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Chapter 6. Fluid Mechanics
Notes:
- Most of the material in this chapter is taken from Young and Freedman, Chap. 12.
6.1 Fluid Statics
Fluids, i.e., substances that can flow, are the subjects of this chapter. But before we can
delve into this topic, we must first define a few fundamental quantities.
6.1.1 Mass Density and Specific Gravity
We have already encountered the mass density (often abbreviated to density ) in previous
chapters. Namely, the mass density is simply the ratio of the mass m of an object to its
volume V
ρ =
m
V
with units of kg/m
3
. Evidently, the density of objects can vary greatly depending of the
materials composing them. For example, the density of water is 1 , 000 kg/m
3
at 4
C , that
of iron is 7 , 800 kg/m
3
, while a neutron star has a mean density of approximately
18
kg/m
3
!
The specific gravity of a substance is defined as the ratio of the mass density to that of
water at 4
ο
C (i.e., 1 , 000 kg/m
3
). It would probably be more precise to use the term
relative density instead of specific gravity, but such is not the custom…
6.1.2 Pressure and Buoyance
A fluid is composed at the microscopic level by molecules and/or atoms that are
constantly wiggling around. When the fluid is contained in a vessel these particles will
collide with the walls of the container, a process that will then change their individual
momenta. The change of momentum that a particle experiences will impart an impulse
over the time interval during which the collision takes place, as a result the walls of the
vessel will “feel” a force. The pressure p at a given point on a wall is defined as the
force component perpendicular to the wall at that point per unit area. That is, if dF ⊥
is
this elemental perpendicular force applied to an infinitesimal area dA^ on a wall, then the
pressure on that area is
p ≡
dF ⊥
dA
When the pressure is the same at all points of a macroscopic, plane surface of area A ,
then the perpendicular force F ⊥
must also be the same everywhere on that surface and
p =
F
⊥
A
The pascal (Pa) is the unit of pressure with
1 Pa = 1 N/m
2
. ( 6. 4 )
Related to the pascal is the bar , which equals 10
5
Pa , and, accordingly, the millibar ,
which equals 100 Pa. The atmospheric pressure p a
, i.e., the average atmospheric
pressure at sea level, is 1 atmosphere (atm) with
1 atm = 101 , 325 Pa
= 1,103.25 millibar.
It is important to note that motion of the particles that cause the pressure is random in
orientation and pressure is therefore isotropic. That is, pressure at one point is the same in
all directions. Also, since the pressure at a point is directly proportional to the force
effected at that point, it should be clear that weight can be a source of pressure. For
example, the pressure in the earth’s atmosphere decreases as one goes to higher altitude
as the weight of the, or the amount of, fluid above is reduced. Similarly, an increase in
pressure is felt by a diver who descends to greater depths in a body of water.
We can quantify this effect by studying how pressure varies within a fluid contained in a
vessel. Accordingly, referring to Figure 1 , we consider a fluid of uniform density ρ
under the effect of gravity g and consider a fluid element of thickness dy and area A.
We assume that the bottom of the vessel is located at y = 0 and the position of the fluid
element is at (^) y ( > 0 ; (^) y thus increases upwards). If the pressure at the bottom of the
element is p , then the pressure immediately on top of it will be p + dp. If we further
assume that the fluid is in equilibrium, then this fluid element must be static and the
different forces, say, at the bottom of the element must cancel each other out. That is,
pA − ⎡( p + dp ) A + dw
where dw is the weight of the fluid element
dw =( ρ Ady ) g. ( 6. 7 )
The quantity between parentheses in equation ( 6. 7 ) is simply the mass of the fluid
element. Equation ( 6. 6 ) then becomes
dp A = − ρ g Ady , ( 6. 8 )
or
It is then convenient to think of Δ y > 0 as the depth in the fluid where the pressure p 1
is
encountered. Equation ( 6. 13 ) also implies that increasing p 0
by some amount will
increase the pressure at any point within the fluid by the same amount. This is the so-
called Pascal’s Law
Pressure applied to an enclosed fluid is transmitted undiminished to every portion of the
fluid and the walls of the containing vessel.
