Pressure as a physical quantity. Pressure formula for air, vapor, liquid or solid

Imagine an air-filled sealed cylinder with a piston mounted on top. If you start to put pressure on the piston, then the volume of air in the cylinder will begin to decrease, the air molecules will collide with each other and with the piston more and more intensively, and the pressure of compressed air on the piston will increase.

If the piston is now abruptly released, then the compressed air will abruptly push it up. This will happen because with a constant piston area, the force acting on the piston from the compressed air will increase. The area of ​​the piston remained unchanged, and the force from the side of the gas molecules increased, and the pressure increased accordingly.

Or another example. A man stands on the ground, stands with both feet. In this position, a person is comfortable, he does not experience inconvenience. But what happens if this person decides to stand on one leg? He will bend one of his legs at the knee, and now he will lean on the ground with only one foot. In this position, a person will feel some discomfort, because the pressure on the foot has increased, and about 2 times. Why? Because the area through which gravity now presses a person to the ground has decreased by 2 times. Here is an example of what pressure is and how easy it is to detect in everyday life.

From the point of view of physics, pressure is a physical quantity numerically equal to the force acting perpendicular to the surface per unit area of ​​this surface. Therefore, in order to determine the pressure at a certain point on the surface, the normal component of the force applied to the surface is divided by the area of ​​the small surface element on which this force acts. And in order to determine the average pressure over the entire area, the normal component of the force acting on the surface must be divided by total area this surface.

Pressure is measured in pascals (Pa). This pressure unit got its name in honor of the French mathematician, physicist and writer Blaise Pascal, the author of the basic law of hydrostatics - Pascal's Law, which states that the pressure exerted on a liquid or gas is transmitted to any point unchanged in all directions. For the first time, the unit of pressure "pascal" was put into circulation in France in 1961, according to the decree on units, three centuries after the death of the scientist.

One pascal is equal to the pressure exerted by a force of one newton, evenly distributed, and directed perpendicular to a surface of one square meter.

In pascals, not only mechanical pressure (mechanical stress) is measured, but also the modulus of elasticity, Young's modulus, bulk modulus of elasticity, yield strength, proportionality limit, tear resistance, shear strength, sound pressure and osmotic pressure. Traditionally, it is in pascals that the most important mechanical characteristics of materials in the strength of materials are expressed.

Atmosphere technical (at), physical (atm), kilogram-force per square centimeter (kgf / cm2)

In addition to the pascal, other (off-system) units are also used to measure pressure. One such unit is the “atmosphere” (at). A pressure of one atmosphere is approximately equal to atmospheric pressure on the Earth's surface at sea level. Today, “atmosphere” is understood as the technical atmosphere (at).

The technical atmosphere (at) is the pressure produced by one kilogram-force (kgf) distributed evenly over an area of ​​one square centimeter. And one kilogram-force, in turn, is equal to the force of gravity acting on a body with a mass of one kilogram under conditions of acceleration free fall, equal to 9.80665 m/s2. One kilogram-force is thus equal to 9.80665 Newton, and 1 atmosphere turns out to be equal to exactly 98066.5 Pa. 1 at = 98066.5 Pa.

In atmospheres, for example, the pressure in car tires, for example, the recommended pressure in the tires of a GAZ-2217 passenger bus is 3 atmospheres.

There is also the "physical atmosphere" (atm), defined as the pressure of a column of mercury, 760 mm high at its base, given that the density of mercury is 13595.04 kg / m3, at a temperature of 0 ° C and under conditions of a gravitational acceleration of 9, 80665 m/s2. So it turns out that 1 atm \u003d 1.033233 atm \u003d 101 325 Pa.

As for the kilogram-force per square centimeter (kgf/cm2), this non-systemic unit of pressure is equal to normal atmospheric pressure with good accuracy, which is sometimes convenient for assessing various effects.

