Chapter Introduction
Lesson 1 Pressure and
Density of
Fluids
Lesson 2 The Buoyant
Force
Lesson 3 Other Effects
of Fluid Forces
Chapter Wrap-Up
In what ways do people
use forces in fluids?
What do you think?
Before you begin, decide if you agree or
disagree with each of these statements.
As you view this presentation, see if you
change your mind about any of the
statements.
Do you agree or disagree?
1. Air is fluid.
2. Pressure is a force acting on a fluid.
3. You can lift a rock easily under water
because there is a buoyant force on
the rock.
Do you agree or disagree?
4. The buoyant force on an object
depends on the object’s weight.
5. If you squeeze an unopened plastic
ketchup bottle, the pressure on the
ketchup changes everywhere in the
bottle.
6. Running with an open parachute
decreases the drag force on you.
Pressure and Density of Fluids
• How do force and area affect
pressure?
• How does pressure change with depth
in the atmosphere and under water?
• What factors affect the density of a
fluid?
Pressure and Density of Fluids
• fluid
• pressure
• atmospheric pressure
What is a fluid?
A fluid is any substance that can flow
and take the shape of the container that
holds it.
fluid
from Latin fluidus, means
“flowing”
What is a fluid? (cont.)
• The volume of a liquid
is constant, regardless
of the container.
• The volume of a gas
always changes to fill
the container.
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Pressure of Fluids
• Pressure is the amount of force per unit
area applied to an object’s surface.
• All fluids (liquids and gases) apply
pressure.
• Pressure applied on an object by a fluid
is related to the weight of the fluid.
Pressure of Fluids (cont.)
• In the equation below, P is pressure, f is
the force applied to a surface, and a is
the surface area over which the force is
applied.
• The unit measurement for pressure is
the pascal (Pa).
Pressure of Fluids (cont.)
Pressure of Fluids (cont.)
A fluid applies pressure perpendicular to all
sides of an object in contact with the fluid.
Pressure of Fluids (cont.)
• Pressure decreases when the surface
area over which a force is applied
increases.
• Pressure increases when the surface
area over which a force is applied
decreases.
Pressure of Fluids (cont.)
How does pressure change
as surface area changes?
Pressure of Fluids (cont.)
The ratio of the
weight of all the air
above you to your
surface area is
atmospheric
pressure.
On land, atmospheric pressure depends
on your elevation.
Under water, the pressure depends on
your depth below the water’s surface.
Pressure of Fluids (cont.)
How does elevation affect
atmospheric pressure?
Density of Fluids
If the volume of two fluids is the same, the
fluid that weighs more is denser.
dense
Science Use a measure of the ratio
of mass to volume
Common Use slow to learn or
understand
Density of Fluids (cont.)
• You can calculate density with the
equation below, where D is density,
m is mass, and v is volume.
• Density is often measured in grams per
cubic centimeter (g/cm3).
Density of Fluids (cont.)
• Materials have different
densities because of
differences in the masses
of their molecules and in
the distances between
them.
• Solids are usually denser
than liquids or gases.
Density of Fluids (cont.)
What factors determine the
density of fluids?
• Pressure is high
when a force is
applied over a
small area.
• Atmospheric
pressure
decreases with
elevation.
• Fluids form layers
depending on their
densities.
Which term refers to the ratio of
the weight of all the air above you
to your surface area?
A. atmospheric pressure
B. density
C. elevation
D. pascal
What happens to pressure
applied by a fluid as depth
increases?
A. decreases
B. increases
C. fluctuates randomly
D. remains the same
By what do you divide mass to
calculate density?
A. volume
B. pressure
C. force
D. area
Do you agree or disagree?
1. Air is fluid.
2. Pressure is a force acting on a fluid.
The Buoyant Force
• How are pressure and the buoyant
force related?
• How does Archimedes’ principle
describe the buoyant force?
• What makes an object sink or float in
a fluid?
The Buoyant Force
• buoyant force
• Archimedes’ principle
What is a buoyant force?
A buoyant force is an upward force
applied by a fluid on an object in the fluid.
buoyant
from Spanish boyar, means “to
float”
What is a buoyant force? (cont.)
