The highest speed is the speed of light in a vacuum, that is, space free of matter. The scientific community accepted its value as 299,792,458 m / s (or 1,079,252,848.8 km / h). At the same time, the most accurate measurement of the speed of light on the basis of a reference meter, carried out in 1975, showed that it is 299 792 458 ± 1.2 m / s. Visible light itself, as well as other types of electromagnetic radiation, such as radio waves, X-rays, and gamma quanta, propagate at the speed of light.

The speed of light in a vacuum is a fundamental physical constant, that is, its value does not depend on any external parameters and does not change with time. This speed does not depend either on the motion of the wave source or on the observer's frame of reference.

What is the speed of sound?

The speed of sound differs depending on the medium in which the elastic waves propagate. It is impossible to calculate the speed of sound in vacuum, since sound cannot propagate under such conditions: there is no elastic medium in vacuum, and elastic mechanical vibrations cannot arise. As a rule, sound propagates more slowly in gas, slightly faster in liquid, and most rapidly in solids.

So, according to the Physical Encyclopedia edited by Prokhorov, the speed of sound in some gases at 0 ° C and normal pressure (101325 Pa) is (m / s):

The speed of sound in some liquids at 20 ° C is (m / s):

Longitudinal and transverse elastic waves propagate in a solid medium, and the velocity of the longitudinal waves is always greater than the transverse ones. The speed of sound in some solids is (m / s):

Today, many new settlers, equipping an apartment, are forced to carry out additional work, including soundproofing their home, tk. the standard materials used make it possible to only partially hide what is happening in your own home, and not be interested in the communication of neighbors against your will.

In solids, at least the density and elasticity of the substance opposing the wave affects. Therefore, when equipping the premises, the layer adjacent to the bearing wall is made soundproof with "overlaps" from above and below. It allows you to reduce in decibels sometimes by more than 10 times. Then basalt mats are laid, and on top - gypsum plasterboard sheets, which reflect the sound outside the apartment. When a sound wave "flies" to such a structure, it is attenuated in the insulator layers, which are porous and soft. If the sound is strong, the materials that absorb it may even heat up.

Elastic substances, such as water, wood, metals, transmit well, therefore we hear the wonderful "singing" of musical instruments. And some nationalities in the past determined the approach, for example, of riders, putting their ear to the ground, which is also quite elastic.

The speed of sound in km depends on the characteristics of the medium in which it propagates. In particular, the process can be influenced by its pressure, chemical composition, temperature, elasticity, density and other parameters. For example, in a steel sheet, a sound wave travels at a speed of 5100 meters per second, in glass - about 5000 m / s, in wood and granite - about 4000 m / s. To convert speed into kilometers per hour, you need to multiply the indicators by 3600 (seconds per hour) and divide by 1000 (meters per kilometer).

The speed of sound in km in an aquatic environment is different for substances with different salinity. For fresh water at a temperature of 10 degrees Celsius, it is about 1450 m / s, and at a temperature of 20 degrees Celsius and the same pressure, it is already about 1490 m / s.

The salty environment, on the other hand, is distinguished by a deliberately higher speed of passage of sound vibrations.

Sound propagation in air also depends on temperature. With a value of this parameter equal to 20, sound waves travel at a speed of about 340 m / s, which is about 1200 km / h. And at zero degrees, the speed slows down to 332 m / s. Returning to our apartment insulators, we can find out that in a material such as cork, which is often used to reduce the level of external noise, the speed of sound in km is only 1800 km / h (500 meters per second). This is ten times lower than this characteristic in steel parts.

A sound wave is a longitudinal vibration of the medium in which it propagates. When, for example, a melody of a piece of music passes through an obstacle, the level of its volume decreases, because changes In this case, the frequency remains the same, due to which we hear a woman's voice as a woman's, and a man's as a man's. The most interesting is the place where the speed of sound in km is close to zero. This is a vacuum in which waves of this type hardly propagate. To demonstrate how this works, physicists place a ringing alarm clock under a bell from which air is pumped out. The more rarefied the air, the quieter the bell is heard.