We can use equation ( 6. 11 ) to explain the behavior of objects submerged (sometimes not
completely) in a fluid, such as water. Let us consider Figure 2 where an objet of mass m ,
horizontal area A , and height h is immersed in a fluid of density ρ ; the whole apparatus
is subject to gravity. We denote by p 1
and p 2
the pressures at the bottom and top surfaces
of the object, respectively, likewise the force components perpendicular to those surfaces
are F 1
and F 2
. But we know from equation ( 6. 11 ) that
p 1
A − p 2
A = F
1
− F
2
= ρ ghA ,
or, while defining the volume of the object with V = hA , we have
F
1
− F
2
= ρ Vg. ( 6. 15 )
Since the F 1
− F
2
is net buoyancy force acting on the body and ρ V is the mass of fluid
displaced by the presence of the body, we are then led to Archimedes’ Principle
The net upward, buoyancy force acting on a partially or completely immersed body
equals the weight of fluid displaced by the body.
It is important to note that the buoyancy force is independent of the weight of the object.
Also, although we derived this result for an object of rectangular volume, it should be
clear that it applies to any possible shape since only the net perpendicular forces on the
areas spanned by the top and bottom surfaces of the object come into play.
1
h
2
1
p = F /A
p = F /A 2
1
Figure 2 - An object immersed in
a fluid.
6.1.3 Exercises
- Hydraulic lift. Consider the representation of a hydraulic lift shown in Figure 3. Use
Pascal’s Law to explain the lift’s functioning.
Solution.
According to equation ( 6. 11 ), two points in fluid located at the same height are subjected
to the same pressure. If we consider points 1 and 2 in Figure 3 both located at the surface
of the fluid, then we can write
p 1
= p 2
F
1
A
1
F
2
A
2
and therefore
F
2
A
2
A
1
F
1
. (^6.^17 )
It follows that we can multiply at point 2 the effect of the force F 1
applied at point 1 over
an area A 1
(with a piston, for example) if we use a surface A 2
> A
1
at point 2. The
resulting force is multiplied by the ratio of the areas, as shown in equation ( 6. 17 ).
- Archimedes’ Principle and Buoyancy. We drop a rectangular piece of cork of mass
m , volume Ah , and density ρ m
= 240 kg/m
3
in a vessel containing water (of density
ρ = 1000 kg/m
3
). Determine the depth at which the object’s bottom surface settles once
equilibrium is reached.
Figure 3 – A hydraulic lift.
From Archimedes’ buoyancy relation we have
p 0
A − p 0
dp 0
dy
h 2
A = ρ air
Ah 2
g , ( 6. 22 )
or, as expected for hydrostatic equilibrium,
dp 0
dy
= − ρ air
g. ( 6. 23 )
Also from the equation of hydrostatic equilibrium (i.e., equation ( 6. 13 )), we can write
p 1
= p 0
Inserting equations ( 6. 23 ) and ( 6. 24 ) into equation ( 6. 21 ), while setting h 2
= h − h 1
, we
find that
p 0
( ) −^ p 0
− ρ air
g h − h 1
⎡ ( ) ⎣
− ρ m
gh = 0 , ( 6. 25 )
and
h 1
ρ m
− ρ air
ρ − ρ air
h
ρ m
ρ
h
0.24 h ,
since the density of air is completely negligible compared to those of water and cork. It
should be clear from this discussion that an object that is completely immersed in a fluid,
and at equilibrium (as in Figure 2 ), must have the same density of the fluid.
- (Prob. 12.33 in Young and Freedman.) A rock is suspended by a light string. When the
rock is in the air, the tension in the string is 39.2 N. When the rock is totally immersed in
water, the tension in the string is 28.4 N. When the rock is totally immersed in an
unknown liquid, the tension is 18.6 N. What is the density of the unknown liquid?
Solution.