The non-systemic unit "bar" is approximately equal to one atmosphere, but is more accurate - exactly 100,000 Pa. In the CGS system, 1 bar is equal to 1,000,000 dynes/cm2. Previously, the name "bar" was carried by the unit, now called "barium", and equal to 0.1 Pa or in the CGS system 1 barium \u003d 1 dyn / cm2. The word "bar", "barium" and "barometer" come from the same Greek word"gravity".

Often, to measure atmospheric pressure in meteorology, the unit mbar (millibar), equal to 0.001 bar, is used. And to measure pressure on planets where the atmosphere is very rarefied - microbar (microbar), equal to 0.000001 bar. On technical pressure gauges, most often the scale has a graduation in bars.

Millimeter of mercury column (mm Hg), millimeter of water column (mm of water column)

The non-systemic unit of measure "millimeter of mercury" is 101325/760 = 133.3223684 Pa. It is designated "mm Hg", but sometimes it is designated "torr" - in honor of the Italian physicist, a student of Galileo, Evangelista Torricelli, the author of the concept of atmospheric pressure.

The unit was formed in connection with convenient way measurement of atmospheric pressure with a barometer, in which the mercury column is in equilibrium under the influence of atmospheric pressure. Mercury has a high density of about 13,600 kg/m3 and is characterized by low saturation vapor pressure under conditions room temperature, therefore, mercury was chosen for barometers at one time.

At sea level, atmospheric pressure is approximately 760 mm Hg, it is this value that is now considered to be normal atmospheric pressure, equal to 101325 Pa or one physical atmosphere, 1 atm. That is, 1 millimeter of mercury is equal to 101325/760 pascals.

In millimeters of mercury, pressure is measured in medicine, meteorology, and aviation navigation. In medicine, blood pressure is measured in mmHg; in vacuum technology, it is graduated in mmHg, along with bars. Sometimes they even just write 25 microns, meaning microns of mercury, if we are talking about evacuation, and pressure measurements are carried out with vacuum gauges.

In some cases, millimeters of water column are used, and then 13.59 mm of water column \u003d 1 mm Hg. Sometimes it is more expedient and convenient. A millimeter of a water column, like a millimeter of a mercury column, is an off-system unit, which in turn is equal to the hydrostatic pressure of 1 mm of a column of water that this column exerts on flat base at a column water temperature of 4°C.

Nobody likes to be under pressure. And it doesn't matter which one. Queen also sang about this along with David Bowie in their famous single "Under pressure". What is pressure? How to understand pressure? In what it is measured, by what instruments and methods, where it is directed and what it presses on. The answers to these and other questions - in our article about pressure in physics and not only.

If the teacher puts pressure on you by asking tricky problems, we will make sure that you can answer them correctly. After all, understanding the essence of things is the key to success! So what is pressure in physics?

A-priory:

Pressure- scalar physical quantity, equal to strength acting per unit surface area.

AT international system SI is measured in Pascals and is marked with the letter p . Pressure unit - 1 Pascal. Russian designation - Pa, international - Pa.

According to the definition, to find pressure, you need to divide the force by the area.

Any liquid or gas placed in a vessel exerts pressure on the walls of the vessel. For example, borscht in a saucepan acts on its bottom and walls with some pressure. Formula for determining fluid pressure:

where g is the acceleration of free fall in the gravitational field of the earth, h- the height of the borscht column in the pan, Greek letter "ro"- the density of borscht.

The most commonly used instrument for measuring pressure is the barometer. But what is pressure measured in? In addition to pascal, there are other off-system units of measurement:

• atmosphere;
• millimeter of mercury;
• millimeter of water column;
• meter of water column;
• kilogram-force.

Depending on the context, different off-system units are used.

For example, when you listen to or read the weather forecast, there is no question of Pascals. They talk about millimeters of mercury. One millimeter of mercury is 133 Pascal. If you drive, you probably know that normal tire pressure passenger car- about two atmospheres.