Sound speed- the speed of propagation of elastic waves in a medium: both longitudinal (in gases, liquids or solids) and transverse, shear (in solids). It is determined by the elasticity and density of the medium: as a rule, the speed of sound in gases is less than in liquids, and in liquids it is less than in solids. Also, in gases the speed of sound depends on the temperature of a given substance, in single crystals - on the direction of wave propagation. Usually does not depend on the frequency of the wave and its amplitude; in cases where the speed of sound depends on frequency, we speak of sound dispersion.

Collegiate YouTube

Already in ancient authors, there is an indication that sound is due to the oscillatory movement of the body (Ptolemy, Euclid). Aristotle notes that the speed of sound is finite and correctly understands the nature of sound. Attempts to experimentally determine the speed of sound date back to the first half of the 17th century. F. Bacon in his "New Organon" pointed out the possibility of determining the speed of sound by comparing the time intervals between a flash of light and the sound of a shot. Using this method, various researchers (M. Mersenne, P. Gassendi, W. Derham, a group of scientists from the Paris Academy of Sciences - D. Cassini, Picard, Huygens, Roemer) determined the value of the speed of sound (depending on the experimental conditions, 350-390 m /with). Theoretically, the question of the speed of sound was first considered by Newton in his "Elements". Newton actually assumed that sound propagation is isothermal, so he received an underestimated estimate. The correct theoretical value for the speed of sound was obtained by Laplace. [ ]

Calculation of Velocity in Liquid and Gas

The speed of sound in a homogeneous liquid (or gas) is calculated by the formula:

c = 1 β ρ (\ displaystyle c = (\ sqrt (\ frac (1) (\ beta \ rho))))

In partial derivatives:

c = - v 2 (∂ p ∂ v) s = - v 2 C p C v (∂ p ∂ v) T (\ displaystyle c = (\ sqrt (-v ^ (2) \ left ((\ frac (\ partial p) (\ partial v)) \ right) _ (s))) = (\ sqrt (-v ^ (2) (\ frac (Cp) (Cv)) \ left ((\ frac (\ partial p) (\ partial v)) \ right) _ (T))))

where β (\ displaystyle \ beta) is the adiabatic compressibility of the medium; ρ (\ displaystyle \ rho) is the density; C p (\ displaystyle Cp) is the isobaric heat capacity; C v (\ displaystyle Cv) is the isochoric heat capacity; p (\ displaystyle p), v (\ displaystyle v), T (\ displaystyle T) - pressure, specific volume and temperature of the medium; s (\ displaystyle s) is the entropy of the environment.

For solutions and other complex physicochemical systems (for example, natural gas, oil), these expressions can give a very large error.

Solid bodies

In the presence of interfaces, elastic energy can be transmitted through surface waves of various types, the speed of which differs from the speed of longitudinal and transverse waves. The energy of these vibrations can be many times greater than the energy of body waves.

Sacor 23-11-2005 11:50

In principle, the question is not as simple as it seems, I found such a definition:

The speed of sound, the speed of propagation of any fixed phase of a sound wave; also called the phase velocity, in contrast to the group velocity. S. z. usually the value is constant for a given substance under given external conditions and does not depend on the frequency of the wave and its amplitude. In those cases when this is not fulfilled and S. z. depends on the frequency, talk about the dispersion of sound.


So what is the speed of sound equal to in winter, in summer, in fog, in rain - these are such incomprehensible things for me now ...

Sergey13 23-11-2005 12:20

at n.u. 320 m / s.

TL 23-11-2005 12:43

The "denser" the medium, the higher the speed of propagation of the disturbance (sound), in the air approx. 320-340 m / s. (Falls with height) 1300-1500 m / s in water (salt / fresh) 5000 m / s in metal, etc. That is, with fog, the speed of sound will be higher, in winter it will also be higher, etc.

StartGameN 23-11-2005 12:48

StartGameN 23-11-2005 12:49

Simultaneously answered

Sacor 23-11-2005 13:00

So the range is 320-340 m / s - I looked at the reference book, there, at 0 Celsius and a pressure of 1 atmosphere, the speed of sound in air is 331 m / s. It means 340 in cold weather, and 320 in hot weather.
And now the most interesting thing, and what is the bullet velocity of subsonic ammunition?
Here is a classification for small-caliber cartridges, for example from ada.ru:
Standard (subsonic) cartridges speed up to 340 m / s
Chucks High velocity (high speed) speed from 350 to 400 m / s
Hyper Velocity or Extra high velocity chucks 400 m / s and above
That is, Eley Tenex 331 m / s Sobol 325 m / s are considered subsonic, and Standard 341 m / s is no longer. Although both those and these, in principle, lie in the same range of sound velocities. Like this?

