the reflection. This reflection makes sound waves may occur multiple reflexes on the surface or inside, and the sound is absorbed in multiple reflexes.

First of all, we must be clear that sound waves will not only reflect, but also absorb. The sound wave itself is the vibration of the sound object. The sound spread in the air is actually the vibration of the air molecules. When the sound wave is exposed to the reflection surface, the kinetic energy or amount of the air molecule will be partially transmitted to the reflection surface. Sound wave absorption.

The sound -absorbing sound insulation wall is to enhance the absorption of sound wave energy with lightweight and loose materials such as foam sponge to achieve the purpose of eliminating sound. The texture of this material is relatively light. When it is transmitted to the material, it is easier to be absorbed. At the same time, its porous structure makes the sound waves continuously reflected inside it, and the energy of the final sound waves will be reduced to a certain extent.

In fact, on the side of the railway, you may also see the sound insulation wall made of metal and other materials, which is a reflected sound insulation wall. As the name implies, it is the purpose of reflecting the sound waves back to achieve sound insulation.

By Frost Bai

Q.e.d.q2 Why is the noodles be cooked over cold water and it is difficult to stick to it? BY anonymous Answer:

This should be because cold water suppresses the paste of starch. At the same time, the dough is used as a polymer composite material. Suddenly when it is cold, it can increase the mechanical properties of the dough, which is a bit similar to quenching.

First introduce the venelization of starch. In the natural starch, starch exists in the form of starch granules, and the crystal area and non -crystalline areas are arranged in the starch granules. In the crystallization area, the starch molecules rely on the combination of hydrogen bonds between the starch molecules. When starch grains are heated in water, the heat will destroy the hydrogen bonds of the crystal area. The original hydrogen bonding point binds to the water, that is, the hydrophobic, and the starch granules will dissolve irreversiblely in the water. The vessels of starch will increase the viscosity of the starch. The structure of the starch particles at room temperature is tight, and the water is not easy to enter. Only under heating can the starch be paste. Natural starch usually starts to pass on after 55 ° C.

The sticky noodles are higher than the dry noodles. This is because the process of cooking noodles itself will occur in starch, and the cooked noodles will be soaked in hot noodle soup. The reason for adhesion. The surface temperature of the noodles decreases after the cooling water. After below the purification temperature, the starch will not be paste, and it is not easy to adhere. At the same time, the dough is used as a starch polymer composite material. Suddenly when it is cold, it is conducive to the increase in the crystallization area of the dough, increasing the mechanical properties of the dough, and making the noodles more tough.

In short, the noodles are overwater YYDS.

Reference materials:

Starch paste

Emperor and aging of starch

What do you think of articles that laughed at the Science Park to cook dumplings and cold water?

By Frost Bai

Q.E.D.Q3 Why is it difficult to hit the flies with your hands? By Anonymous Answer:

Of course it is because it is too "fast"! And explaining why it is so fast, then it must be mentioned! Reunion is the main visual organs of insects, usually with outstanding positions on the head of insects. Most insects are round, oval or kidney -shaped. It consists of most small eyes. Each small eye has corneal, crystal vertebral, pigment cells, retinal cells, and visual rods. It is an independent photosensitive unit. For the reopening eye of the flies, it is composed of about 4,000 small eyes. The role of compound eye allows the flies to effectively calculate the orientation of itself and observe objects, so as to make faster judgments and reactions.

Fly Flutes | Wikipedia

How fast can the fly's brain be calculated? As a physics researcher, it is indispensable to accurately describe it intuitively: a person's flash fusion critical frequency (when looking at a flash with a low interval frequency, you get the flashing feeling of light and dark alternate. The flash interval frequency increases, and it will no longer be flashing light, but a stable and continuous light. This phenomenon is called flash fusion, which can be understood as the processing speed of the brain receiving image.) 250 times in seconds, four times higher than people. This means that our speed is 4 times slower in the eyes of the flies.

Of course, it is not only fast brain, but also fast! The flies will avoid danger by ejection. The specific method is: the flies will jump away from its wings and its legs, instead of flying. This means that when the flies perceive the danger of the surroundings, the brain starts the procedure that escapes the danger. The legs and wings will be sent to the legs and then jump away.

Flies starting | Baidu Encyclopedia

Wait, do you think of another pest, mosquito? In fact, the compound eye of mosquitoes is less than 100, and its nervous system and flying exercise ability are not far as strong as flies (please see the references in detailed analysis), so the possibility of catching mosquitoes with bare hands will be much higher than catching flies. ~ Try it next time

Reference materials:

Why can't you hit a fly?

