11-fluids mekanik
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u su s
Dr. MuhammadDr. Muhammad SyukriSyukri, MT, MT
Department of Physics, Faculty of Math and Natural SciencesDepartment of Physics, Faculty of Math and Natural Sciences
UnsyiahUnsyiah
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A f ui is a substance that flows under the action of shearing
forces. If a fluid is at rest, we know that the forces on it are in
balance.
A gas is a fluid that is easily compressed. It fills any vessel in
which it is contained.
A liquid is a fluid which is hard to compress. A given mass of
liquid will occupy a fixed volume, irrespective of the size of
t e conta ner.
A free surface is formed as a boundary between a liquid and a
gas a ove .
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Li uids and ases have the abilit to flow
They are called fluids “ ”
obey
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The ensity of a fluid is defined as its mass per unit
volume. It is denoted by the Greek symbol, ρ .
ρ =
V m3kgm-3
water = 998 kgm-3
ρ air =1.2kgm-3
gm
If the density is constant (most liquids), the flow is
incompressible.
If the density varies significantly (eg some gas
flows), the flow is compressible.
(Although gases are easy to compress, the flow may be treated as
incompressible if there are no large pressure fluctuations)
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Re ardless of form solid, li uid, as we
can define how much mass is squeezedinto a articular s ace
density =
mass
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A measure of the amount of force
exerted on a surface area
pressure =
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Pressure is the force per unit area, where the force is
perpendicular to the area.
p=
A m2 Nm-2
F pa= 105 Nm-2
1psi =6895Pa
This is the Absolute pressure, the pressure compared to
a vacuum.The pressure measured in your tyres is the gauge pressure,
- a
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The ressure is ust the wei ht of all the
fluid above you Atmos heric ressure is ust the wei ht
of all the air above on area on the surface
of the earth
In a swimming pool the pressure on your
water above you (plus the air pressure
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So, the onl thin that counts in fluid
pressure is the gravitational force actingon the mass ABOVE ou
The deeper you go, the more weight
above ou and the more ressure
Go to a mountaintop and the air pressure
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Pressure in a fluid acts equally in all directions
Pressure in a static liquid increases linearly with depth
Δ p=
increase inde th m pressure
ρ g Δ h
The pressure at a given depth in a continuous, static body of
liquid is constant.
p1 p3 p1 = p2 = p3
2
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Pressure acts perpendicular
to the surface
and increasesat greater
depth.
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Net u ward
force iscalled the
buoyant
force!!!
Easier to
a rocin water!!
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The amount ofwater dis laced is
equal to the
volume of therock.
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Measuring pressure (1)Measuring pressure (1)
ManometersManometers
p1 =
p1 = px
=
(negligible pressure
change in a gas)
(since they are at
h x
z
pz= p2 = pa
t e same eig t)
- =
liquid
density ρ
y z
p1 - pa = ρ gh
So a manometer measures gauge pressure.
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Measuring Pressure (2)Measuring Pressure (2)
BarometersBarometers
A barometer is used to measure p1 = 0vacuum
.
The simplest type of barometer
consists of a column of fluid.h
p2 = pa
p2 - p1 = ρ gh
a
examples
= = 5 3* ~ a .
mercury: h = pa/ ρ g =105
/(13.4*103
*9.8)~800mm
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’’ An immersed bod is buo ed u b a force
equal to the weight of the fluid it displaces. If the buo ant force on an ob ect is
greater than the force of gravity acting on
the ob ect the ob ect will float
The apparent weight of an object in a
the buoyant force
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A floatin ob ect dis laces a wei ht of fluid
equal to its own weight.
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The rimar difference between a li uid
and a gas is the distance between themolecules
In a gas, the molecules are so widely
se arated that there is little interactionbetween the individual moledules
Independent of what the molecules are
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’’
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’’ Pressure de ends on densit of the as
Pressure is just the force per unit areaexerted b the molecules as the collide
with the walls of the container
,
collisions with the wall and this doubles
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’’
Density is massdivided by
volume.
volume and you
density and thus
.
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’’ At a iven tem erature for a iven
quantity of gas, the product of theressure and the volume is a constant
P V = P V
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ust the wei ht of the air above ou
Unlike water, the density of the airdecreases with altitude since air is
compressible and liquids are only very
sli htl com ressible
Air pressure at sea level is about 105
2
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An ob ect surrounded b air is buo ed u b
a force equal to the weight of the airdis lace.
Exactly the same concept as buoyancy in
water. ust substitute air for water in thestatement
weight of the object, it will rise in the air
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Since air ets less
dense with altitude,
the buo ant forcedecreases with
altitude. So helium
balloons don’t riseforever!!!
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’’
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’’ Flow is faster when the i e is narrower
Put your thumb over the end of a gardenhose
Energy conservation requires that the
faster
compress a gas (PV is energy, more later)
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’’ When the s eed of a fluid increases, internal
pressure in the fluid decreases.
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’’
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’’
quite complicated and relates to the air
.
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’’