Kostya 23-11-2005 13:39

IMHO you shouldn't bother with this, you are not fond of acoustics, but shooting.

Sacor 23-11-2005 13:42

quote: Originally posted by Kostya:
IMHO you shouldn't bother with this, you are not fond of acoustics, but shooting.

Yes, it's just interesting, otherwise everything is subsonic, but how I dug it turned out to be completely ambiguous.

By the way, what is the subsonic speed for silent firing at x54, x39, 9PM?

John JACK 23-11-2005 13:43

The cartridges also have a spread in the initial speed, and it also depends on the temperature.

GreenG 23-11-2005 14:15


Sound is an elastic longitudinal wave, the propagation speed of which depends on the properties of the environment. Those. higher terrain - lower air density - lower speed. Unlike light, it is a transverse wave.
It is accepted to consider V = 340 m / s (approximately).

However, this is off

StartGameN 23-11-2005 14:40




Current, light has a transverse electromagnetic wave, and sound has a mechanical longitudinal. If I understand correctly they are related by the description of the same mathematical function.

However, this is off

Hunt 23-11-2005 14:48

Here's what I'm interested in, the maximum atmospheric pressure (in general for a month) while having a rest in the Urals never rose to the parameters of the local ones. At the moment there are 765 t-32. And what is interesting is the temperature is lower and the pressure is lower. Well ... this, as far as I have noted for myself, ... I do not conduct constant observations. I have a score. the tables were last year for a pressure of 775 mm \ Hg \ Art. Maybe the lack of oxygen in our area is partially compensated by the increased atmospheric pressure. I asked at my department a question, it turns out there is NO DATA !. And these are the people who create decompression tables for people like me! And for the military, jogging (on physical exercises) in our Palestinians is prohibited, because lack of oxygen. I think, if there is a lack of oxygen, then what is replaced ... by nitrogen, that is, the density is different. And if you look at all this and count, you have to be a galactic-class shooter. I for myself (while the Señor is poring over the calculator, and the customs over my parcels) decided: For 700 no, no, fig whether to shoot cartridges.
So I wrote and thought. After all, he spat and made a promise more than once, well, nafig all this. What to go to the Chepionat? Compete with whom?
... You read the forum and again bears. Where to get bullets, matrices, etc.
CONCLUSION: A terrible dependence on communication with people like them who love weapons - homo ... (I propose to find a continuation of the expression)

GreenG 23-11-2005 16:02

quote: Originally posted by StartGameN:


I can develop off - my diploma was called "Nonlinear acoustoelectromagnetic interactions in crystals with quadratic electrostriction"

StartGameN 23-11-2005 16:24

I'm not a theoretical physicist, so there weren't any "experiments". An attempt was made to take into account the second derivative and explain the occurrence of resonance.
But the idea is correct


Khabarovsk 23-11-2005 16:34

Can I stand here from the edge to listen? I will not interfere, honestly. Sincerely, Alex

Antti 23-11-2005 16:39

quote: Originally posted by GreenG:


the main experimental method was, apparently, knocking a magnet on a crystal?

A square magnet along a curved crystal.

Sacor 23-11-2005 19:03

Then another question, why does the sound of a shot seem louder in winter than in summer?

SVIREPPEY 23-11-2005 19:27

I'll tell you all this.
From ammunition to the speed of sound is close. 22lr. We put a modernization on the barrel (to remove the sound background) and fire a hundred, for example. And then all the cartridges can be easily divided into subsonic (you can hear how it flies into the target - there is such a light "bunch") and supersonic - when it hits the target, it bounces so that the whole idea with the moder flies down the drain. From the sub-sound I can note the tempo, biathlon, from the imported ones - RWS Target (well, I don't know much about them, and in stores the choice is not that). From supersonic - for example, Lapua Standard, cheap, interesting, but very noisy cartridges. Then we take the initial speeds from the manufacturer's website - and here's an approximate range of where the speed of sound is at a given shooting temperature.