Visually Mediated Motor Planning in the Escape Response of DROSOPHILA

Why is mosquito better than flies?

by Justu

Q.E.D.Q4 Is there a negative pressure? BY two answers:

Usually we think that the pressure is not negative.

Pressure is the law of the interaction of a certain section of interaction in an object. Generally speaking, the interaction between molecules and molecular movements contribute to the pressure. Needless to mention the interaction between molecules. Briefly introduce molecular movements, which is also the source of gas pressure. We consider the pressure of a certain section. Due to the molecular movement, there will always be molecules through this section. These molecules will carry the amount of driving volume. The amount of rushing theorem can be known that the molecules carrying the amount of mobility through this section means This section is a powerful effect, so the force on the unit area is the pressure of this section. According to this principle, we can derive the ideal gas pressure formula:

We know that for the particles of high -speed exercise, the Galileo transformation under the classic mechanics system is no longer applicable, and the Lorentz transformation needs to be introduced:

< /g> g data-mml-node = "mo" transform = "translate (725.2, 0)" /> < /g> < /g> g data-mml-node = "mo" transform = "translate (0, 258.3) scale (0.707)"/>

In the formula, n is the molecular number density, P is the molecular momentum, V is the molecular speed, and the latter item is the average of the internal accumulation of the momentum and speed. For detailed derivation, you can view the reference.

From the essence of pressure, it is not difficult to see that aside from some special cases, the pressure cannot be negative, the minimum is 0, which corresponds to the situation of no molecular.

So what is the special case? This is about negative pressure.

The negative pressure in the usual sense is a relative concept, which means that the actual pressure is lower than the reference value. If we use the atmospheric pressure as the reference value, the 0.5 atmospheric pressure is the negative pressure in the relative meaning.

But when we pull some block -shaped solids or liquids, it may appear as a net tension. This is because the molecules are attractive. When this attractiveness is high than the thermal energy, this can occur. This can be regarded as an absolute negative pressure.

There is also the authentic Casimir effect. The Casimir effect refers to the small attraction of objects in real vacuum due to the rise in vacuum energy. This may also be regarded as a negative pressure.

The last case is that for the non -biased stress in the rigid body, if we choose different surface directions, the same force may have positive pressure components along a certain surface, and have negative pressure ingredients on the other surface.

references:

[1] Zhao Kaihua, Luo Weiyin. New Conceptual Physics Tutorial, Hot Study [M]. Higher Education Press, 1998.

Presure

BY Le Superman

Q.E.D.Q5 What is the principle of solar panels? By Anonymous Answer:

Photosal Vodidal effect.

The core component of solar panels is PN knot. Taking the silicon -based battery as an example, it is based on the SI semiconductor material, mixed with boron or other outer electrons with less than 4 at the end of one end (called the P end), and the other end is doped or other outermost. Atoms with a layer electrical number greater than 4 (referred to as N -terminal). The P end is mixed with the impurities of the lack of electrons, so some of the positions that should have become a vacant position can be regarded as the cave with positive electricity (but note that at this time, the P -end is still electrical neutral); The N -terminal is mixed with the impurities of multiple electrons, and there are surplus free electrons (N -terminals are still electrical neutral at this time) after the surrounding SI formation bonds. Under such a difference in concentration, the electronics will run from the N -terminal to the P end, and the acupuncture will run from the P end to the N -end. Therefore, on the interface of the PN end, the PN terminal is negatively charged and the N -end is positive. With the spread of electronics and cavities, this area will gradually increase, and the built -in electric field will gradually increase, and the electronics and holes outside the region will no longer be further spread. Since this stable area in the end no longer has free movement electrons and caves, we call it exhaustive.

Okay, now it seems that the P end comes with negative power, and the N -end is positive, so will there be a current directly to connect the P end and N -end?

The difference between the electrostatic difference generated by the diffusion is equivalent to the difference between the Fermi energy of different materials

The answer is not, because on the interface between the wires and the P and N -terminal contact, there will still be a difference in potential caused by diffusion, and the three potential differences will just offset.

At this time, we need our great sun to help. Light (in fact, it is not necessarily sunlight, as long as the frequency is appropriate), after being illuminated on the PN knot, the energy of the light will be absorbed by the depletion of the material, producing a new electronic cave pair. This new electronics will float to the N -terminal under the action of built -in electric field, and the new acupoint will float to the P end, which will increase the P -end potential, the N -end potential decreases, break the original potential difference, form a P positive n negative N negatives Light battery.