According to Archimedes’ Principle the buoyancy force acting on the rock equals the
weight of the displaced volume of liquid. That is,
F
b
= ρ Vg , ( 6. 27 )
where V is the volume of the rock. According to Newton’s Second Law
T + F
b
− mg = 0 , ( 6. 28 )
with T^ the tension in the string and m^ the mass of the rock. Applying equation ( 6. 28 ) to
the cases of air and water and then equating them, we have
T
air
Vg = T water
Vg ( 6. 29 )
and
V =
T
air
− T
water
ρ water
− ρ air
( ) g
( 39.2^ −^ 28.4)N
( 1000 − 1.2)kg/m
3
⋅ 9.81 m/s
2
= 1.10 × 10
− 3
m
3
.
One way to proceed is to insert this result in equation ( 6. 28 ) to find the mass of the rock
(let us choose the case where it is immersed in water)
m =
T
water
g
V
28.4 N
9.81 m/s
2
3
⋅1.10 × 10
− 3
m
3
= 4.00 kg.
Equation ( 6. 28 ) for the unknown liquid then yields
ρ u
mg − T u
Vg
4.00 kg ⋅ 9.81 m/s
2
− 18.6 N
1.10 × 10
− 3
m
− 3
⋅ 9.81 m/s
2
= 1907 kg/m
3
.
Alternatively, we could have written
T
air
Vg = T u
Vg , (^6.^33 )
and, with a similar outcome,
ρ u
= ρ air
T
air
− T
u
Vg
Equation ( 6. 37 ) is the so-called continuity equation for steady flows (of incompressible
fluids). Generally, we can state that the volume flow rate is conserved
dV
dt
= Av
= constant.
6.2.2 Bernoulli’s Equation
We now seek to apply the mass continuity equation while taking into account any
changes of pressure that can accompany the flow of fluids. Such changes in pressure are
to be expected whenever the cross-sectional area A changes along a flow. This is
because as the area varies, the velocity must also change according to the continuity
equation ( 6. 37 ); if the flow speed changes, then there must be forces acting on the flow to
cause this acceleration. Finally, pressure variations must also occur since pressure is
defined as the force per cross-sectional area.
To derive the equation that relates these quantities, we will use the work-energy theorem
defined in Chapter 2, which we write her for convenience
W
other
= Δ K + Δ U
grav
where W other
is the work done by all forces other than gravity, Δ K is the change in
kinetic energy, and Δ U grav
is the change in gravitational potential energy. In our case we
will substitute W other
→ W
pressure
≡ W
p
. Let us consider the tube of changing cross-section
shown in Figure 6. We first concentrate on the section on the left of width v 1
dt and cross-
section A 1
, through which the flow speed is v 1
and the pressure p 1
. We can ask what
amount of work dW 1
was done by the pressure on a fluid that has traveled from the
entrance to the exit of that section? The important fact to remember is that pressure is
isotropic, meaning that the force p 1
A
1
at the entrance has the same magnitude as the force
at the exit but of opposite direction. It therefore follows that
dW p, 1
= F
net, 1
⋅ d r 1
= p 1
A
1
v 1
( dt )
entrance
A
1
v 1
( dt )
exit
The pressure does no work when cross-section of the flow is constant. The same result
dW p, 2
= 0 would be found for the section, of width (^) v 2
dt and cross-section^ A 2
, through
which the flow speed is v 2
and the pressure p 2
, on the right of the tube.
The same cannot be said for the middle section of the tube, where the cross-section
changes from A 1
to A 2
. In this case we find
dW p, 12
= F
net, 12
⋅ d r 12
= p 1
A
1
v 1
( dt )
entrance
A
2
v 2
( dt )
exit
where dt is an infinitesimal time interval such that v 1
dt and v 2
dt are much smaller than
the width of the section. We now use equation ( 6. 37 ) for mass continuity and transform
equation ( 6. 41 ) to
dW p, 12
= p 1
− p 2
( ) dV ,^ (^6.^42 )
with dV the volume element spanned in the interval dt (see equation ( 6. 38 )). This
equation can be integrated over between any two points along a tube, and generalized to
(for an incompressible and non-viscous fluid)
W
p
= p 1
− p 2
( ) dV.^ (^6.^43 )
We can now write down the corresponding changes in kinetic and gravitational potential
energies (if there is a change in vertical position y between point 1 and 2)
Δ K =
ρ v 2
2
− v 1
2
( ) dV
Δ U
grav
= ρ g y 2
− y 1
( ) dV.