StartGameN 23-11-2005 19:56


Then another question, why does the sound of a shot seem louder in winter than in summer?

In winter, a mustache in hats walks and therefore hearing is dulled

STASIL0V 23-11-2005 20:25

But seriously: for what purpose is it required to know the real speed of sound for a specific condition (in the sense from a practical point of view)? the purpose usually defines the means and methods / accuracy of the measurement. For me, it seems like you don't need to know the speed to hit the target or when hunting (unless, of course, without a silencer) ...

Parshev 23-11-2005 20:38

Generally speaking, the speed of sound is to some extent the limit for the stabilized flight of a bullet. If you look at the accelerated body, then before the sound barrier the air resistance grows, in front of the barrier quite sharply, and then, after passing the barrier, it drops sharply (that's why the aviators were so eager to achieve supersonic sound). When braking, the picture is built in the reverse order. That is, when the speed ceases to be supersonic, the bullet experiences a sharp jump in air resistance and can go somersault.

vyacheslav 23-11-2005 20:38




everything turned out to be completely ambiguous.

The most interesting conclusion in the whole argument.

q123q 23-11-2005 20:44

And so, comrades, the speed of sound directly depends on the temperature, the higher the temperature, the greater the speed of sound, and not the other way around, as noted at the beginning of the topic.
*************** /------- |
the speed of sound a = \ / k * R * T (this is the root so designated)

For air, k = 1.4 is the adiabatic exponent
R = 287 - specific gas constant for air
T - temperature in Kelvin (0 degrees Celsius corresponds to 273.15 degrees Kelvin)
That is, at 0 Celsius, a = 331.3 m / s

Thus, in the range of -20 +20 Celsius, the speed of sound changes in the ranges from 318.9 to 343.2 m / s

I think there will be no more questions.

As for why all this is needed, it is necessary in the study of flow regimes.

Sacor 24-11-2005 10:32

Exhaustively, but doesn't the speed of sound depend on density, pressure?

BIT 24-11-2005 12:41

[B] If you look at the accelerated body, then before the sound barrier the air resistance grows, in front of the barrier quite sharply, and then, after passing the barrier, it drops sharply (that's why the aviators were so eager to achieve supersonic sound).

I've already pretty much forgotten physics, but as far as I remember, air resistance grows with increasing speed and before "sound" and after. Only at subsonic sound, the main contribution is made by overcoming the frictional force against the air, while at supersonic this component sharply decreases, but the energy losses for creating a shock wave increase. A. in general, energy losses increase, and the further, the more progressive.


Blackspring 24-11-2005 13:52

I agree with q123q. We were taught - the norm at 0 Celsius is 330 m / s, plus 1 degree - plus 1 m / s, minus 1 degree - minus 1 m / s. Quite a working scheme for practical use.
Probably, the rate can change with pressure, but the change will still be about a degree-meter per second.
BS

StartGameN 24-11-2005 13:55

quote: Originally posted by Sacor:


Depends-Depends. But: there is Boyle's law, according to which at constant temperature p / p1 = const, i.e. the change in density is directly proportional to the change in pressure

Parshev 24-11-2005 14:13


Originally posted by Parshev:
[B]
I have already quite forgotten physics, but as far as I remember, air resistance grows with increasing speed and before "sound" and after. ...

And I never knew.

It grows both before and after the sound, and in different ways at different speeds, but falls on the sound barrier. That is, 10 m / s before the speed of sound, the resistance is higher than when it is 10 m / s after the speed of sound. Then it grows again.
Of course, the nature of this resistance is different, so objects of different shapes cross the barrier in different ways. Before sound, drop-shaped objects fly better, after sound - with a sharp nose.


BIT 24-11-2005 14:54

Originally posted by Parshev:
[B]

That is, 10 m / s before the speed of sound, the resistance is higher than when it is 10 m / s after the speed of sound. Then it grows again.

