2. OBJECTIVES:
At the end of the chapter, the students should be able to:
1. Explain the nature and scope of fluids, particularly:
a) Fluids at rest
b) Fluids in motion
2. Define term like density, specific gravity, pressure, buoyancy,
viscosity, streamline, hydrostatics, and hydrodynamics.
3. Interpret and apply Archimedes’ principle, Pascal’s Law, and
Bernoulli’s principle.
3. 2. State reasons why a body floats, sinks or is suspended in a
liquid.
3. Explain why some places have high water pressure while
other places have low water pressure.
4. Solve problems on specific gravity, buoyancy, and pressure.
4. TERMS TO REMEMBER
Fluid – A gas or liquid that flows.
Density – Mass per unit volume.
Specific gravity – The ratio of the density of a
substance to that of a standard.
Pressure – Force applied per unit area.
Standard pressure – Pressure exerted by a column
of mercury exactly 760 mm high.
Total force – The force acting against the entire area
of a surface
Buoyant force – The upward force which is exerted
by any fluid on a body immersed in it.
5. Gauge pressure – Pressure above or below atmospheric
pressure.
Hydraulic pressure – A hydraulic device for multiplying
force or speed.
Manometer – An instrument for measure pressure
Hydrometer – A device for measuring the specific
quantity of a liquid.
Venturi meter – A device used to calculate the speed of
fluid in a horizontal tube from the difference in pressure
in the vertical tube.
6. FLUIDS AT REST
Matter is classified as solid, liquid or gas.
Liquid and gases are frequently grouped together as fluids,
since they flow readily and do not resist shearing stresses.
Fluids at rest are liquids or gases in a container that are not
allowed to flow.
If the fluids are at rest, the study of them is called fluid
statics.
If the fluids are in motion, where pressure forces are not
considered, the study of them is called fluid Kinematics.
If the fluids are in motion and the pressure forces are
considered, the study of them is called fluid dynamics.
7. Density (D)
The “mass per unit volume” is mass density. Hence,
it has units of kilograms per cubic meter
The mass density of water at 4oC is 1000 kg/m3
while it is 1.20 kg/m3 for air at 20oC at standard
pressure.
VVDm
V
m
D
8. Density: Example #1
A quantity of helium gas at 0°C with a volume of 4.00 m3 has a
mass of 0.712 kg at standard atmospheric pressure.
Determine the density of this sample of helium gas?
?
712.0
00.4 3
D
kgm
mV
3178.0
00.4
712.0
3 m
kg
m
kg
D
V
m
D
9. Specific Weight or Weight Density (γ)
It is the ratio between the weight if a fluid to its volume.
It is also weight per unit volume of a fluid.
Its unit is N/m3.
Water at 20 oC has a specific weight of 9.79 kN/m3
g r g ( in terms of mass density)
VVDm
V
w
Y
10. Specific Volume
It is defined as the volume of a fluid occupied by a unit mass
or volume per unit mass of a fluid is called specific volume.
Specific Volume = Volume of the Fluid / Mass of the Fluid
= 1/mass of the fluid/volume of the fluid
= 1 / r
11. Specific Gravity (Sp.Gr)
The ratio of specific weight of a given liquid to the
specific weight of water at a standard reference
temperature (4oC)is defined as specific gravity, Sp.Gr.
The specific weight of water at atmospheric pressure is
9810 N/m3.
𝑆𝑃. 𝐺𝑅 =
𝛾 𝑠𝑢𝑏𝑠.
𝛾 𝑤𝑎𝑡𝑒𝑟
=
𝜌 𝑠𝑢𝑏𝑠.
𝜌 𝑤𝑎𝑡𝑒𝑟
12. Example #2
Carbon tetrachloride with a mass of 500 kg is placed
in a container with 0.325 m3 in volume.
a)Calculate the density (1538.46 kg/m3)
b)Calculate its specific weight (15.1 kN/m3)
c)Determine also its weight (4.9 kN)
13. Example #3
A certain liquid has a unit weight of 56 kN/m3.
a)Compute the mass density (5708 kg/m3)
b)Compute its specific volume (0.000175 m3/kg)
c)Compute its specific gravity (5.71)
14. Pressure
Is defined as force per unit area. The force is always
perpendicular to the surface of the container.
Pressure units are force units divided by area units,
such as nt/m2, dyne/cm2, lb/ft2, and lb/in2.
Pressure can also be expressed in atmospheres,
where one atmosphere(atm) is equal to normal
atmospheric pressure, which is approximately 1.013
x 105 nt/m2.
VVDm
A
F
P
15. Rules in changing the height of liquid to another
liquid
1. To change the height of any liquid to height of water.
Rule : Multiply the height of the liquid by its own specific gravity.
2. To change the height of water to height of any liquid.
Rule : Divide the height of water by the specific gravity of liquid where
the height is to be found.
3. To change the height of the liquid in terms of another liquid.
Rule : Multiply the height of one liquid by its own specific gravity and
divide the product by the specific gravity of the liquid where the height
is to be found.
16. Assignment
1. A pressure gauge at elevation 8 m. at the side of the tank containing
a liquid reads 80 kPa. Another gauge at elevation 3 m. 120 kPa.
Compute the following:
a) Specific weight
b) Density
c) Specific gravity
2) The pressure in the tank is 50 psi.
a) Find the equivalent pressure head of water.
b) Find the equivalent pressure head of fuel oil with sp.gr = 0.92
c) Find the equivalent pressure head of nectar sp.gr = 2.94
17. Example # 4
A pressure in a given tank reads 277 mm of Hg. Specific
gravity of Mercury(Hg) is 13.6.
a)Determine the equivalent height of column of water.
(3.77m)
b)Determine the equivalent height of column of kerosene
sp.gr = 0.82. (4.59m)
c) Determine the equivalent height of column of nectar
sp.gr = 2.94. (1.28m)