Chapter VIII Fluid Mechanics

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24 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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Chapter VIII

Fluid Mechanics


Density and Pressure


Pressures in a fluid at Rest


Buoyancy


Fluid Flow



A
fluid

is a collection of molecules that are randomly arranged and
held together by weak cohesive forces and by forces exerted by
the walls of a container.


Both liquids and gases are
fluids
.

A. Density and Pressure

F

=

force

A
=

surface area

p = Pressure

T
he density of a substance is
defined

as its mass per unit
volume



= m/V




= dencity

m = mass

V = Volume

T
he pressure
p

at a depth h below the surface
of a liquid open to the

atmosphere is greater
than atmospheric pressure

(p
o
)

by an amount

gh

B. Pressures in a Fluid at Rest

Pascal’s law: A change in the pressure

applied to a
fluid

is
transmitted undiminished to every point of the
fluid

and to the
walls of the container.

Because the increase in
pressure is the same on

the
two sides, a small force F
l

at
the left produces a much
greater force F
2

at the right

C. Buoyancy


Have you ever tried to push a beach ball under water?


This is extremely
difficult

to

do because of the large upward force
exerted by the water on the ball.


The upward

force exerted by water on any immersed object is
called a buoyant force
.

1. Totally Submerged Object

B = Buoyant force

F
g

= Force of gravity/weight

2. Floating Object


F
g

= B


o

V
o

g =

f

V
f

g

V
f


is the volume of the
fluid

displaced by the
object (this

volume is the same as the
volume of that part of the


object that is
beneath the

fluid

level)


Ideal fluid

D. Fluid Flow

1.
The
fluid

is
nonviscous
. In a
nonviscous

fluid
, internal friction
is neglected.

An object moving through the
fluid

experiences no
viscous force.

2.
The
flow

is steady. In steady (laminar)
flow
, the velocity of the
fluid

at each

point remains constant.

3.
The
fluid

is incompressible. The density of an incompressible
fluid

is constant.

4.
The
flow

is
irrotational
. In
irrotational

flow
, the
fluid

has no
angular momentum about any point


A
1

v
1

= A
2

v
2

= constant

The equation of continuity

Bernoulli’s Equation


MEKANIKA FLUIDA

Fluida Statik

Fluida merupakan istilah untuk zat alir.

Fluida statik adalah fluida yang tidak bergerak.

Contoh fluida statik misalnya air di gelas, air di
kolam renang, dan air danau.

1. Fluida ideal

Fluida ideal adalah fluida yang memiliki ciri
-
ciri sebagai berikut.

a. tidak kompresibel (volumenya tidak berubah karena perubahan
tekanan)

b. berpindah tanpa mengalami gesekan (viskositasnya nol)


2. Fluida sejati

Fluida sejati memiliki ciri
-
ciri sebagai berikut.

a. kompresibel

b. berpindah dengan mengalami gesekan (viskositasnya tertentu)

1. Kohesi dan Adhesi


Gaya kohesi adalah gaya tarik
-
menarik antarmolekul sejenis.


Sedangkan gaya adhesi adalah gaya tarik
-
menarik antarmolekul
yang tidak sejenis.

o
Meniskus cembung menimbulkan sudut kontak tumpul (> 90)

o
Meniskus cekung menimbulkan sudut kontak lancip (< 90
°
)