Not certainly in that way. When the sound barrier is crossed, the TOTAL resistance force increases, and abruptly, due to a sharp increase in energy consumption for the formation of a shock wave. The contribution of the FRICTION FORCE (more precisely, the drag force due to turbulence behind the body) sharply decreases due to a sharp decrease in the density of the medium in the boundary layer and behind the body. Therefore, the optimal body shape at subsonic levels becomes suboptimal at supersonic levels, and vice versa. A drop-shaped body streamlined at supersonic creates a very powerful shock wave, and experiences a much greater TOTAL resistance force, compared to a pointed but with a "blunt" rear part (which practically does not matter at supersonic). During the reverse transition, the rear non-streamlined part creates a large, in comparison with the drop-shaped body, turbulence and, consequently, a drag force. In general, a whole section of general physics - hydrodynamics - is devoted to these processes, and it is easier to read a textbook. And the scheme outlined by you, as far as I can judge, does not correspond to reality.

Sincerely. BIT

GreenG 24-11-2005 15:38

quote: Originally posted by Parshev:


Before sound, drop-shaped objects fly better, after sound - with a sharp nose.

Uraaaa!
It remains to come up with a bullet that can fly nose-first on top of the sound and well .. sing after crossing the barrier.

In the evening I'll tap brandy for my bright head!

Machete 24-11-2005 15:43

Inspired by the discussion (off).

Gentlemen, did you drink a cockroach?

BIT 24-11-2005 15:56

Recipe, pliz.

Antti 24-11-2005 16:47




In general, a whole section of general physics is devoted to these processes - hydrodynamics ...

Hydra has something to do with it?


Parshev 24-11-2005 18:35




Hydra has something to do with it?

And the name is beautiful. Of course, it has nothing to do with different processes in water and in air, although there is something in common.

Here you can see what happens to the coefficient of drag on the sound barrier (3rd graph):
http://kursy.rsuh.ru/aero/html/kurs_580_0.html

In any case, there is a sharp change in the flow pattern at the barrier, disturbing the movement of the bullet, and for this it may be useful to know the speed of sound.


STASIL0V 24-11-2005 20:05

Returning again to the practical plane, it turns out that when switching to subsonic sound, additional unpredictable "disturbances" appear, leading to destabilization of the bullet and an increase in the spread. Therefore, in order to achieve sports goals, a supersonic small-sized cartridge should by no means be used (and the maximum possible accuracy will not interfere with hunting). What, then, is the advantage of supersonic cartridges? More (not much) energy and therefore destructive power? And this is due to the accuracy and more noise. Is it worth using the supersonic 22lr at all?

gyrud 24-11-2005 21:42

quote: Originally posted by Hunt:
And for the military, jogging (on physical exercises) in our Palestinians is prohibited, because lack of oxygen. I think if there is a lack of oxygen, then what is replaced ... by nitrogen,

It is impossible to speak about any substitution of oxygen for oxygen by nitrogen. there is simply no substitute for it. The percentage of atmospheric air is the same at any pressure. Another thing is that with a reduced pressure in the same liter of inhaled air, there is actually less oxygen than at normal pressure, so oxygen deficiency develops. That is why pilots at altitudes above 3000m breathe through masks an air mixture enriched with up to 40% oxygen.


q123q 24-11-2005 22:04

quote: Originally posted by Sacor:
Exhaustively, but doesn't the speed of sound depend on density, pressure?

Only through the temperature.

Pressure and density, or rather, their ratio is rigidly related to temperature
pressure / density = R * T
what is R, T see in my post above.

That is, the speed of sound is an unambiguous function of temperature.

Parshev 25-11-2005 03:03

It seems to me that the ratio of pressure and density is rigidly related to temperature only in adiabatic processes.
Are the climatic changes in temperature and atmospheric pressure such?

StartGameN 25-11-2005 03:28

Correct question.
Answer: Climate change is not an adiabatic process.
But you need to use some kind of model ...

BIT 25-11-2005 09:55

quote: Originally posted by Antti:


Hydra has something to do with it?
Chevy, I suspect that the picture may be slightly different in air and water due to compressibility / incompressibility. Or not?

At our university, we had a combined course in hydro- and aerodynamics, as well as a department of hydrodynamics. Therefore, I have abbreviated this section. You are of course right, processes in liquids and gases can proceed in different ways, although there are a lot in common.


BIT 25-11-2005 09:59


What, then, is the advantage of supersonic cartridges? More (not much) energy and therefore destructive power? And this is due to the accuracy and more noise. Is it worth using the supersonic 22lr at all?