Atmosphere pressure

The atmosphere is a gas, more precisely, a mixture of gases that is held near the Earth due to gravity. The atmosphere passes into interplanetary space gradually, and its height is approximately 100 kilometers.

How to understand the expression "atmospheric pressure"? over each square meter Earth's surface is a hundred-kilometer column of gas. Of course, the air is transparent and pleasant, but it has a mass that presses on the surface of the earth. This is atmospheric pressure.

Normal atmospheric pressure is considered to be equal to 101325 Pa. This is the pressure at sea level at 0 degrees Celsius. Celsius. The same pressure at the same temperature is exerted on its base by a column of mercury with a height 766 millimeters.

The higher the altitude, the lower the atmospheric pressure. For example, on top of a mountain Chomolungma it is only one-fourth of normal atmospheric pressure.

Blood pressure

Another example where we face pressure in Everyday life is a measurement of blood pressure.

Blood pressure is blood pressure, i.e. The pressure that blood exerts on the walls of blood vessels, in this case arteries.

If you have measured your blood pressure and you have it 120 on the 80 , then all is well. If a 90 on the 50 or 240 on the 180 , then it will definitely not be interesting for you to figure out what this pressure is measured in and what it generally means.

However, the question arises: 120 on the 80 what exactly? Pascals, millimeters of mercury, atmospheres or some other units of measurement?

Blood pressure is measured in millimeters of mercury. It determines the excess pressure of the liquid in circulatory system above atmospheric pressure.

Blood exerts pressure on the vessels and thereby compensates for the effect of atmospheric pressure. Otherwise, we would simply be crushed by a huge mass of air above us.

But why in the dimension blood pressure two numbers?

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The fact is that the blood moves in the vessels not evenly, but in jolts. The first digit (120) is called systolic pressure. This is the pressure on the walls of blood vessels at the time of contraction of the heart muscle, its value is the largest. The second digit (80) defines smallest value and called diastolic pressure.

When measuring, the values ​​​​of systolic and diastolic pressures are recorded. For example, for healthy person a typical blood pressure value is 120 to 80 millimeters of mercury. This means that the systolic pressure is 120 mm. rt. Art., and diastolic - 80 mm Hg. Art. The difference between systolic and diastolic pressure is called pulse pressure.

physical vacuum

Vacuum is the absence of pressure. More precisely, its almost complete absence. Absolute vacuum is an approximation, like an ideal gas in thermodynamics and material point in mechanics.

Depending on the concentration of the substance, low, medium and high vacuum are distinguished. The best approximation to the physical vacuum is space, in which the concentration of molecules and pressure are minimal.

Pressure is the main thermodynamic parameter of the state of the system. It is possible to determine the pressure of air or another gas not only by instruments, but also using equations, formulas and laws of thermodynamics. And if you don’t have time to figure it out, the student service will help you solve any problem of determining pressure.

Why does a person standing on skis not fall into loose snow? Why does a car with wide tires have more flotation than a car with regular tires? Why does a tractor need caterpillars? We will find out the answer to these questions by getting acquainted with the physical quantity called pressure.

Solid body pressure

When a force is applied not to one point of the body, but to many points, then it acts on the surface of the body. In this case, one speaks of the pressure that this force creates on the surface of a solid body.

In physics, pressure is a physical quantity that is numerically equal to the ratio of the force acting on a surface perpendicular to it to the area of ​​this surface.

p = F/S ,

where R - pressure; F - force acting on the surface; S - surface area.

So, pressure occurs when a force acts on a surface perpendicular to it. The magnitude of the pressure depends on the magnitude of this force, and is directly proportional to it. The greater the force, the greater the pressure it creates per unit area. The elephant is heavier than the tiger, so it exerts more pressure on the surface. The car pushes against the road with more force than the pedestrian.

The pressure of a solid body is inversely proportional to the surface area on which the force acts.

