#### Transcript Physics Ch 8 PPT

```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,
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.