Transcript Section 2 Buoyant Force

Chapter 7
Section 1 Fluids and Pressure
Fluids Exert Pressure
• A fluid is any material that can flow and that takes
the shape of its container. Fluids include liquids and
gases.
• All fluids exert pressure, which is the amount of
force exerted per unit area of a surface.
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Chapter 7
Section 1 Fluids and Pressure
Fluids Exert Pressure, continued
• In the image below, the force of the air particles
hitting the inner surface of the tire creates pressure,
which keeps the tire inflated.
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Chapter 7
Section 1 Fluids and Pressure
Fluids Exert Pressure, continued
• Calculating Pressure Pressure can be calculated
by using the following equation:
pressure =
force
area
• The SI unit for pressure is the pascal. One pascal
(1 Pa) is the force of one newton exerted over an
area of one square meter (1 N/m2).
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Chapter 7
Section 1 Fluids and Pressure
Pressure, Force, and Area
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Chapter 7
Section 1 Fluids and Pressure
Fluids Exert Pressure, continued
• Pressure and Bubbles Soap bubbles get rounder
as they get bigger because fluids exert pressure
evenly in all directions.
• Since air is a fluid, adding air to an air bubble
causes it to expand in all directions at once.
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Chapter 7
Section 1 Fluids and Pressure
Atmospheric Pressure
• The atmosphere is the thin layer of nitrogen, oxygen,
and other gases that surrounds Earth.
• Atmospheric pressure is the pressure caused by
the weight of the atmosphere.
• Atmospheric pressure is exerted on everything on
Earth, including you.
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Chapter 7
Section 1 Fluids and Pressure
Atmospheric Pressure, continued
• The air inside
this balloon exerts
pressure that
keeps the balloon
inflated against
atmospheric
pressure.
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Chapter 7
Section 1 Fluids and Pressure
Atmospheric Pressure, continued
• Variation of Atmospheric Pressure The
atmosphere stretches about 150 km above the
Earth’s surface, but about 80% of the atmosphere’s
gases are found within 10 km. At the top of the
atmosphere, pressure is almost nonexistent.
• Atmospheric Pressure and Depth As you travel
through the atmosphere, atmospheric pressure
changes. The further down through the atmosphere
you go, the greater the pressure is.
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Chapter 7
Section 1 Fluids and Pressure
Atmospheric Pressure, continued
• Pressure Changes and Your Body If you travel to
higher or lower points in the atmosphere, the fluids in
your body have to adjust to maintain equal pressure.
• You may have experienced this adjustment is your
ears have “popped” when you were in a plane taking
off or in a car traveling down a steep mountain road.
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Chapter 7
Section 1 Fluids and Pressure
Water Pressure
• Water is a fluid. So, it exerts pressure like the
atmosphere does.
• Water Pressure and Depth Like atmospheric
pressure, water pressure depends on depth.
• Density Makes a Difference Because water is
more dense than air, a certain volume of water has
more mass—and weighs more—than the same
volume of air. Water exerts more pressure than air.
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Chapter 7
Section 1 Fluids and Pressure
Pressure Differences and Fluid Flow
• Just by drinking through a straw you can observe
an important property of fluids: Fluids flow from
areas of high pressure to areas of low pressure.
• Pressure Difference and Breathing The next
slide shows how exhaling causes fluids to flow from
high to low pressure.
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Chapter 7
Section 1 Fluids and Pressure
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Chapter 7
Section 1 Fluids and Pressure
Pressure Differences and Fluid Flow,
continued
• Pressure Differences and Tornadoes The air
pressure inside a tornado is very low. Because the
air pressure outside of the tornado is higher than the
pressure inside, air rushes into the tornado.
• The rushing air causes the tornado to be like a
giant vacuum cleaner.
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Chapter 7
Section 2 Buoyant Force
Buoyant Force and Fluid Pressure,
continued
• There is more pressure at the bottom of an object
because pressure increases with depth. This results
in an upward buoyant force on the object.
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Chapter 7
Section 2 Buoyant Force
Weight Versus Buoyant Force
• Sinking An object in a fluid will sink if its weight is
greater than the buoyant force.
