The SI unit of pressure is newton every square metre (N/m2). Push can additionally be to express in pascals (Pa);


Atmospheric push is occasionally expressed together mmHg, cmHg or atmospheres.

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For a offered amount the force, the smaller the area of contact the better the push exerted. This explains why it would be less complicated for a sharp pin to pass through a piece of cardboard than a blunt one when the same pressure is used.

A solid relaxing on a horizontal surface ar exerts a normal contact force equates to to that weight. The press of the solid on the surface depends on the area that contact.

The pressure in between two hard surfaces counts on 2 things: (a) the force between the surfaces (b) the area of contact in between the 2 surfaces.


The better the pressure or the smaller the area the better the pressure.


A man whose massive is 90kg stand on a floor. If the area the contact in between his feet and the floor is 0.0368m2,a) determine how much push he able to exert top top the floor. solutionP= F/A = 900N/0.0368m2

=24,456.5217N/m2. B) What push will the exert ~ above the floor if now he was standing on one foot?

P=900N/(0.0368/2) =48,913.0435N/m2

Pressure in fluids

A fluid is a ax that refers to either liquids and also gases. The set up listed below can be supplied to show pressure in fluids:


Pressure in fluids boosts with depth i.e. The better the depth the higher the push it exerts.

This describes why the wall surfaces of a dam room made more comprehensive downwards.

A diver under water experiences pressure as result of the load of water above him add to the atmospheric pressure over the water surface. The depths the diver, the greater the pressure.


When a liquid is poured into a set of associated tubes of different shapes, the rises up till the levels room the exact same in every the tubes.


The fluid pressure formula

Consider a liquid of density ρ in a container of uniform cross-section area A, such the the depth the the liquid in the container is h


Volume the the liquid = A*h

Mass = volume * density


Weight that the fluid = mass * gravitational ar intensity= pressure exerted


From the meaning of pressure, P= F/A

= Ahρg/A

= hρg

It is therefore clear that push in fluids is straight proportional to the height of the column h, the thickness of the fluid ρ and also the gravitational ar strength g.

Note: press in fluids go not rely on the cross ar area that the container i beg your pardon holds it.


Calculate the press exerted through a obelisk of kerosene that 0.85m ( take the density of kerosene= 800kgm-3).

P=hρg =0.85m x 800kgm-3x10N/kg

= 6800Pa

Transmission of press in fluids

The figure listed below shows a fluid under pressure as result of the pressure F exhilaration on the plunger.


Assuming that the holes room identical, as soon as the plunger is propelled forward, the fluid squirts out v the holes with equal force. If the piston area is A, then the pressure occurred is F/A. This pressure is sent same to all components of the liquid. This is called Pascal’s principle. The principle says that pressure used at one part of a fluid is sent same to all other parts that the attached liquid. Gases can additionally transmit push in a comparable way noted they room incompressible.see a clip in the connect below

The working of hydraulic equipments is based upon Pascal’s principle.


Atmospheric pressure

The term setting refers come the air surrounding the earth. The load of air above the earth’s surface exerts press on the earth. This pressure is called atmospheric pressure. The presence of atmospheric pressure deserve to be demonstrated by the crushing deserve to experiment;


The deserve to is filled v water climate heated for several minutes. After sometime, the can is sealed and also then cooled by to run cold water over it.

When the water is heated, steam is created which displaces waiting in the can. When cold water is run over it, vapor condenses leave a vacuum in the can. Pressure inside is therefore reduced listed below the external atmospheric pressure. Therefore the have the right to crushes inwards.

Atmospheric press is likewise very critical when making use of a drink straw. Through sucking v a drinking straw the pressure inside is reduced. The atmospheric push acting on the surface of the fluid overcomes the push inside the straw. The push difference and hence the resultant force pushes the fluid up the straw.

Measurement the pressure

Atmospheric push is measured making use of an instrument referred to as a barometer. The complying with are some species of barometers:


Atmospheric pressure have the right to support a liquid tower in a tube. One end of the tube is closed and the pipe is filled through mercury. As soon as inverted and with the open end below the liquid surface in the container. The atmospheric press (Patm ) on the open surface istransfer through the liquid to the base of the liquid column and also supports that weight.

With the liquid pillar h, the atmospheric pressure can be figured out from the equation;

Pressure = hρg

Where h- is the elevation of the fluid column

ρ- thickness of the liquid (mercury)

g- Gravitational field strength

At sea level atmospheric pressure deserve to support about 760mm pillar of mercury tantamount to around 10m obelisk of water. Mercury is thus preferred as a barometric fluid since it gives a much shorter and measurable column contrasted to water.


In general, atmospheric press decreases through altitude. The worth of atmospheric push at sea level is dubbed the typical atmospheric pressure and is in ~ times described as one atmosphere.

pressure at sea level =hρg =0.76m*13600kg/m3 *10N/kg

= 103,360N/m2

If there is wait trapped in the room above the mercury tower then the barometer is faulty. This an are above the mercury tower is dubbed toricellian vacuum. To test whether this room has part air trapped, the test pipe is tilted until it is in ~ the very same level through the mercury tower when the tube is upright. If the room is important a vacuum, the test tube will be totally filled v mercury while if it has actually trapped wait a an are will still continue to be at the top.


Normally the toricellian vacuum contains some tiny mercury vapour.

Note the this barometer is not readily portable.

A manometer

This is a U-shaped tube open up on both ends. One finish is linked to a source of gas whose push is to be determined. The various other arm is open up to the atmosphere. This create a pressure difference which displaces the manometer liquid.


The clues 2 and 3 are at the exact same level and also as such experience the very same amount that pressure. The press at 2 is the gas pressure while that at 3 equals the pressure due to the liquid column plus atmospheric pressure;

Pg = PA + h2ρg

Fortin barometer

The fixed ivory index v a sharp point acts asthe zero note of the main scale. Before taking any kind of reading the level of the mercury in the reservoir must first be adjusted until the tip of the cream color index just touches the surface ar of mercury.

The height of mercury pillar is then read from the main scale .this analysis is then offered to calculate the press at the place. Any change in the atmospheric pressure reasons the level the mercury in the reservoir to move up or down, hence the convey of the cream color index is necessary.

Aneroid barometer

This form of barometer is more portable.


When the pressure exterior the vacuum chamberbox is reduced, the box expands setup the levers into motion. However, once the pressure exterior increases, package reduces in volume. The resultant motions of the springs and also levers move the pointer across the scale recording the worth of the atmospheric pressure.

The aneroid barometer can also be used to measure heights. Because that instance, altimeters space aneroid barometers supplied in aircrafts to measure heights.

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Pressure gauge

Pressure gauges are additionally easily portable. The is typically used to measure gas pressure, tire pressure, etc. It consists of a coiled flexible metal tube. As soon as the pressure inside the tube increases, the tube uncoils. The motion of the pipe is amplified by the lever and gear mechanism which climate moves the pointer throughout the scale.