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Lecture Outline
Chapter 9
College Physics, 7th Edition
Wilson / Buffa / Lou
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Units of Chapter 9
Solids and Elastic Moduli
Fluids: Pressure and Pascal’s Principle
Buoyancy and Archimedes’ Principle
Fluid Dynamics and Bernoulli’s Equation
Surface Tension, Viscosity, and Poiseuille’s
Law
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9.1 Solids and Elastic Moduli
All solids are elastic to some degree, due to
the spring-like structure of the
intermolecular bonds holding them together.
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9.1 Solids and Elastic Moduli
Stress is defined as the force per unit area:
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9.1 Solids and Elastic Moduli
The stress results in a change in the shape of
the solid, called the strain:
The strain is related to the stress; how much
strain a particular stress causes depends on
the material.
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9.1 Solids and Elastic Moduli
Changes in length, shape, and volume are
described by Young’s modulus, the shear
modulus, and the bulk modulus, respectively.
Young’s modulus:
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9.1 Solids and Elastic Moduli
Stress is proportional to strain until the strain
gets too large. Then a material becomes
permanently deformed, and finally breaks.
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9.1 Solids and Elastic Moduli
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9.1 Solids and Elastic Moduli
Shear modulus:
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9.1 Solids and Elastic Moduli
Bulk modulus (the only one relevant for
fluids—why?)
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9.2 Fluids: Pressure and Pascal’s
Principle
Pressure is defined as the force per unit area:
If the force is at an
angle to the
surface, the more
general form (blue
box) is used.
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9.2 Fluids: Pressure and Pascal’s
Principle
Unit of pressure: the Pascal (Pa)
Density is defined as mass per unit volume:
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9.2 Fluids: Pressure and Pascal’s
Principle
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9.2 Fluids: Pressure and Pascal’s
Principle
The pressure in a
fluid increases with
depth, due to the
weight of fluid
above it.
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9.2 Fluids: Pressure and Pascal’s
Principle
Pascal’s principle:
Pressure applied to an
enclosed fluid is transmitted
undiminished to every point
in the fluid and to the walls
of the container.
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9.2 Fluids: Pressure and Pascal’s
Principle
Hydraulic lifts and shock absorbers take
advantage of Pascal’s principle.
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9.2 Fluids: Pressure and Pascal’s
Principle
Since the pressure is constant, a small force
acting over a small area can become a large
force acting over a large area.
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9.2 Fluids: Pressure and Pascal’s
Principle
There are a number of methods used to
measure pressure.
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9.2 Fluids: Pressure and Pascal’s
Principle
Absolute pressure is the total force per unit
area. We often measure the gauge pressure,
which is the excess over atmospheric pressure.
Atmospheric pressure historically was
measured using a mercury barometer.
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9.2 Fluids: Pressure and Pascal’s
Principle
The pressure corresponding to 1 mm of
mercury is called the torr (in honor of
Torricelli).
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9.3 Buoyancy and Archimedes’ Principle
A body immersed wholly or partially in a fluid
experiences a buoyant force equal in magnitude to the
weight of the volume of fluid that is displaced:
An object’s density will tell you whether it will
sink or float in a particular fluid.
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9.4 Fluid Dynamics and Bernoulli’s
Equation
In an ideal fluid, flow is
steady, irrotational,
nonviscous, and
incompressible.
Steady flow means that all the
particles of a fluid have the same
velocity as they pass a given
point.
Steady flow can be
described by streamlines.
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9.4 Fluid Dynamics and Bernoulli’s
Equation
Nonviscous flow means that viscosity is negligible.
Viscosity produces drag, and retards fluid flow.
Incompressible flow means that the fluid’s density is
constant. This is generally true for liquids, but not
for gases.
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9.4 Fluid Dynamics and Bernoulli’s
Equation
Equation of continuity:
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9.4 Fluid Dynamics and Bernoulli’s
Equation
If the density is constant,
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9.4 Fluid Dynamics and Bernoulli’s
Equation
Bernoulli’s equation is a consequence of the
conservation of energy.
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9.4 Fluid Dynamics and Bernoulli’s
Equation
One consequence of Bernoulli’s equation, that
the pressure is lower where the speed is higher,
can be counterintuitive.
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9.4 Fluid Dynamics and Bernoulli’s
Equation
The flow rate from a tank with a hole is given by
Bernoulli’s equation; the pressure at open areas
is atmospheric pressure.
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9.5 Surface Tension, Viscosity, and
Poiseuille’s Law
Surface tension is due to the forces that
molecules in a liquid exert on each other. There
is a net inward force at the surface.
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9.5 Surface Tension, Viscosity, and
Poiseuille’s Law
All real fluids
have some
viscosity, which
causes drag.
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9.5 Surface Tension, Viscosity, and
Poiseuille’s Law
The higher a
fluid’s
viscosity, the
more it resists
flow.
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9.5 Surface Tension, Viscosity, and
Poiseuille’s Law
Poiseuille’s law describes viscous flow in a
tube or pipe of length L and radius r.
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Review of Chapter 9
Stress is a measure of the force causing a
deformation; strain is a measure of the
deformation itself.
Elastic modulus is the ratio of stress to
strain.
Pressure is defined as force per unit area.
Pressure varies with depth in a fluid:
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Review of Chapter 9
Pressure in an enclosed fluid is transmitted
unchanged to every part of the fluid.
The buoyant force is equal to the weight of
displaced fluid.
An object will float if its average density is
less than that of the fluid; if it is greater,
the object will sink.
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Review of Chapter 9
Equation of continuity:
Flow rate equation:
Bernoulli’s law:
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Review of Chapter 9
Surface tension is due to intermolecular forces.
Viscosity is a fluid’s internal resistance to flow.
Poiseuille’s law:
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36.
A cylinder has a diameter of 15 cm. The water level in the
cylinder is maintained at a constant height of 0.45 m.
If the diameter of the spout pipe is 0.50 cm, how high is h , the
vertical stream of water? (Assume the water to be an ideal
fluid.)
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14.
Two metal plates are held together by two steel rivets,
each of diameter 0.20 cm and length 1.0 cm.
How much force must be applied parallel to the plates to
shear off both rivets?
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© 2010 Pearson Education, Inc.
© 2010 Pearson Education, Inc.
© 2010 Pearson Education, Inc.
© 2010 Pearson Education, Inc.