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3.Force and Pressure3.1 Pressure

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3.1 Pressure

Pressure is defined as normal force per

unit area

Unit for pressure is Nm-2 orPascal(Pa)

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Applications of High Pressure

A sharp knife can be used to cut an object

because very small area produce a large

pressure

Spiked running shoes can provide a better

grip

Ice-skates are designed to have a small

area. The weight of skater produces a

large pressure on ice

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3.2 Understanding Pressure in

Liquid

The Pressure Formula

P = hpg

WhereP = Pressure (Nm-2)

h = depth (m)

p = density (kgm-3)g = accelerations due the gravity (ms-2)

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Applications of Pressure in Liquids

1.Public water supply

A water tank positioned high above theground

2.A patient receiving drips of a liquid fromthe bottle, the bottle placed at a height

3.The Walls of a dam

the wall of a dam increase in thicknessdownwards

4.Fire house used by a fire-fighter

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3.3 Understanding Gas Pressure

and Atmospheric Pressure Gas Pressure

1. According to the Kinetic Theory Of Gases,molecules in the gas are always movingrandomly and constantly colliding with the wall

its container2. Collisions of gas molecules on anysurface/wall produce an impulsive force,creating the gas pressure

3. The force unit area produced the GasPressure

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Atmospheric Pressure

The weight of the mass of the atmosphere

on the Earths surface cause atmospheric

pressure

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Altitude and the Magnitude of

Atmospheric Pressure

The Atmospheric pressure is decreases

with altitude

At high altitude, the density of air become

lower

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Instruments for measuring pressure

Gas Pressure

(1) Manometer

P gas = P atm + hpg

(2) Bourdon Gauge

Atmospheric Pressure

(1) Mercury Barometer

(2) Aneroid Barometer

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Application of Atmospheric

Pressure

1. Drinking straw

When air is sucked out from a straw.the airpressure inside becomes lower,then the higher

atm. Pressure acting on the surface of thedrinking pushes the water into the straw.

2. Syringe

When the piston of a syringe is pulled

upwards, a partial vacuum is created in thesyringe. The atm. pressure is greater will forcethe water to flow up

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3, Rubber Sucker

When we push a rubber plunger againsta wall, air pushed out from plunger. The

atm. Pressure inside decreases.The atmpressure greater than inside the plungerand it is able to support the weight

4. Siphon 5. Vacuum cleaner

6. Lift pump

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3.4 PASCALS PRINCIPLE

Pascals principle state that when pressureis applied to an enclosed fluid, thepressure will be transmitted equally

throughout the whole enclosed fluid.

Pressure acting to = pressure acting to

smaller syringe larger syringeF1 = F2

A1 A2

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Applications of Pascals principle

1. Hydraulic jack

2. Car hyrdraulic system (brakes)

3. Hydraulic lift

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3.5 ARCHIMEDES PRINCIPLE

States that an on object Which is partially

or wholly immersed in a fluid is acted upon

by an upward buoyant force equal to

the weight of the fluid it displace

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Buoyant Force

Force acting on upper surface F1= h1pgA

Force acting on lower surface ,F2 = h2pgA

Buoyant force = the resultant force acting

upwards= F2 F1

= (h2h1)Apg=Vpg=mg

= mass liquid displaced x g

= weight of liquid displaced by the

object

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Buoyant force

A hot air balloon rises from the surface of

the earth due to buoyant force acting on it

A ship made of iron can float

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Applications of Archimedes

Principle Submarine

A submarine has large ballast tanks to control itspositions and the depths it can submerge below sealevel

When afloat , water is drive out from the ballast tanks bycompressed air to reduce its weight and produce a largerbuoyant force.

When submerged the ballast tanks are filled with wateragain to increase the weight of the submarine

The submarine can submerge to a depth where thebuoyant force is equal to its weight

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Application of Archimedes

Ship

Hot-air balloons

If the buoyant force is equal to the totalweight of the balloon it remains stationary

or move upwards with constant velocity

Hydrometer

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3.6Bernoullis Principle

Bernoullis principle states that for uniform

flow of a fluid, region of high velocity

corresponds to low pressure

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Applications of Bernoulli principle

Aerofoil

Bunsen Burner

Carburettor