StartGameN 25-11-2005 12:44

The "accuracy" of the small cartridge is explained by the extremely weak heating of the barrel and the shellless lead bullet, and not by the speed of its departure.

BIT 25-11-2005 15:05

Heating is clear. And the shelllessness? Greater manufacturing precision?

STASIL0V 25-11-2005 20:48

quote: Originally posted by BIT:


IMHO - ballistics, you mean the trajectory. Less flight time means less external disturbances. In general, the question arises: Since when switching to subsonic air resistance sharply decreases, then the overturning moment should also sharply decrease, and consequently the stability of the bullet should increase? Isn't that why the small cartridge is one of the most accurate?

Machete 26-11-2005 02:31

quote: Originally posted by STASIL0V:


Opinions were divided. According to you, a supersonic bullet comes out when you switch to subsonic sound, it stabilizes. And according to Parshev, on the contrary, there is an additional disturbing effect that worsens stabilization.

Dr. Watson 26-11-2005 12:11

Exactly.

BIT 28-11-2005 12:37

And I didn't think to argue. He simply asked questions and, opening his mouth, listened.

Sacor 28-11-2005 14:45

quote: Originally posted by Machete:


In this case, Parshev is absolutely right - with the reverse transonic transition, the bullet is destabilized. That is why the maximum firing range for each specific cartridge in Long Range is determined by the distance of the reverse transonic transition.

It turns out that a small-caliber bullet fired at a speed of 350 m / s is strongly destabilized somewhere by 20-30 m? And the accuracy deteriorates significantly.

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1 kilometer per hour [km / h] = 0.0001873459079907 speed of sound in fresh water

Initial value

Converted value

meter per second meter per hour meter per minute kilometer per hour kilometer per minute kilometer per second centimeter per hour centimeter per minute centimeter per second millimeter per hour millimeter per minute millimeter per second foot per hour foot per minute foot per second yard per hour yard in minute yard per second mile per hour mile per minute mile per second knot knot (UK) speed of light in vacuum first space speed second space speed third space speed speed of rotation of the Earth speed of sound in fresh water speed of sound in sea water (20 ° C, depth 10 meters) Mach number (20 ° C, 1 atm) Mach number (SI standard)

American wire gauge

More about speed

General information

Speed ​​is a measure of the distance traveled in a specified time. The speed can be a scalar or a vector - this takes into account the direction of movement. The speed of movement in a straight line is called linear, and along a circle - angular.

Measuring speed

Average speed v found by dividing the total traveled distance ∆ x for the total time ∆ t: v = ∆x/∆t.

In the SI system, speed is measured in meters per second. Metric kilometers per hour and miles per hour are also widely used in the United States and Great Britain. When, in addition to the magnitude, the direction is also indicated, for example 10 meters per second to the north, then we are talking about the vector speed.

The speed of bodies moving with acceleration can be found using the formulas:

• a, with an initial speed u during the period ∆ t, has a final speed v = u + a×∆ t.
• A body moving with constant acceleration a, with an initial speed u and final speed v, has an average speed ∆ v = (u + v)/2.

Average speeds

The speed of light and sound

According to the theory of relativity, the speed of light in a vacuum is the fastest speed at which energy and information can move. It is denoted by the constant c and is equal c= 299 792 458 meters per second. Matter cannot move at the speed of light, because it will require an infinite amount of energy, which is impossible.

The speed of sound is usually measured in an elastic medium, and is equal to 343.2 meters per second in dry air at a temperature of 20 ° C. The speed of sound is lowest in gases and highest in solids. It depends on the density, elasticity, and shear modulus of a substance (which indicates the degree of deformation of a substance under shear load). Mach number M is the ratio of the speed of a body in a liquid or gas medium to the speed of sound in this medium. It can be calculated using the formula:

M = v/a,

where a is the speed of sound in the medium, and v- body speed. The Mach number is commonly used in determining speeds close to the speed of sound, such as the speeds of airplanes. This value is not constant; it depends on the state of the environment, which, in turn, depends on pressure and temperature. Supersonic speed is a speed exceeding Mach 1.

Vehicle speed

Below are some of the vehicle speeds.