A buoyant force acts
on any object in a
liquid. Objects in a
gas also experience
a buoyant force.
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The buoyant force on a diver is the
difference between the force from pressure
above and below the diver.
What is a buoyant force? (cont.)
How is pressure related to
buoyant force?
What is a buoyant force? (cont.)
• The depth of an object completely
submerged in a fluid has no effect on the
buoyant force.
• The buoyant force depends on an
object’s volume.
Archimedes’ Principle
• The greater the volume of an object in a
fluid, the greater the buoyant force on it.
• Archimedes’ principle states that the
buoyant force on an object is equal to
the weight of the fluid that the object
displaces.
The buoyant force is greater on the balloon
than on the tennis ball or the billiard ball
because the balloon displaces more water.
Archimedes’ Principle (cont.)
What is the buoyant force on
you if you displace 400 N of
water as you dive under water?
Sinking and Floating
• When the buoyant force on an object is
less than the gravitational force, the
object sinks.
• An object floats if the buoyant force
acting on it is equal to the object’s
weight.
If the weight of an object is greater than
the buoyant force acting on it, the object
sinks.
Sinking and Floating (cont.)
If an object is more dense than the fluid in
which it is placed, then the buoyant force
on that object will be less than the object’s
weight, and the object will sink.
If an object weighing 14 N
experiences a 12-N buoyant
force, will it sink or float?
The boat on the left floats because it is
filled with air instead of water.
As a balloon loses helium, its density
increases and its buoyant force decreases.
• A buoyant force
results from the
difference in
pressure
between the top
and the bottom
of an object?
• Objects that have the same volume
in a fluid experience the same
buoyant force.
• When the density of a balloon
becomes greater than the density of
air, the balloon sinks.
What is the direction of a buoyant
force?
A.
B.
C.
D.
upward
horizontal
downward
diagonal
How does increasing the depth of
an object in a fluid affect the
buoyant force on the fluid?
A. It has no effect.
B. It decreases the buoyant force.
C. It increases the buoyant force.
D. The effect depends on the
object’s depth.
An object floats if the buoyant
force is which of these?
A. equal to the object’s weight
B. greater than the object’s weight
C. less than atmospheric pressure
D. less than the object’s weight
Do you agree or disagree?
3. The buoyant force on an object
depends on the object’s weight.
4. You can lift a rock easily under water
because there is a buoyant force on
the rock.
Other Forces in Fluids
• How are forces transferred through a
fluid?
• How does Bernoulli’s principle
describe the relationship between
pressure and speed?
• What affects drag forces?
Other Forces in Fluids
• Pascal’s principle
• Bernoulli’s principle
• drag force
Fluid Forces—Benefits and
Challenges
• You produce a force when you squeeze
a plastic ketchup bottle.
• You make use of a buoyant force when
you float on a raft.
• Fluid forces from floods, tornadoes, and
hurricanes can cause damage.
Pascal’s Principle
Pascal’s principle states that when
pressure is applied to a fluid in a closed
container, the pressure increases by the
same amount everywhere in the container.
Pascal’s Principle (cont.)
How does pressure change
when force is applied to a
fluid in a closed container?
Pascal’s Principle (cont.)
• A piston uses a small force in one area
to apply a larger force in another area.
• According to Pascal’s principle,
pushing on the piston increases the
pressure equally throughout the fluid in
the piston.
• Car mechanics rely on Pascal’s principle
when they raise a car using a hydraulic
lift.
• Pushing down
with a small force
in the narrow
tube generates
a force under the
car that is large
enough to lift
the car.
• The piston on the left will have to be
pushed down further than the distance
the car will be raised.
• Since work is
force times
distance, the
work done by
the two pistons
is equal.
Bernoulli’s Principle
Bernoulli’s principle states that the
pressure of a fluid decreases when the
speed of that fluid increases.
Bernoulli’s Principle (cont.)
What is the relationship
between speed and pressure
in a fluid?
Bernoulli’s Principle (cont.)
Bernoulli’s Principle (cont.)
How does Bernoulli’s
principle explain how wind
can take the roof off a house?
Because air moves from areas of high to
low pressure, the higher pressure on the
left side of the soccer ball causes the ball
to curve right.