Everyone knows that walking in deep snow is difficult due to the fact that the legs constantly fall through. But skiing is pretty easy. The thing is that in both cases a person acts on the snow with the same force - the force of gravity. But this force is distributed over surfaces with different area. Since the surface area of ​​the skis is larger than the area of ​​the soles of the boots, the weight of a person in this case is distributed over a larger area. And the force acting per unit area is several times smaller. Therefore, a person standing on skis puts less pressure on the snow and does not fall into it.

By changing the surface area, you can increase or decrease the amount of pressure.

When going on a hike, we choose a backpack with wide straps to reduce pressure on the shoulder.

To reduce the pressure of the building on the ground, increase the area of ​​\u200b\u200bthe foundation.

Tires trucks make wider than tires cars so that they exert less pressure on the ground. For the same reason, a tractor or tank is made on tracks, and not on wheels.

Knives, blades, scissors, needles are sharpened sharply so that they have the smallest possible area of ​​\u200b\u200bthe cutting or piercing part. And then even with the help of a small applied force, a lot of pressure is created.

For the same reason, nature has provided animals with sharp teeth, fangs, and claws.

Pressure - scalar. In solids, it is transmitted in the direction of the force.

The unit of force is newton. The area unit is m 2 . Therefore, the unit of pressure is N/m 2 . This value in the international system of units SI is called pascal (Pa or Ra). It got its name in honor of the French physicist Blaise Pascal. A pressure of 1 pascal causes a force of 1 newton acting on a surface of 1 m 2 .

1 Pa = 1N/m2 .

Other systems use units such as bar, atmosphere, mmHg. Art. (millimeters of mercury), etc.

Pressure in liquids

If in a solid body pressure is transmitted in the direction of the force, then in liquids and gases, according to Pascal's law, " any pressure exerted on a liquid or gas is transmitted in all directions without change ».

Let's fill a ball with tiny holes connected to a narrow tube in the form of a cylinder with liquid. Let's fill the ball with liquid, insert the piston into the tube and start moving it. The piston presses on the surface of the liquid. This pressure is transmitted to every point of the fluid. Liquid begins to pour out of the holes in the ball.

Filling the balloon with smoke, we will see the same result. This means that in gases pressure is also transmitted in all directions.

The force of gravity acts on the liquid, as on any body on the surface of the Earth. Each layer of liquid in the container creates pressure with its own weight.

This is confirmed by the following experiment.

If water is poured into a glass vessel, instead of the bottom of which has a rubber film, then the film will sag under the weight of water. And the more water there is, the more the film will bend. If we gradually immerse this vessel with water into another container, also filled with water, then as it sinks, the film will straighten. And when the water levels in the vessel and container are equal, the film will straighten completely.

At the same level, the pressure in the liquid is the same. But with increasing depth, it increases, since the molecules upper layers put pressure on the molecules of the lower layers. And those, in turn, put pressure on the molecules of the layers located even lower. Therefore, at the lowest point of the tank, the pressure will be the highest.

The pressure at depth is determined by the formula:

p = ρ g h ,

where p - pressure (Pa);

ρ - liquid density (kg / m 3);

g - free fall acceleration (9.81 m/s);

h - height of the liquid column (m).

It can be seen from the formula that the pressure increases with depth. The lower the submersible descends in the ocean, the more pressure it will experience.

Atmosphere pressure

Evangelista Torricelli

Who knows, if in 1638 the Duke of Tuscany had not decided to decorate the gardens of Florence with beautiful fountains, atmospheric pressure would not have been discovered in the 17th century, but much later. We can say that this discovery was made by chance.

In those days, it was believed that the water would rise behind the piston of the pump, because, as Aristotle said, "nature does not tolerate emptiness." However, the event was not successful. The water in the fountains really rose, filling the resulting "void", but at a height of 10.3 m it stopped.

