Chapter 14
Forces in Fluids
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Section 1 Fluids and Pressure
Section 2 Buoyancy and Density
Concept Map
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Chapter 14
Section 1 Fluids and Pressure
Fluids and Pressure
• A fluid is a nonsolid state of matter in which the
atoms or molecules are free to move past each other.
• A fluid is any material that can flow and that takes the
shape of its container. Liquids and gases are fluids.
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Chapter 14
Section 1 Fluids and Pressure
Fluids and Pressure, continued
• Pressure is the amount of force exerted on a given
area.
• Moving particles of matter create pressure by
colliding with one another and with the walls of their
container.
• Fluids exert pressure evenly in all directions.
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Chapter 14
Section 1 Fluids and Pressure
Fluids and Pressure, continued
• Any force, such as the weight of an object, acting on
an area creates pressure.
• 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 14
Section 1 Fluids and Pressure
Atmospheric Pressure
• The atmosphere is the layer of nitrogen, oxygen, and
other gases that surrounds Earth.
• The pressure caused by the weight of the
atmosphere is called atmospheric pressure.
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Chapter 14
Section 1 Fluids and Pressure
Atmospheric Pressure, continued
• The atmosphere extends about 150 km above
Earth’s surface. Atmospheric pressure changes as
you travel through the atmosphere.
• At the top of the atmosphere, pressure is almost
nonexistent because there are fewer gas particles
and they rarely collide.
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Chapter 14
Forces in Fluids
Atmospheric Pressure
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Chapter 14
Section 1 Fluids and Pressure
Water Pressure
• Water is a fluid. So, like the atmosphere, water exerts
pressure.
• Water pressure increases as the depth of the water
increases.
• Water pressure depends on depth, not on the total
amount of fluid present.
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Chapter 14
Section 1 Fluids and Pressure
Water Pressure, continued
• A person swimming 3 m below the surface of a small
pond feels the same pressure as a person swimming
3 m below the surface of a large lake.
• Because water is about 1,000 times denser than air,
water exerts more pressure than air does.
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Chapter 14
Section 1 Fluids and Pressure
Pressure Differences and Fluid Flow
• Fluids flow from areas of high pressure to areas of
low pressure.
• Differences in air pressure help explain the way your
body breathes and the way tornadoes create
damaging winds.
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Chapter 14
Forces in Fluids
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Chapter 14
Section 2 Buoyancy and Density
Buoyant Force and Fluid Pressure
• All fluids exert an upward force called buoyant force.
• Buoyant force is the upward force that keeps an
object immersed in or floating on a liquid.
• Buoyant force is caused by differences in fluid
pressure.
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Chapter 14
Forces in Fluids
Buoyant Force
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Chapter 14
Section 2 Buoyancy and Density
Buoyant Force and Fluid Pressure,
continued
• A Greek mathematician named Archimedes
discovered how to find buoyant force.
• Archimedes’ principle states that the buoyant force
on an object in a fluid is an upward force equal to the
weight of the volume of fluid that the object displaces.
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Chapter 14
Forces in Fluids
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Chapter 14
Section 2 Buoyancy and Density
Weight Versus Buoyant Force
• An object in a fluid will sink if the object’s weight is
greater than the buoyant force (the weight of the fluid
that the object displaces).
• An object floats only when the buoyant force on the
object is equal to the object’s weight.
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Chapter 14
Forces in Fluids
Buoyant Force on Floating Objects
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Chapter 14
Section 2 Buoyancy and Density
Density and Floating
• Density is mass per unit volume.
• Any object that is denser than the surrounding fluid
will sink.
• An object that is less dense than the surrounding fluid
will float.
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Chapter 14
Section 2 Buoyancy and Density
Determining Density
• To determine the density of an object, you need to
know the object’s mass and volume.
• A balance can be used to find the mass of an object.
• To find the volume of a regular solid, such as a cube,
multiply the lengths of the three sides together.
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Chapter 14
Section 2 Buoyancy and Density
Determining Density, continued
• To find the volume of an irregular solid, use water
displacement.
• By measuring the volume of water that the object
displaces, or pushes aside, you find the volume of
the object itself.
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Chapter 14
Section 2 Buoyancy and Density
Changing Overall Density
• The overall density of an object can be changed by
changing the object’s shape, mass, or volume.
• Submarines use ballast tanks to change their overall
density and dive under water.
• As water is added to the tanks, the submarine’s mass
increases, but its volume stays the same.
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Chapter 14
Section 2 Buoyancy and Density
Changing Overall Density, continued
• Most bony fishes have an organ called a swim
bladder that allows them to adjust their overall
density.
• An inflated swim bladder increases the fish’s volume,
which decreases the fish’s overall density so the fish
does not sink.
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Chapter 14
Forces in Fluids
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