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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
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