They turned to Galileo Galilei for help. Since he could not find a logical explanation, he instructed his students - Evangelista Torricelli and Vincenzo Viviani conduct experiments.

Trying to find the cause of the failure, Galileo's students found out that different liquids rise behind the pump to different heights. The denser the liquid, the lower the height it can rise. Since the density of mercury is 13 times that of water, it can rise to a height 13 times less. Therefore, they used mercury in their experiment.

In 1644 the experiment was carried out. The glass tube was filled with mercury. Then it was thrown into a container, also filled with mercury. After some time, the column of mercury in the tube rose. But he did not fill the entire tube. There was an empty space above the mercury column. It was later called the "Torricellian void". But mercury did not pour out of the tube into the container either. Torricelli explained this by the fact that mercury presses atmospheric air and keeps it in the tube. And the height of the mercury column in the tube shows the magnitude of this pressure. This was the first time atmospheric pressure was measured.

The atmosphere of the Earth is its air shell, held near it by gravitational attraction. The gas molecules that make up this shell are constantly and randomly moving. Under the influence of gravity, the upper layers of the atmosphere press on the lower layers, compressing them. The lowest layer near the Earth's surface is compressed the most. Therefore, the pressure in it is the greatest. According to Pascal's law, it transmits this pressure in all directions. It is experienced by everything that is on the surface of the Earth. This pressure is called atmospheric pressure .

Since atmospheric pressure is created by the overlying layers of air, it decreases with increasing altitude. It is known that high in the mountains it is less than at the foot of the mountains. And deep underground it is much higher than on the surface.

Normal atmospheric pressure is the pressure equal to the pressure of a column of mercury 760 mm high at a temperature of 0 o C.

Atmospheric pressure measurement

Because atmospheric air has different densities different height, then the value of atmospheric pressure cannot be determined by the formulap = ρ · g · h . Therefore, it is determined using special instruments called barometers .

Distinguish between liquid barometers and aneroids (non-liquid). The operation of liquid barometers is based on the change in the column of liquid level under the pressure of the atmosphere.

The aneroid is a sealed container made of corrugated metal, inside which a vacuum is created. The container contracts when the atmospheric pressure rises and straightens when it is lowered. All these changes are transmitted to the arrow with the help of a spring metal plate. The end of the arrow moves along the scale.

By changing the readings of the barometer, one can assume how the weather will change in the coming days. If the atmospheric pressure rises, then clear weather can be expected. And if it goes down, it will be cloudy.

In diving practice, one often encounters the calculation of mechanical, hydrostatic and gas pressure of a wide range of values. Depending on the value of the measured pressure, different units are used.

In the SI and ISS systems, the unit of pressure is the pascal (Pa), in the MKGSS system - kgf / cm 2 (technical atmosphere - at). Torah (mm Hg), atm (physical atmosphere), m of water are used as non-systemic units of pressure. Art., and in English measures - pounds / inch 2. Relationships between different pressure units are given in Table 10.1.

Mechanical pressure is measured by the force acting perpendicular to the unit surface area of ​​the body:

where p - pressure, kgf / cm 2;
F - force, kgf;
S - area, cm 2.

Example 10.1. Determine the pressure that the diver exerts on the deck of the vessel and on the ground under water when he takes a step (i.e. stands on one leg). The weight of a diver in equipment in the air is 180 kgf, and under water 9 kgf. The area of ​​the sole of the diving galoshes is taken to be 360 ​​cm 2. Decision. 1) The pressure transmitted by the diving boots to the deck of the ship, according to (10.1):

P \u003d 180/360 \u003d 0.5 kgf / cm

Or in SI units

P \u003d 0.5 * 0.98.10 5 \u003d 49000 Pa \u003d 49 kPa.

Table 10.1. Relationships between different units of pressure

2) The pressure transmitted by diving galoshes to the ground under water:

or in SI units

P \u003d 0.025 * 0.98 * 10 5 \u003d 2460 Pa \u003d 2.46 kPa.

hydrostatic pressure liquid everywhere perpendicular to the surface on which it acts, and increases with depth, but remains constant in any horizontal plane.

