3
3
Overview
READI NG WARM-U P
In this section, students learn
about Bernoulli’s principle. They
then explore how heavier-thanair objects can achieve flight.
Students also learn about the
basic aspects of flight. Finally,
students learn about Pascal’s
principle.
Objectives
•
•
•
•
Describe the relationship between
pressure and fluid speed.
Analyze the roles of lift, thrust, and
wing size in flight.
Describe drag, and explain how it
affects lift.
Explain Pascal’s principle.
Terms to Learn
Bernoulli’s principle
lift
thrust
drag
Pascal’s principle
Bellringer
Pose the following problem to
your students: “You have been
asked to design two kites. One
kite will be flown in areas where
there is almost always a good
breeze. The other kite will be
flown in areas with very little
wind.” What differences in
design and materials are there
between your two kites?
READI NG STRATEGY
Reading Organizer As you read this
section, create an outline of the section.
Use the headings from the section in
your outline.
Bernoulli’s principle the principle
that states that the pressure in a
fluid decreases as the fluid’s velocity
increases
Demonstration --------------g
Magic Water Place a straw
upright in a glass of water. Hold
a second straw at a right angle at
the top of the first so that the
straws are just touching. Blow
very hard through the horizontal straw. Water will rise up in
the vertical straw and form a
spray. Tell students they will
learn why this occurs after reading this section. l Visual
Figure 1 This ball is pushed
by the higher pressure of the air
into an area of reduced pressure—
the water stream.
CHAPTER RESOURCES
Chapter Resource File
CRF
• Lesson Plan
• Directed Reading A b
• Directed Reading B s
Technology
Transparencies
• Bellringer
• Wing Design and Lift
192
Chapter 7 • Forces in Fluids
Fluids and Motion
Hold two sheets of paper so that the edges are hanging in
front of your face about 4 cm apart. The flat faces of the
paper should be parallel to each other. Now, blow as hard
as you can between the two sheets of paper.
What’s going on? You can’t separate the sheets by blowing
between them. In fact, the sheets move closer together the
harder you blow. You may be surprised that the explanation
for this unusual occurrence also includes how wings help birds
and planes fly and how pitchers throw curve balls.
Fluid Speed and Pressure
The strange reaction of the paper is caused by a property of
moving fluids. This property was first described in the 18th
century by Daniel Bernoulli (ber NOO lee), a Swiss mathematician. Bernoulli’s principle states that as the speed of a moving
fluid increases, the fluid’s pressure decreases. In the case of
the paper, air speed between the two sheets increased when
you blew air between them. Because air speed increased, the
pressure between the sheets decreased. Thus, the higher
pressure on the outside of the sheets pushed them together.
Science in a Sink
Bernoulli’s principle is at work in Figure 1. A table-tennis ball is
attached to a string and swung into a stream of water. Instead
of being pushed out of the water, the ball is held in the water.
Why? The water is moving faster than the air around it, so
the water has a lower pressure than the surrounding air. The
higher air pressure pushes the ball into the area of lower pressure—the water stream. Try this at home to see for yourself!
Figure 2
Wing Design and Lift
v----------------------------b
a Airplane wings are made so
that the air speed above the
wing is greater than the air
speed below the wing.
b According to Bernoulli’s principle,
a difference in air speed means a
difference in pressure. The result
is an upward force that contributes to lift.
c Another feature of wing
design is that the shape
of the wing forces the
air downward. So, the air
pushes the wing upward.
l Verbal/Logical
v--------------------------------------a
Factors That Affect Flight
A common commercial airplane in the skies today is the
Boeing 737 jet. Even without passengers, the plane weighs
350,000 N. How can something so big and heavy get off the
ground and fly? Wing shape plays a role in helping these big
planes—as well as smaller planes and birds—achieve flight, as
shown in Figure 2.
According to Bernoulli’s principle, the fast-moving air
above the wing exerts less pressure than the slow-moving air
below the wing. The greater pressure below the wing exerts
an upward force. This upward force, known as lift, pushes the
wings (and the rest of the airplane or bird) upward against
the downward pull of gravity.
✓Reading Check
Wing Shape Ask students to
examine the wing shape shown
in Figure 2. Have students use
their knowledge of Bernoulli’s
principle to hypothesize about
what type of wings might work
in flight. Does the wing have to
be curved? Is flight possible
without wings? l Logical/Visual
lift an upward force on an object
that moves in a fluid
What is lift? (See the Appendix for answers to
Reading Checks.)
MISCONCEPTION
ALERT
More Than Bernoulli When teaching
about airplane flight, emphasize that
there is more to understanding lift than
can be explained by Bernoulli’s principle. Newton’s third law also plays a part.
A tilted wing deflects horizontal airflow
downward (the action force exerted by
the wing on the air). The reaction force
is the upward force the air exerts on the
wing. This force also contributes to lift.
Pressure Analogy Before you
discuss Bernoulli’s principle, it
may help some students to
imagine the pressure of a fluid
as the combined pressure of
many particles striking a surface.
