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Chapter 8
Section 1 Fluids and Buoyant
Force
Defining a Fluid
• A fluid is a nonsolid state of matter in which the
atoms or molecules are free to move past each other,
as in a gas or a liquid.
• Both liquids and gases are considered fluids because
they can flow and change shape.
• Liquids have a definite volume; gases do not.
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Chapter 8
Section 1 Fluids and Buoyant
Force
Density and Buoyant Force
• The concentration of matter of an object is called the
mass density.
• Mass density is measured as the mass per unit
volume of a substance.
m

V
mass
mass density 
volume
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Chapter 8
Section 1 Fluids and Buoyant
Force
Mass Density
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Chapter 8
Section 1 Fluids and Buoyant
Force
Density and Buoyant Force, continued
• The buoyant force is the upward force exerted by a
liquid on an object immersed in or floating on the
liquid.
• Buoyant forces can keep objects afloat.
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Chapter 8
Section 1 Fluids and Buoyant
Force
Buoyant Force and Archimedes’ Principle
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Chapter 8
Section 1 Fluids and Buoyant
Force
Displaced Volume of a Fluid
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Chapter 8
Section 1 Fluids and Buoyant
Force
Density and Buoyant Force, continued
• Archimedes’ principle describes the magnitude of a
buoyant force.
• Archimedes’ principle: Any object completely or
partially submerged in a fluid experiences an upward
buoyant force equal in magnitude to the weight of the
fluid displaced by the object.
FB = Fg (displaced fluid) = mfg
magnitude of buoyant force = weight of fluid displaced
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Chapter 8
Section 1 Fluids and Buoyant
Force
Buoyant Force on Floating Objects
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Chapter 8
Section 1 Fluids and Buoyant
Force
Buoyant Force
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Chapter 8
Section 1 Fluids and Buoyant
Force
Density and Buoyant Force, continued
• For a floating object, the buoyant force equals the
object’s weight.
• The apparent weight of a submerged object depends
on the density of the object.
• For an object with density O submerged in a fluid of
density f, the buoyant force FB obeys the following
ratio:
Fg (object) O

FB
f
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Chapter 8
Section 1 Fluids and Buoyant
Force
Sample Problem
Buoyant Force
A bargain hunter purchases a “gold” crown at a flea
market. After she gets home, she hangs the crown
from a scale and finds its weight to be 7.84 N. She
then weighs the crown while it is immersed in water,
and the scale reads 6.86 N. Is the crown made of
pure gold? Explain.
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Chapter 8
Section 1 Fluids and Buoyant
Force
Sample Problem, continued
Buoyant Force
1. Define
Given:
Fg = 7.84 N
apparent weight = 6.86 N
f = pwater = 1.00  103 kg/m3
Unknown:
O = ?
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Chapter 8
Section 1 Fluids and Buoyant
Force
Sample Problem, continued
Buoyant Force
Diagram:
1. Define, continued
TIP: The use of a diagram
can help clarify a problem
and the variables involved.
In this diagram, FT,1 equals
the actual weight of the
crown, and FT,2 is the
apparent weight of the
crown when immersed in
water.
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Chapter 8
Section 1 Fluids and Buoyant
Force
Sample Problem, continued
Buoyant Force
2. Plan
Choose an equation or situation: Because the
object is completely submerged, consider the ratio of
the weight to the buoyant force.
Fg – FB  apparent weight
O

FB f
Fg
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Chapter 8
Section 1 Fluids and Buoyant
Force
Sample Problem, continued
Buoyant Force
2. Plan, continued
Rearrange the equation to isolate the unknown:
FB  Fg –  apparent weight 
O 
Fg
FB
f
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Chapter 8
Section 1 Fluids and Buoyant
Force
Sample Problem, continued
Buoyant Force
3. Calculate
Substitute the values into the equation and solve:
FB  7.84 N – 6.86 N = 0.98 N
Fg

7.84 N
O  f 
1.00  103 kg/m3
FB
0.98 N

O  8.0  103 kg/m3
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Chapter 8
Section 1 Fluids and Buoyant
Force
Sample Problem, continued
Buoyant Force
4. Evaluate
From the table, the density of
gold is 19.3  103 kg/m3.
Because 8.0  103 kg/m3 <
19.3  103 kg/m3, the crown
cannot be pure gold.
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Chapter 8
Section 2 Fluid Pressure
Pressure
• Pressure is the magnitude of the force on a surface
per unit area.
F
P
A
force
pressure =
area
• Pascal’s principle states that pressure applied to a
fluid in a closed container is transmitted equally to
every point of the fluid and to the walls of the
container.
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Chapter 8
Section 2 Fluid Pressure
Pascal’s Principle
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Chapter 8
Section 2 Fluid Pressure
Pressure, continued
• Pressure varies with depth
in a fluid.
• The pressure in a fluid
increases with depth.
P  P0   gh
absolute pressure =
atmospheric pressure +
 density  free-fall acceleration  depth 
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Chapter 8
Section 2 Fluid Pressure
Fluid Pressure as a Function of Depth
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Chapter 8
Section 3 Fluids in Motion
Fluid Flow
• Moving fluids can exhibit laminar (smooth) flow or
turbulent (irregular) flow.
• An ideal fluid is a fluid that has no internal friction or
viscosity and is incompressible.
• The ideal fluid model simplifies fluid-flow analysis.
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Chapter 8
Section 3 Fluids in Motion
Characteristics of an Ideal Fluid
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Chapter 8
Section 3 Fluids in Motion
Principles of Fluid Flow
• The continuity equation
results from conservation of mass.
• Continuity equation
A1v1 = A2v2
Area  speed in region
1 = area  speed in
region 2
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Chapter 8
Section 3 Fluids in Motion
Principles of Fluid Flow, continued
• The speed of fluid flow
depends on crosssectional area.
• Bernoulli’s principle
states that the pressure
in a fluid decreases as
the fluid’s velocity
increases.
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Chapter 8
Section 3 Fluids in Motion
Bernoulli’s Principle
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