If the surface of the liquid does not experience external pressure (for example, air pressure) or it is not taken into account, then the pressure inside the liquid is called excess pressure.

where p is the liquid pressure, kgf/cm 2 ;
p is the density of the liquid, gf "s 4 / cm 2;
g - free fall acceleration, cm/s 2 ;
Y- specific gravity liquids, kg/cm 3 , kgf/l;
H - depth, m.

If the surface of the liquid experiences external pressure the pressure inside the liquid

If atmospheric air pressure acts on the surface of a liquid, then the pressure inside the liquid is called absolute pressure(i.e. pressure measured from zero - full vacuum):
where B - atmospheric (barometric) pressure, mm Hg. Art.
In practical calculations for fresh water accept
Y \u003d l kgf / l and atmospheric pressure p 0 \u003d 1 kgf / cm 2 \u003d \u003d 10 m of water. Art., then the excess water pressure in kgf / cm 2
and the absolute water pressure
Example 10.2. Find the absolute pressure of sea water acting on a diver at a depth of 150 m if the barometric pressure is 765 mm Hg. Art., and the specific gravity of sea water is 1.024 kgf / l.

Decision. Absolute pressure of the ox by (10/4)

estimated value of absolute pressure according to (10.6)
AT this example the use of the approximate formula (10.6) for the calculation is quite justified, since the calculation error does not exceed 3%.

Example 10.3. In a hollow structure containing air under atmospheric pressure p a \u003d 1 kgf / cm 2, located under water, a hole was formed through which water began to flow (Fig. 10.1). What pressure force will the diver experience if he tries to close this hole with his hand? The area "At the cross section of the hole is 10X10 cm 2, the height of the water column H above the hole is 50 m.

Rice. 9.20. Observation chamber "Galeazzi": 1 - eye; 2 - cable recoil device and cable cut; 3 - fitting for telephone input; 4 - hatch cover; 5 - upper porthole; 6 - rubber attachment ring; 7 - lower porthole; 8 - camera body; 9 - oxygen cylinder with a pressure gauge; 10 - emergency ballast return device; 11 - emergency ballast; 12 - lamp cable; 13 - lamp; 14 - electric fan; 15-phone-microphone; sixteen - accumulator battery; 17 - regenerative working box; 18 - hatch cover porthole

Decision. Overpressure water at the hole according to (10.5)

P \u003d 0.1-50 \u003d 5 kgf / cm 2.

Pressure force on the diver's hand from (10.1)

F \u003d Sp \u003d 10 * 10 * 5 \u003d 500 kgf \u003d 0.5 tf.

The pressure of the gas contained in the vessel is distributed evenly, if we do not take into account its weight, which, given the dimensions of the vessels used in diving practice, has an insignificant effect. The magnitude of the pressure of a constant mass of gas depends on the volume it occupies and the temperature.

The relationship between the pressure of a gas and its volume at a constant temperature is established by the expression

P 1 V 1 = p 2 V 2 (10.7)

Where p 1 and p 2 - initial and final absolute pressure, kgf / cm 2;

V 1 and V 2 - initial and final volume of gas, l. The relationship between the pressure of a gas and its temperature at a constant volume is established by the expression

where t 1 and t 2 are the initial and final gas temperatures, °C.

At constant pressure, a similar relationship exists between the volume and temperature of the gas

The relationship between pressure, volume and temperature of a gas is established by the combined law of the gaseous state

Example 10.4. The capacity of the cylinder is 40 l, the air pressure in it is 150 kgf / cm 2 according to the manometer. Determine the volume of free air in the cylinder, i.e., the volume reduced to 1 kgf / cm 2.

