03. force and pressure 1

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PressurePressure

Pressure is defined as the force acting normally per unit area.

(Here, the word "normally" means perpendicularly.)

The SI unit of pressure is Pascal (Pa). 1 Pa is eaual to 1 newton

per metre 2 (N/m 2).

Factors that affect the pressure acting on a surface.

Magnitude of the force .

The larger the force, the higher the pressure.

Contact area .

The larger the contact area, the lower the pressure.

Examples of application of high pressure are :

a. Sharp Knifeb. Ice-Skatec. Sole of shoes with spike

Examples of application of low pressure are

a. Foundation of Buildingb. Snow Shoesc. Tyre of Tractor

d. Feet of Elephant

Note
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Liquid Pressure Pressure in Liquids1. Pressure in liquid is owing to the weight of the

liquid acting on the surface of any objects in theliquid.

2. Pressure of a liquid is directly proportional toa. the gravitational field strength

b. the depthc. density of the liquid.

Formulae:

Pressure caused by a liquid P = hg

Pressure in a liquid

P = hg + P atm

U-Tube Usually, a U-tube is used to compare and measure densityof liquids.

The density of the 2 liquids is related by the equation:

Application of Liquid Pressure1. Dam2. Submarine

3. Measuring Blood Pressure4. Intravenous Transfusion5. Water Tower


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Gas & Atmospheric Pressure Q: Explain how gas pressure is produced in a closedcontainer?

A:

Gas molecules in the container are in constant and random motion.As a result, the gas molecules collide on theinner wall of the container.After colliding on the wall, the gas molecules

bounce off, and the direction change creating achange of momentum to the molecule of the gas.The change of momentum produces a force onthe wall.The force per unit area is the pressure on thewall .

Atmospheric Pressure

1. On the surface of the earth, there is a thick

layer of gas called the atmosphere. Theatmosphere consists of various types of gascalled the atmospheric gas.

2. The atmospheric gases collide on the surfaceof the earth and hence exert a pressure on thesurface of the earth, called the atmospheric

pressure.3. The atmospheric pressure can be measured in

the unit of atm, mmHg or Pa. The atmospheric pressure at sea level is taken to be 1 atm,which is approximately 760 mmHg or 101,000Pa.

Characteristics of

Atmospherics Pressure

Decreases with altitude

The atmospheric pressure changes accordingly to the altitude. Altitude is

the height above sea level. The greater the altitude, the lower theatmospheric pressure.

Act equally in all direction

The atmospheric pressure acts on every object in

the atmosphere. It acts equally in all direction.

Atmospheric pressure is ~ 100,000Pa at sea level

On the surface of the earth, the atmospheric pressure

can be as high as 101,000 Pa.

Proof of Existence of Atmospheric Pressure

The existence of the atmospheric pressure can be proved

by the following experiments.a. Crushing can experiment

b. Water cover with cardboard does not flow outc. Magdeburg Hemisphere

Instruments Used to Measure Atmospheric Pressure

Atmospheric pressure can be measured by either

a. a simple barometer, b. a Fortin barometerc. an Aneroid barometer.

Unit Used to Measure Atmospheric Pressure

1. Pascal (Pa)

2. StandardAtmosphericPressure (atm)

3. mmHg (alsoknown as torr)

1 Pa = 1 N/m 2

1 atm = Atmospheric Pressure at

sea level ( = 101,325 Pa)

1 mmHg = 1/760 atm (roughly

equal to the liquid pressure

exerted by a millimetre of

mercury). In SPM, usually we use

the unit cmHg, instead of mmHg


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Gas & Atmospheric Pressure Simple Barometer

2 Cases

No air trapped in the tube Some air trapped in the tube

Pressure at A = x cmHg

Atmospheric pressure = h cmHgPressure at A = x cmHg + P gas

Atmospheric pressure = h cmHg + P gas

Characteristics of the Mercury Barometer


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Application of Atmospheric Pressure

Instruments Used to Measure Gas PressureThe pressure of the gas in a container can be measured

by usinga. Bourdon gauge

b. Manometer

P gas = P atm + P liquid .


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Pascals PrinciplePascal's principle states that in a

confined fluid, an externallyapplied pressure is transmitted

uniformly in all directions .

Pascal's principle is also known as

the principle of transmission of

pressure in a liquid.

Experiment

When the plunger is pushed in, the watersquirts equally from all the holes . This

shows that the pressure applied to the

plunger has been transmitted uniformly

throughout the water.

Hydraulic System

A hydraulic system applies Pascal's principle in its

working mechanism. It can be used as a force

multiplier .

According to Pascals principle,

Change of Oil Level in a Hydraulic System

In the diagram to the left, when piston-X is pressed

down, piston-Y will be push up. The change of the piston

levels of the 2 pistons is given by the following equation:


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Archimedes Principle Archimedes Principle

1. Archimedes Principle states that when a body

is wholly or partially immersed in a fluid it

experiences an upthrust equal to the weight of

the fluid displaced.

2. Upthrust/Buoyant force is an upward force

exerted by a fluid on an object immersed in it.

F = Upthrust/Buoyant Force

= Density of the liquidV = Volume of the displaced liquid

g = Gravitational field strength

Revision Note:

Density = mass per unit volume

Principle of Floatation

F = W

1. The principle of floatation states that when an

object floats in a liquid the buoyant

force/upthrust that acts on the object is equal

to the weight of the object.

2. If the weight of the object > upthrust, the

object will sink into the fluid.

Application of Archimedes Principle

1. Plimsoll Line2. Airship3. Hot Air Balloon

4. Hydrometers5. Submarine


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03. force and pressure 1

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