Have students imagine a swarm
of bees trapped in a short section of a long piece of pipe. As
the bees fly around inside the
pipe, they bounce off each other
and off the walls of the pipe,
creating pressure. Then, have
students imagine that the bees
are suddenly able to fly the
entire length of the pipe.
Explain that, because the bees
have more room, they bounce
against the walls of the pipe
much less frequently, creating
less pressure inside the pipe.
Answer to Reading Check
Lift is an upward force on an object that
is moving in a fluid.
Demonstration --------------g
Flying Ball Point the airflow of
a portable hair dryer straight up,
and suspend a table-tennis ball
in the airstream. Change the
direction of the airflow slightly
to maneuver the ball. Have students speculate on the forces
that are at work in this
demonstration. l Visual
Section 3 • Fluids and Motion 193
Figure 3
Increased Thrust Versus Increased Wing Size
INCLUSION
Strategies
• Hearing Impaired
• Learning Disabled
• Developmentally Delayed
The concept of airplane lift
is complicated for students
with language delays to
understand. Use this experiment to give them a chance
to experience the idea of lift.
Organize the students into
small groups. Give each
group an 8 1/2 in.  11 in.
sheet of paper and an
11 in.  17 in. sheet of paper.
Ask each team to make two
paper airplanes that are alike
except that one has much
larger wings. Ask students to
note the lift of each plane as
they do the following: Throw
the two planes with the same
force. Throw the short-winged
plane with light force and
then with heavy force. Throw
the long-winged plane with
light force and then with
heavy force.
l Kinesthetic
e
v--------------------------------------a
Wind Tunnels Have students
research how engineers use
wind tunnels to test the design
of airplane wings. Then, have
students use what they have
learned to build their own wings
and wind tunnel, and show the
class how to test the wing designs.
l Kinesthetic
Thrust and Lift
thrust the pushing or pulling force
exerted by the engine of an aircraft
or rocket
The amount of lift created by a plane’s wing is determined
partly by the speed at which air travels around the wing. The
speed of a plane is determined mostly by its thrust. Thrust is
the forward force produced by the plane’s engine. In general, a
plane with a large amount of thrust moves faster than a plane
that has less thrust does. This faster speed means air travels
around the wing at a higher speed, which increases lift.
Wing Size, Speed, and Lift
The First Flight The first
successful flight of an enginedriven machine that was
heavier than air happened in
Kitty Hawk, North Carolina, in
1903. Orville Wright was the
pilot. The plane flew only 37 m
(about the length of a 737
jet) before landing, and the
entire flight lasted only 12 s.
Research another famous pilot
in the history of flight. Make a
poster that includes information about the pilot as well as
pictures of the pilot and his or
her airplane.
Cultural
Awareness
The amount of lift also depends partly on the size of a plane’s
wings. Look at the jet plane in Figure 3. This plane can fly
with a relatively small wing size because its engine gives a large
amount of thrust. This thrust pushes the plane through the sky
at great speeds. So, the jet creates a large amount of lift with
small wings by moving quickly through the air. Smaller wings
keep a plane’s weight low, which also helps it move faster.
Compared with the jet, the glider in Figure 3 has a large
wing area. A glider is an engineless plane. It rides rising air
currents to stay in flight. Without engines, gliders produce
no thrust and move more slowly than many other kinds of
planes. Thus, a glider must have large wings to create the lift
it needs to stay in the air.
Bernoulli and Birds
Birds don’t have engines, so birds must flap their wings to push
themselves through the air. A small bird must flap its wings at
a fast pace to stay in the air. But a hawk flaps its wings only
occasionally because it has larger wings than the small bird
has. A hawk uses its large wings to fly with very little effort.
Fully extended, a hawk’s wings allow the hawk to glide on
wind currents and still have enough lift to stay in the air.
g
Boomerangs More than 8,000 years
ago, Australian aborigines discovered
the aerodynamic qualities of a type of
hunting stick called a boomerang. Have
students research boomerangs and compare a boomerang’s flight with an airplane’s flight. Ask students to present
their findings in a poster. l Visual
194
Chapter 7 • Forces in Fluids
This glider has no engine and
therefore no thrust. So, its wings
must be large in order to maximize the amount of lift achieved.
The engine of this jet creates a
large amount of thrust, so the
wings don’t have to be very big.
CHAPTER RESOURCES
Technology
Transparencies
• Bernoulli’s Principle and the Screwball
Figure 4
CONNECTION v
Language Arts -------------------------g
Bernoulli’s Principle and the Screwball
Bird Story Have students imagine that they
are a hawk or an albatross. Have students write a onepage story describing how the
principles of flight apply to them
as they travel through the sky.
Students may need to research
the bird of their choice before
writing their stories. l Verbal
Writing
a Air speed on the left side of the ball is decreased
because air around the ball moves in the opposite
direction of the airflow. So, there is a region of
increased pressure on the left side of the ball.
c Because air pressure on the
left side is greater than air
pressure on the right side, the
ball is pushed toward the right
in a curved path.