Decision. Initial absolute pressure p \u003d 150 + 1 \u003d 151 kgf / cm 2, final p 2 \u003d 1 kgf / cm 2, initial volume V 1 \u003d 40 l. Free air volume from (10.7)

Example 10.5. The manometer on the oxygen cylinder in a room with a temperature of 17 ° C showed a pressure of 200 kgf / cm 2. This cylinder was transferred to the deck, where the next day at a temperature of -11 ° C, its readings decreased to 180 kgf / cm 2. An oxygen leak was suspected. Check if the suspicion is correct.

Decision. Initial absolute pressure p 2 \u003d 200 + 1 \u003d \u003d 201 kgf / cm 2, final p 2 \u003d 180 + 1 \u003d 181 kgf / cm 2, initial temperature t 1 \u003d 17 ° C, final t 2 \u003d -11 ° C. Estimated final pressure from (10.8)

Suspicions are unfounded, since the actual and calculated pressures are equal.

Example 10.6. A diver under water consumes 100 l / min of air compressed to a pressure of a diving depth of 40 m. Determine the flow rate of free air (i.e., at a pressure of 1 kgf / cm 2).

Decision. Initial absolute pressure at immersion depth according to (10.6)

P 1 \u003d 0.1 * 40 \u003d 5 kgf / cm 2.

Final absolute pressure P 2 \u003d 1 kgf / cm 2

Initial air flow Vi = l00 l/min.

Free air flow according to (10.7)

Let's do an experiment. Let us take a small board with four nails driven into the corners, and place it with the points up on the sand. We put a weight on top of it (Fig. 81). We will see that the nail heads are only slightly pressed into the sand. If we turn the board over and put it again (together with the weight) on the sand, now the nails will go into it much deeper (Fig. 82). In both cases, the weight of the board was the same, but the effect was different. Why? The whole difference in the cases under consideration was that the surface area on which the nails rested was larger in one case and smaller in the other. After all, at first the heads of the nails touched the sand, and then their points.

We see that the result of the impact depends not only on the force with which the body presses on the surface, but also on the area of ​​this surface. It is for this reason that a person who is able to slide on loose snow on skis immediately falls into it as soon as he takes them off (Fig. 83). But it's not just the area. The magnitude of the applied force also plays an important role. If, for example, on the same. board (see Fig. 81) put another weight, then the nails (with the same area of ​​\u200b\u200bsupport) will sink even deeper into the sand.

The force applied perpendicular to the surface is called force of pressure to this surface.

Pressure force should not be confused with pressure. Pressure- this is a physical quantity equal to the ratio of the pressure force applied to a given surface to the area of ​​\u200b\u200bthis surface:

p - pressure, F - pressure force, S - area.

So, to determine the pressure, it is necessary to divide the pressure force by the surface area on which the pressure is applied.

With the same force, the pressure is greater when the area of ​​support is smaller, and vice versa than more area supports, the less pressure.

In cases where the pressure force is the weight of the body on the surface (F = P = mg), the pressure exerted by the body can be found by the formula

If the pressure p and the area S are known, then the pressure force F can be determined; To do this, you need to multiply the pressure by the area:

F = pS (32.2)

The pressure force (like any other force) is measured in newtons. Pressure is measured in pascals. Pascal(1 Pa) is the pressure that a pressure force of 1 N produces when applied to a surface of 1 m 2:

1 Pa \u003d 1 N / m 2.

Other pressure units are also used - hectopascal (hPa) and kilopascal (kPa):

1 hPa = 100 Pa, 1 kPa = 1000 Pa.

1. Give examples showing that the result of the action of a force depends on the area of ​​\u200b\u200bthe support on which this force acts. 2. Why doesn't a skier fall into the snow? 3. Why does a sharp button go into wood more easily than a blunt one? 4. What is called pressure? 5. What units of pressure do you know? 6. What is the difference between pressure and pressure force? 7. How can you find the pressure force, knowing the pressure and the surface area to which the force is applied?