The pressure in a fluid

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Lectures for

University Physics, Twelfth Edition




Hugh D. Young and Roger A. Freedman

Lectures by James Pazun

Chapter 14

Fluid Mechanics

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Goals for Chapter 14


To study density and pressure


To consider pressures in a fluid at rest


To shout “Eureka” with Archimedes and overview
buoyancy


To turn our attention to fluids in motion and calculate
the effects of changing openings, height, density,
pressure, and velocity

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Introduction


Submerging bath toys and
watching them pop back up
to the surface is an
experience with Archimedes
Principle.


Fish move through water
with little effort and their
motion is smooth. Consider
the shark at right … it must
keep moving for its gills to
operate properly.

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Density does not depend on the size of the object


Density is a measure of
how much mass occupies
a given volume.


Refer to Example 14.1
and Table 14.1 (on the
next slide) to assist you.


Density values are
sometimes divided by the
density of water to be
tabulated as the unit less
quantity, specific gravity.

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Densities of common substances

Table 14.1

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The pressure in a fluid


Pressure in a fluid is
force per unit area. The
Pascal is the given SI unit
for pressure.


Refer to Figures 14.3 and
14.4.


Consider Example 14.2.


Values to remember for
atmospheric pressure
appear near the bottom of
page 458.

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Pressure, depth, and Pascal’s Law


Pressure is everywhere equal in a uniform fluid of equal depth.


Consider Figure 14.7 and a practical application in Figure 14.8.

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Finding absolute and gauge pressure


Pressure from the fluid and pressure from the air above it
are determined separately and may or may not be combined.


Refer to Example 14.3 and Figure 14.9.

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There are many clever ways to measure pressure


Refer to Figure 14.10.


Follow Example 14.4.

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Measuring the density of a liquid


Have you ever
seen the
barometers made
from glass spheres
filled with various
densities of liquid?
This is their
driving science.


Refer to Figure
14.13.

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Buoyancy and Archimedes Principle


The buoyant force is equal to the weight of the displaced fluid.


Refer to Figure 14.12.

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Buoyancy and Archimedes Principle II


Consider
Example 14.5.


Refer to Figure
14.14 as you
read Example
14.5.

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Surface tension


How is it that
water striders can
walk on water
(although they are
more dense than
the water)?


Refer to Figure
14.15 for the
water strider and
then Figures 14.16
and 14.17 to see
what’s occurring
from a molecular
perspective.

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Fluid flow I


The flow lines at left in Figure 14.20 are laminar.


The flow at the top of Figure 14.21 is turbulent.

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Fluid flow II


The
incompressibility
of fluids allows
calculations to be
made even as pipes
change.


Refer to Figure
14.22 as you
consider Example
14.6.

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Bernoulli’s equation


Bernoulli’s equation allows
the user to consider all
variables that might be
changing in an ideal fluid.


Refer to Figure 14.23.


Consider Problem
-
Solving
Strategy 14.1.

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Water pressure in a home (Bernoulli’s Principle II)


Consider
Example 14.7.

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Speed of efflux (Bernoulli’s Equation III)


Refer to
Example 14.8.

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The Venturi meter (Bernoulli’s Equation IV)


Consider Example 14.9.

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Lift on an airplane wing


The first time I saw lift
from a flowing fluid, a man
was holding a Ping
-
Pong
ball in a funnel while
blowing out. A wonderful
demonstration to go with
the lift is by blowing across
the top of a sheet of paper.


Refer to Conceptual
Example 14.10.

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Viscosity and turbulence

Figures 14.28, 14.29


When we cease to treat
fluids as ideal, molecules
can attract or repel one
another

they can interact
with container walls and
the result is turbulence.

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A curve ball (Bernoulli’s equation applied to sports)


Bernoulli’s equation allows us to explain why a curve ball
would curve, and why a slider turns downward.


Consider Figure 14.31.