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The Fluid States
Section 13.1
Physics
Objectives
 Describe
how fluids create pressure and
relate Pascal’s principle to some everyday
occurrences.
 Apply Archimedes’ and Bernoulli’s
principles.
 Explain how forces within liquids cause
surface tension and capillary action, and
relate the kinetic model to evaporation and
condensation.
Properties of Fluids
 Fluids:
materials that flow and have no
definite shape of their own.
 For the most part, Newton’s Laws and the
Laws of Conservation can be applied to
fluids.
 When we apply a force to a fluid, what do
we call it?
Pressure
 We
call a force applied to a fluid:
pressure.
 Pressure is represented by the following
equation:


P = F/A
Pressure equals force divided by area.
Pressure
 The
force is assumed to be perpendicular
to the affected surface area.
 Pressure is measured in Pascals, Pa.
 The Pascal: 1N/m².
 Which has a greater pressure?


The area under an elephant’s foot or the area
under a woman’s high heel shoe?
The area under a high heel shoe.
Pressure from a Gas?
 It
is easy to see how solids and fluids exert
pressure on a surface.
 But how does a gas exert pressure on an
area?
 The tiny gas particles are constantly hitting
surfaces and applying a pressure value.
Pressure from a Gas
 Are
we under pressure right now?
 Yep.
 In fact, on every square centimeter of
Earth’s surface at sea level, the
atmosphere exerts a force of
approximately 10 N = 1 kg.
 Blue Barometer Demo.
http://www.cabelas.com/cabelas/en/content/Item/61/60/81/i616081sq02.jpg
Practice Problems
 Pg
 1-4
303
Fluids at Rest
 Water
also exerts pressure just as the
atmosphere exerts pressure.
 Think of a diver, the pressure felt when
under water does not depend on whether
the diver is upright, sideways, or up-sidedown.
 Pascal’s Principle accounts for this
pressure.
 Pascal’s
Principle: any change in
pressure applied at any point on a
confined fluid is transmitted undiminished
throughout the fluid.
 How does toothpaste demonstrate this
principle?
http://www.gauweiler.net/photos/art/toothpaste.jpg
Pascal’s Principle
Pascal’s Principle
 So
why is this important?
 There are more important reasons than
that of toothpaste.
 Hydraulic systems use this principle
 Force Exerted by lift = F = (F1A2)/A1
Practice Problem
 Pg
5
304
Swimming Under Pressure
 When
you dive under water, your body
feels increasing pressure as you dive
deeper.
 The downward pressure of water is
illustrated by the following equation.
 P = ρhg
 Rho; density of fluid, h; depth, g; gravity.
Swimming Under Pressure
 While
swimming, you may also notice the
upward force from the water you are
immersed in.
 This force is the buoyant force.
 Fbuoyant = ρVg
 Rho; density, V; volume, g; gravity.
 The net upward force is equal to the
weight of the fluid displaced by the object.
Archimedes’ Principle
 The
buoyant force and weight of displaced
fluid relationship was discovered by the
Greek scientist Archimedes.
 Archimedes’ Principle: states that an
object immersed in a fluid has an upward
force on it equal to the weight of the fluid
displaced by the object; it does not depend
on the weight of the object.
So Will It Sink Or Float?
 When
an object is placed in a fluid, it has
the force of gravity pressing down and the
buoyant force pressing up.
 The difference between the two
determines if the object will sink or float.
 Fapparent = Fg – Fbuoyant
 A negative value for Fapparent indicates the
object will float.
Practice Problems
 Pg
309
 6-7
http://home.earthlink.net/~divegeek/archimedes_files/buoyancy.gif
Fluids in Motion
 Ok,
we have looked at static fluids, but
fluids do not just stay still.
 Bernoulli’s Principle: describes the
relationship between the velocity and
pressure exerted by a moving fluid.
 Bernoulli’s Principle states that as the
velocity of a fluid increases, the pressure
exerted by that fluid decreases.
Fluids in Motion
 Most
airplanes get part of their lift by
utilizing this principle.
 The curvature of the top wing is greater
than that of the bottom.
 As the wing travels through the air, ;the air
moving over the top surface travels faster
than air moving past the bottom surface.
Fluids in Motion
 The
decreased air pressure created on the
top surface results in a net upward
pressure that produces an upward force
on the wings, or lift, which helps hold the
plane up.
 Race cars use this same principle…How?
Forces Within Liquids
 All
of the fluids examined thus far have
been ideal fluids.
 In real liquids, particles exert
electromagnetic forces of attraction on
each other.
 These forces affect the behavior of liquids.
Forces Within Liquids
 These
forces are cohesive forces.
 Cohesive forces are the forces of
attraction between particles of a fluid.
 These forces account for water’s surface
tension.
Forces Within Liquids
 Another
force that affects the way a fluid
behaves is adhesion.
 Adhesive forces are the attraction between
particles of a fluid and the particles of
another surface.
 This is why capillary action occurs.