Combining equations ( 6. 39 ), ( 6. 43 ), and ( 6. 44 ) we get the so-called Bernoulli’s
Equation
p 1
ρ v 1
2
= p 2
ρ v 2
2
2
1
p 1
p 1
A 1
v dt 1
v dt 2
p 2
p 2
p 2
A
p
Figure 6 – A tube of changing cross-section,
through which an incompressible and non-
viscous fluid is flowing.
- (Prob. 12.55 in Young and Freedman.) A dam has the shape of a rectangular solid. The
side facing the lake has an area A^ and a height h^. The surface of the fresh water lake
behind the dam is at the top of the dam. (a) Show that the net horizontal force exerted by
the water on the dam is ρ gAh 2 − that is, the average gauge pressure across the face of
the dam times the area. (b) Show that the torque exerted by the water about an axis along
the bottom of the dam is ρ gAh
2
Solution.
(a) The pressure at a given depth h − y ( y = 0 is at the bottom of the dam) is given by
p = p 0
+ ρ g ( h − y ) ( 6. 50 )
with p 0
the atmospheric pressure at the top of the lake (and dam). The force exerted by
the water on a horizontal strip of the dam of width dy at that depth is
dF ⊥
= p
A
h
dy
A
h
p 0
⎡ + ρ g ( h − y )
dy ,
with A h the width of the dam since it is rectangular. The total force exerted by the
water on the dam will then be
F
⊥
= dF ⊥ 0
h
∫
A
h
p 0
( + ρ gh ) dy
0
h
∫
− ρ g y dy
0
h
∫
A
h
p 0
( +^ ρ gh ) h^ −^
ρ gh
= p 0
A +
ρ gAh.
However, the dam “feels” a force equal to p 0
A on its side opposing the lake from the
atmosphere. The total force on the dam is therefore
F
total
ρ gAh. ( 6. 53 )
(b) The torque about the axis at y = 0 acting on a horizontal strip of the wall at depth
h − y is
d τ = dF ⊥
− p 0
A
h
dy
⋅ y
A
h
ρ gy ( h − y ) dy.
The total torque on the dam is thus
τ = d τ
0
h
A
h
ρ g h y dy
0
h
− y
2
dy
0
h
A
h
ρ g
h
3
h
3
ρ gAh
2
.
- (Prob. 12.98 in Young and Freedman.) A siphon , as shown in Figure 7 , is a convenient
device for removing liquids from containers. To establish the flow, the tube must be
initially filled with fluid. Let the fluid have a density ρ , and let the atmospheric pressure
be p atm
. Assume that the cross-sectional area of the tube is the same at all points along it.
(a) If the lower end of the siphon is at a distance h below the surface of the liquid in the
container, what is the speed of the fluid as it flows out of the lower end of the siphon?
(Assume that the container has a very large diameter, and ignore any effect of viscosity.)
(b) A curious feature of a siphon is that the fluid initially flows “uphill.” What is the
greatest height H that the high point of the tube can have if flow is still to occur?
Solution.
(a) The pressure at the top of the liquid in the container is p 0
= p atm
, the same as it is at
the lower end of the tube. Applying Bernoulli’s equation with points “1” and “2” at the
top of the liquid in the container and the lower end of the tube, respectively, we have
p 0
ρ v 1
2
ρ v 2
2
, ( 6. 56 )
or
v 2
2
= v 1
2
But with a very large container we can assume that v 1
≈ 0 and
v 2
= 2 gh. ( 6. 58 )