• Floating An object will float only when the buoyant
force on the object is equal to the object’s weight.
• Buoying Up When the buoyant force on an object
is greater than the object’s weight, the object is
buoyed up (pushed up) in water.
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Chapter 7
Section 2 Buoyant Force
Weight Versus Buoyant Force, continued
• Will an object sink or float? That depends on the
whether the buoyant force is less than or equal to
the object’s weight.
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Chapter 7
Section 2 Buoyant Force
Floating, Sinking, and Density
• More Dense Than Air Ice floats on water because
it is less dense than water. Ice, like most substances,
is more dense than air. So, ice does not float in air.
• Less Dense Than Air One substance that is less
dense than air is helium gas. A given volume of
helium displaces an equal volume of air that is much
heavier than itself. So, helium floats in air.
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Chapter 7
Section 2 Buoyant Force
Finding Density
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Chapter 7
Section 2 Buoyant Force
Changing Overall Density
• Changing Shape The secret of how a ship floats
is in the shape of the ship. Ships made of steel
float because their overall density is less than the
density of water.
• The next slide demonstrates how a ship made out
of steel, which is almost 8 times denser than water,
is able to float in water.
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Chapter 7
Section 2 Buoyant Force
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Chapter 7
Section 2 Buoyant Force
Changing Overall Density, continued
• Changing Mass A submarine is a special kind of
ship that can travel both on the surface of the water
and underwater.
• Submarines have ballast tanks that can be opened
to allow sea water to flow in.
• As water is added, the submarine’s mass increases,
but its volume stays the same.
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Chapter 7
Section 2 Buoyant Force
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Chapter 7
Section 2 Buoyant Force
Changing Overall Density, continued
• Changing Volume Like
a submarine, some fish
adjust their overall density
to stay at a certain depth
in the water.
• Most bony fishes have
an organ called a swim
bladder which helps them
change volume.
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Chapter 7
Section 3 Fluids and Motion
Fluid Speed and Pressure
• Bernoulli’s principle states that as the speed of a
moving fluid increases, the fluid’s pressure
decreases.
• Science in a Sink A table-tennis ball is attached to
a string and swung into a stream of water, where it is
held. Because the water is moving faster than air, the
ball is pushed by the higher pressure of the air into
an area of reduced pressure—the water stream.
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Chapter 7
Section 3 Fluids and Motion
Factors That Affect Flight
• Thrust and Lift Thrust is the forward force
produced by a plane’s engine. Lift is the upward force
on the wing as it moves through the air.
• Wing Size, Speed, and Lift Smaller wings keep a
plane’s weight low, which also helps it move faster.
• Bernoulli and Birds A small bird must flap its small
wings at a fast pace to stay in the air, but a large bird
flaps less.
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Chapter 7
Section 3 Fluids and Motion
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Chapter 7
Section 3 Fluids and Motion
Factors That Affect Flight, continued
• Bernoulli and Baseball The next slide shows how
a baseball pitcher can take advantage of Bernoulli’s
principle to throw a curveball.
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Chapter 7
Section 3 Fluids and Motion
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Chapter 7
Section 3 Fluids and Motion
Drag and Motion in Fluids
• Drag is the force that opposes or restricts motion
in a fluid. It is a force that is parallel to the velocity
of the flow.
• Drag is usually caused by an irregular flow of air,
known as turbulence.
• Turbulence and Lift Lift is often reduced when
turbulence causes drag.
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Chapter 7
Section 3 Fluids and Motion
Pascal’s Principle
• What Is Pascal’s Principle? Pascal’s principle
states that a change in pressure at any point in an
enclosed fluid will be transmitted equally to all parts of
that fluid.
• Pascal’s Principle and Motion Hydraulic devices
use Pascal’s principle to move or lift objects. Liquids
are used in hydraulic devices because liquids cannot
be easily compressed into a smaller space.
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Chapter 7
Section 3 Fluids and Motion
Pascal’s Principle, continued
• Because of Pascal’s principle, the touch of a foot
can stop tons of moving metal.
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