• Passenger aircraft with turbofan engines: the cruising speed of passenger aircraft is from 244 to 257 meters per second, which corresponds to 878-926 kilometers per hour or M = 0.83-0.87.
• High-speed trains (like the Shinkansen in Japan): These trains reach top speeds of 36 to 122 meters per second, that is, 130 to 440 kilometers per hour.

Animal speed

The maximum speeds of some animals are approximately equal:



Human speed

• People walk at about 1.4 meters per second, or 5 kilometers per hour, and run at speeds up to about 8.3 meters per second, or 30 kilometers per hour.

Examples of different speeds

Four-dimensional speed

In classical mechanics, vector velocity is measured in three-dimensional space. According to the special theory of relativity, space is four-dimensional, and the measurement of speed also takes into account the fourth dimension - space-time. This speed is called four-dimensional speed. Its direction can change, but the value is constant and equal to c, that is, the speed of light. Four-dimensional speed is defined as

U = ∂x / ∂τ,

where x represents the world line - a curve in space-time along which the body moves, and τ - "proper time", equal to the interval along the world line.

Group speed

The group velocity is the velocity of propagation of waves, which describes the velocity of propagation of a group of waves and determines the rate of transfer of wave energy. It can be calculated as ∂ ω /∂k, where k is the wave number, and ω - angular frequency. K measured in radians / meter, and the scalar frequency of the waves ω - in radians per second.

Hypersonic speed

Hypersonic speed is a speed exceeding 3000 meters per second, that is, many times the speed of sound. Rigid bodies moving at such a speed acquire the properties of liquids, since due to inertia, the loads in this state are stronger than the forces that hold the molecules of matter together during collisions with other bodies. At ultra-high hypersonic speeds, two colliding solids turn into gas. In space, bodies move at exactly this speed, and engineers who design spaceships, orbital stations and spacesuits must take into account the possibility of a collision of a station or an astronaut with space debris and other objects when working in outer space. In such a collision, the skin of the spacecraft and the spacesuit suffer. Equipment designers are conducting hypersonic collision experiments in special laboratories to determine how strong the spacesuits, as well as the hull and other parts of the spacecraft, such as fuel tanks and solar panels, can withstand collisions for durability. For this, spacesuits and casing are subjected to impacts by various objects from a special installation at supersonic speeds exceeding 7500 meters per second.

The first attempts to understand the origin of sound were made more than two thousand years ago. In the writings of the ancient Greek scientists Ptolemy and Aristotle, correct assumptions are made that sound is generated by body vibrations. Moreover, Aristotle argued that the speed of sound is measurable and finite. Of course, in Ancient Greece there was no technical capability for any accurate measurements, so the speed of sound was relatively accurately measured only in the seventeenth century. For this, a comparison method was used between the time the flash was detected from the shot and the time after which the sound reached the observer. As a result of numerous experiments, scientists have come to the conclusion that sound travels in the air at a speed of 350 to 400 meters per second.

The researchers also found that the value of the speed of propagation of sound waves in a particular medium directly depends on the density and temperature of this medium. So, the thinner the air, the slower the sound travels through it. In addition, the higher the temperature of the medium, the higher the speed of sound. Today it is generally accepted that the speed of propagation of sound waves in air under normal conditions (at sea level at a temperature of 0 ° C) is 331 meters per second.

Mach number

In real life, the speed of sound is a significant parameter in aviation, however, at altitudes where it is usual, the characteristics of the environment are very different from normal. That is why aviation uses a universal concept called the Mach number, named after the Austrian Ernst Mach. This number is the speed of the object divided by the local speed of sound. Obviously, the lower the speed of sound in a medium with specific parameters, the larger the Mach number will be, even if the speed of the object itself does not change.

The practical application of this number is due to the fact that movement at a speed that is higher than the speed of sound differs significantly from movement at subsonic speeds. Basically, this is due to changes in the aerodynamics of the aircraft, deterioration of its controllability, heating of the body, and also with the resistance of waves. These effects are observed only when the Mach number exceeds one, that is, the object overcomes the sound barrier. At the moment, there are formulas that allow you to calculate the speed of sound for certain air parameters, and, therefore, calculate the Mach number for different conditions.