By Muyu

Q.E.D.Q6 In space, is there still pressure inside the liquid? Will the pressure inside the same type of liquid increase with the depth? by snowman Answer:

Yes, but it will not change with depth.

First of all, it needs to be clear, what is the space here? If it means that the vacuum or vacuum of strict significance is very high, the liquid will obviously boil immediately, which can be seen from the phase diagram (taking water as an example). So even in space, as long as it is still kept liquid, it can be inferred that there is still certain pressure internally.

Stream: Wikipedia

Where can this pressure come from? From the previous question, the pressure comes from two aspects, between molecules interaction and molecular heat movement. The stress we see on the ground and its characteristics with deep linearity come from gravity. The contribution of gravity to pressure is much higher than other factors. However, in space, after no gravity, other factors have to be considered. For example, molecular heat exercise, surface tension, and possible environmental air pressure.

However, in a gravity environment, the characteristics of the pressure as deep as the depth of depth are gone.

We can analyze the stress of the static fluid and get the equilibrium equation. Here, the molecular heat movement is not considered as a small amount. We establish a right -angle coordinate system in a certain point in the fluid. The Z axis is vertically upward, and then considers a small square with a coordinate axis as the edge. The pressure of the fluid. The quality here is brought by the quality of the fluid itself, including but not limited to gravity. Static fluids must be balanced, so we can get the final balance equation:

Some derivatives are omitted here, only the final balance equation is given. Interested students can click on the reference materials to view.

In the equilibrium equation, X, Y, and Z are the weight of the quality force in the direction of the coordinate axis.

If you consider gravity, there are: x = y = 0, z = -g. Basic equations of fluid static at gravity under the action of the division of the procedure:

< /g> is p = γ (c-z), γ = ρg, note: Here Z is the coordinate value in the direction of the z axis. If we use the depth H instead of Z, then the H = -z upper form is turned into p = ρGh, and this formula is very familiar, right? However, on the ground, you must also consider environmental air pressure, that is, atmospheric pressure. For non -compressed fluids, this will bring a constant item.

Therefore, when the gravity is considered, the pressure has not changed with the depth.

Reference materials:

Fluid static ● Above

By Frost Bai

Q.E.D.Q7 The faster the flow rate of a fluid in the physical textbook of junior high school, the smaller the pressure. However, the principle of Bernunley does not match this point of view, and the flow rate of high -pressure water guns is fast, and it is painful to hit people. This proves that its pressure is strong. BY Xinchu Third Answer:

First of all, the expression of the junior high school textbook "the faster the flow speed, the smaller the pressure, the less the pressure" is the simplified version of the principle of Berinley.

The complete expression of the principle of Bernoulli is [1] [2]: When the non -friction -free non -compressed fluid is determined to flow along the streamline, its internal satisfaction is satisfied

< /g> < /g> Of course it can be written

Among them, V is the speed of a fluid at a certain point. H is the height of the relative reference point (the reference point is not necessarily the liquid surface). P is the liquid pressure at this point. CONST represents the constant. If the pressure P is regarded as a potential field that drives the fluid micro -motion -the potential field that drives the charging motion in the ratio -then, then the principle of Berinley is an energy conservation with a lot of restricted conditions. Of course, this is a very rough understanding. As for the restrictions of the Bernunley principle, compared to the noun in the explanation of the attributive, I prefer to explain in a descriptive spoken language: a more ideal fluid that is more ideal than the "vacuum chicken in the vacuum" in the paragraph, which is extremely extreme. Rule flow -rules to observers do not even see whether it is flowing: neither vortex nor friction to the wall of the tube, and the nature of the flow does not change with time, and there will be no small part of the fluid invading the neighbors of the neighbors. Walk the route. In this way, keeping the height H is unchanged, and the flow velocity V is relatively large, the pressure P naturally is relatively small. As for high -pressure water guns, although it is no longer in line with the assumptions of Bernonley's principle, as a rough approximate approximate, the principle of Bernunley can also qualitatively explain your confusion about water guns. The moment when the human body was impacted, the speed of the water flow became 0. Apply the second formula above, < /g> , then < /g> Natural P₂ will become very large. If you switch to the more familiar language of the middle school, the water flow impacted the pressure that the body felt, not the P in the above formula, but the original liquid pressure and the water flow were blocked by the human body. Essence And because the rate of water in the water gun is fast, the part of the rush is the main factor that makes people "feel pain".