The SI unit of push is newton per square metre (N/m2). Press can also be to express in pascals (Pa);


Atmospheric pressure is periodically expressed as mmHg, cmHg or atmospheres.

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For a provided amount of force, the smaller the area of call the higher the press exerted. This describes why it would be simpler for a sharp pin to penetrate a item of cardboard 보다 a blunt one once the same pressure is used.

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

The pressure in between two solid surfaces relies on two things: (a) the force in between the surfaces (b) the area of contact between the two surfaces.


The greater the pressure or the smaller sized the area the greater the pressure.


A guy whose fixed is 90kg stands on a floor. If the area of contact between his feet and also the floor is 0.0368m2,a) determine exactly how much push he able come exert ~ above the floor. solutionP= F/A = 900N/0.0368m2

=24,456.5217N/m2. B) What pressure will he exert top top the floor if now he stand on one foot?

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

Pressure in fluids

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


Pressure in fluids rises with depth i.e. The better the depth the higher the press it exerts.

This defines why the wall surfaces of a dam are made broader downwards.

A diver under water experiences pressure due to the load of water over him add to the atmospheric pressure above the water surface. The deeper the diver, the better the pressure.


When a liquid is poured right into a set of linked tubes of various shapes, the rises up until the levels room the same in all the tubes.


The fluid pressure formula

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


Volume of the fluid = A*h

Mass = volume * density


Weight of the fluid = fixed * gravitational ar intensity= pressure exerted


From the definition of pressure, P= F/A

= Ahρg/A

= hρg

It is for this reason clear that press in fluids is directly proportional come the elevation of the shaft h, the thickness of the liquid ρ and also the gravitational ar strength g.

Note: pressure in fluids does not rely on the cross section area of the container which holds it.


Calculate the push exerted by a shaft of kerosene the 0.85m ( take the thickness of kerosene= 800kgm-3).

P=hρg =0.85m x 800kgm-3x10N/kg

= 6800Pa

Transmission of push in fluids

The figure below shows a fluid under pressure because of the force F acting on the plunger.


Assuming the the holes room identical, as soon as the plunger is thrust forward, the liquid squirts out v the holes through equal force. If the piston area is A, climate the pressure emerged is F/A. This push istransfer equally to all components of the liquid. This is referred to as Pascal’s principle. The principle claims that pressure used at one part of a liquid is sent equally to all other parts that the fastened liquid. Gases can likewise transmit push in a similar way provided they space incompressible.see a clip in the link below

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


Atmospheric pressure

The term environment refers to the air bordering the earth. The weight of air over the earth’s surface ar exerts press on the earth. This push is referred to as atmospheric pressure. The existence of atmospheric pressure have the right to be prove by the crushing can experiment;


The deserve to is filled v water then heated for numerous minutes. ~ sometime, the can is sealed and also then cooled by running cold water over it.

When the water is heated, steam is developed which displaces waiting in the can. As soon as cold water is run over it, vapor condenses leaving a vacuum in the can. Pressure inside is hence reduced below the outside atmospheric pressure. For this reason the can crushes inwards.

Atmospheric pressure is also very crucial when utilizing a drinking straw. By sucking through a drink straw the press inside is reduced. The atmospheric pressure acting ~ above the surface ar of the fluid overcomes the push inside the straw. The push difference and hence the resultant pressure pushes the fluid up the straw.

Measurement that pressure

Atmospheric press is measured utilizing an instrument referred to as a barometer. The adhering to are some species of barometers:


Atmospheric pressure have the right to support a liquid obelisk in a tube. One finish of the tube is closed and also the tube is filled through mercury. When inverted and also with the open up end listed below the liquid surface in the container. The atmospheric push (Patm ) on the open up surface is sent by the fluid to the base of the liquid column and supports the weight.

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

Pressure = hρg

Where h- is the elevation of the liquid column

ρ- thickness of the liquid (mercury)

g- Gravitational ar strength

At sea level atmospheric pressure can support approximately 760mm tower of mercury tantamount to approximately 10m pillar of water. Mercury is thus preferred as a barometric fluid because it gives a much shorter and measurable column compared to water.


In general, atmospheric pressure decreases v altitude. The value of atmospheric push at sea level is referred to as the traditional atmospheric pressure and also is in ~ times described as one atmosphere.

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

= 103,360N/m2

If over there is air trapped in the room above the mercury tower then the barometer is faulty. This room above the mercury column is referred to as toricellian vacuum. To check whether this space has some air trapped, the test pipe is tilted till it is in ~ the exact same level through the mercury pillar when the pipe is upright. If the an are is truly a vacuum, the check tube will be totally filled through mercury when if it has trapped waiting a an are will still stay at the top.


Normally the toricellian vacuum contains some small mercury vapour.

Note that this barometer is not easily portable.

A manometer

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


The points 2 and also 3 are at the exact same level and also as such endure the exact same amount the pressure. The push at 2 is the gas press while that at 3 equates to the pressure because of the liquid obelisk plus atmospheric pressure;

Pg = PA + h2ρg

Fortin barometer

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

The elevation of mercury tower is then check out from the key scale .this reading is then used to calculation the pressure at the place. Any change in the atmospheric pressure causes the level that mercury in the reservoir to relocate up or down, thus the adjustment of the cream color index is necessary.

Aneroid barometer

This kind of barometer is an ext portable.


When the pressure external the vacuum chamberbox is reduced, the box expands setup the levers into motion. However, as soon as the pressure exterior increases, package reduces in volume. The resultant movements of the springs and also levers moves the pointer throughout the range recording the worth of the atmospheric pressure.

The aneroid barometer can also be offered to measure heights. For instance, altimeters room aneroid barometers supplied in aircrafts to measure heights.

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Pressure gauge

Pressure gauges are additionally easily portable. It is generally used to measure up gas pressure, tyre pressure, etc. It consists of a coiled flexible steel tube. Once the pressure inside the pipe increases, the pipe uncoils. The movement of the pipe is intensified by the lever and also gear mechanism which then moves the pointer throughout the scale.