11-Fluids mekanik

7/26/2019 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



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 .



Li uids and ases have the abilit to flow

They are called fluids “ ”

obey



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)



Re ardless of form solid, li uid, as we

can define how much mass is squeezedinto a articular s ace

density =

mass



A measure of the amount of force

exerted on a surface area

pressure =



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



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



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



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



Pressure acts perpendicular

to the surface

and increasesat greater

depth.



Net u ward

force iscalled the

buoyant

force!!!

Easier to

a rocin water!!



The amount ofwater dis laced is

equal to the

volume of therock.



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.



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



’’ 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



A floatin ob ect dis laces a wei ht of fluid

equal to its own weight.



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



’’



’’ 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



’’

Density is massdivided by

volume.

volume and you

density and thus

.



’’ 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



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



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



Since air ets less

dense with altitude,

the buo ant forcedecreases with

altitude. So helium

balloons don’t riseforever!!!



’’



’’ 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)



’’ When the s eed of a fluid increases, internal

pressure in the fluid decreases.



’’



’’

quite complicated and relates to the air

.



’’

11-Fluids mekanik

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