Drag Forces
Drag force is a force that opposes the
motion of an object through a fluid.
drag force
from Old Norse draga, means
“to draw”; and Latin fortis,
means “force”
Drag Forces (cont.)
• As the speed of an object in a fluid
increases, the drag force on that object
also increases.
• If two objects move in the same
direction, the object with the greater
surface area toward the direction of the
motion has a greater drag force on it.
• Drag forces increase when the density
of a fluid increases.
Drag Forces (cont.)
What affects the drag force
on an object?
• People rely on Pascal’s principle
when they use hydraulic lifts.
• The imbalance of
pressures in fluids
can cause a roof
to lift off a house
in a severe
windstorm.
• A soccer player who kicks a curved
ball makes use of Bernoulli’s
principle.
Which states that pressure in a
closed container is applied
equally?
A. Archimedes’ principle
B. Bernoulli’s principle
C. drag force principle
D. Pascal’s principle
What is the relationship between
the amount of work on the two
pistons of a closed fluid system?
A. More work is done on the larger side.
B. More work is done on the smaller
side.
C. No work is done on either side.
D. The amount of work is the same on
both sides.
Which states that the pressure of
a fluid decreases when the speed
of that fluid increases?
A. Archimedes’ principle
B. Bernoulli’s principle
C. drag force principle
D. Pascal’s principle
Do you agree or disagree?
5. If you squeeze an unopened plastic
ketchup bottle, the pressure on the
ketchup changes everywhere in the
bottle.
6. Running with an open parachute
decreases the drag force on you.
Key Concept Summary
Interactive Concept Map
Chapter Review
Standardized Test Practice
People use forces in
fluids to float objects
on water and in air, to
lift objects, and to
affect the motions of
objects.
Lesson 1: Pressure and Density of Fluids
• Pressure is the ratio of force to area.
• Atmospheric pressure decreases
with elevation. Pressure under water
increases with depth.
• The density of a fluid depends on
the mass of the fluid and its volume.
Lesson 2: The Buoyant Force
• The change in pressure between
the top and the bottom of an object
results in an upward force called
the buoyant force.
• Archimedes’ principle states that the
weight of the fluid displaced by an
object is equal to the buoyant force
on that object.
• An object sinks if its weight is greater
than the buoyant force on it. An object does not sink
if the buoyant force on it is equal to its weight.
Lesson 3: Other Forces from Fluids
• Pascal’s principle states that when pressure is
applied to a fluid in a closed container, the pressure
increases by the same amount everywhere in the
container.
• Bernoulli’s principle states that when the speed in a
fluid increases, the pressure decreases.
• Speed, size, and shape of an
object, as well as the density
of the fluid in which the object
moves, affect the drag force
on that object.
Which term describes the amount
of force per unit area applied to
an object’s surface?
A. atmospheric pressure
B. drag force
C. fluid
D. pressure
By what do you divide force to
calculate pressure?
A. area
B. mass
C. volume
D. weight
Which describes an upward force
applied by a fluid on an object in
the fluid?
A. pressure
B. gravity
C. density
D. buoyant force
The buoyant force on a diver is the
difference between the upward
force from pressure and which of
these?
A. the depth of the water
B. the downward force from pressure
C. the density of the diver
D. the density of the fluid
What happens to the atmospheric
pressure as you hike a trail to the
top of a mountain?
A. it decreases
B. it increases
C. it first decreases, then increases
D. if first increases, then decreases
Which term describes any
substance that can flow and take
the shape of the container that
holds it?
A. fluid
B. gas
C. liquid
D. solid
Which is the unit measurement
for pressure?
A. grams per cubic centimeter
B. joule
C. pascal
D. newton
Archimedes principle states that the
buoyant force on an object is equal
to what?
A. the depth of the object
B. the surface area of the object
C. the weight of the displaced fluid
D. the weight of the object
Which describes a force that
opposes the motion of an object
through a fluid?
A. atmospheric pressure
B. buoyant force
C. drag force
D. friction
What happens to the drag force on
an object in a fluid as the speed of
the object increases?
A. It decreases.
B. It depends on the shape of the
object.
C. It increases.
D. It remains constant.