Direction
of spin
Direction
of airflow
Group
b Air speed on the right side of the ball is increased
because air around the ball moves in the same
direction as the airflow. So, there is a region of
decreased pressure on the right side of the ball.
Safety Caution: Caution students to wear goggles, gloves,
and aprons while doing this
activity.
Floating Bubbles Prepare a
solution consisting of 250 mL of
dishwashing liquid, 50–60 drops
of glycerin, and 4.5 L of water.
Give small groups of students
containers of the solution and
straws or other bubble-blowing
tools. You may also want to provide students with index cards
to help create a breeze. Ask the
groups to devise ways to keep
the bubbles from hitting the
floor. Have groups describe
methods that increase the
pressure below the bubbles
or decrease the pressure
above them. l Kinesthetic
Bernoulli and Baseball
You don’t have to look up at a bird or a plane flying through
the sky to see Bernoulli’s principle in your world. Any time
fluids are moving, Bernoulli’s principle is at work. Figure 4
shows how a baseball pitcher can take advantage of Bernoulli’s
principle to throw a confusing screwball that is difficult for
a batter to hit.
Drag and Motion in Fluids
Have you ever walked into a strong wind and noticed that the
wind seemed to slow you down? It may have felt like the wind
was pushing you backward. Fluids exert a force that opposes
the motion of objects moving through the fluids. The force
that opposes or restricts motion in a fluid is called drag.
In a strong wind, air “drags” on your body and makes it
difficult for you to move forward. Drag also works against the
forward motion of a plane or bird in flight. Drag is usually
caused by an irregular flow of air. An irregular or unpredictable flow of fluids is known as turbulence.
✓Reading Check
v -------g
drag a force parallel to the velocity
of the flow; it opposes the direction
of an aircraft and, in combination
with thrust, determines the speed
of the aircraft
Answer to Reading Check
An irregular or unpredictable flow
of fluids is known as turbulence.
What is turbulence?
h -----------------------------g
o
Traveling Faster Than Sound In the
1940s, pilots of high-speed airplanes
reported that as they approached the speed
of sound (343 m/s at 20°C), their planes
began to shake and the controls did not
function properly. At these speeds, shock
waves formed a cone of turbulence around
the plane, interrupting the airflow over the
wings. Some scientists thought that an airplane could not go faster than the speed of
sound because the turbulence from shock
waves would tear the wings apart. Other
scientists thought that with better designs,
planes could exceed this speed. Jet planes
with swept-back wings and stronger frames
eventually exceeded the speed of sound.
Pascal’s Principle and
Hydraulics Have students
research hydraulic lifts that are
used in auto repair shops. Ask
students to explain what the
lifts have in common with
power brakes in automobiles.
Ask students to make a diagram
that illustrates how a hydraulic
lift system works. l Visual
Section 3 • Fluids and Motion 195
Reteaching -------------------------------------b
Seeing Turbulence Give pairs
of students a shallow pan of
water and an index card. Tell
students to slowly drag the
index card through the water
and to watch the water behind
the card. Tell students that the
ripples behind the card and the
swirls that come off the edge of
the card are examples of
turbulence. l Visual
Figure 5 The pilot of this airplane
can adjust these flaps to help
increase lift when the airplane
lands or takes off.
Turbulence and Lift
Lift is often reduced when turbulence causes drag. Drag can
be a serious problem for airplanes moving at high speeds. So,
airplanes are equipped with ways to reduce turbulence as much
as possible when in flight. For example, flaps like those shown
in Figure 5 can be used to change the shape or area of a wing.
This change can reduce drag and increase lift. Similarly, birds
can adjust their wing feathers in response to turbulence.
Quiz --------------------------------------------------------------------g
1. What forces act on an
aircraft? (lift, thrust, drag,
and gravity)
✓Reading Check
2. When an airplane is flying,
Pascal’s Principle
how does the air pressure
above a wing compare with
that below the wing? (Air pressure above the wing is lower.)
3. How is thrust related to the
Pascal’s principle the principle
speed of an airplane? (The
speed of an airplane increases as
its thrust increases.)
that states that a fluid in equilibrium
contained in a vessel exerts a
pressure of equal intensity in all
directions
Imagine that the water-pumping station in your town increases
the water pressure by 20 Pa. Will the water pressure be increased
more at a store two blocks away or at a home 2 km away?
Believe it or not, the increase in water pressure will be the
same at both locations. This equal change in water pressure is
explained by 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. This principle was
discovered by the 17th-century French scientist Blaise Pascal.
Pascal’s Principle and Motion
Alternative
Assessment ---------------------------g
Hydraulic (hie DRAW lik) devices use Pascal’s principle to move
or lift objects. Liquids are used in hydraulic devices because
liquids cannot be easily compressed, or squeezed, into a smaller
space. Cranes, forklifts, and bulldozers have hydraulic devices
that help them lift heavy objects.
Hydraulic devices can multiply forces. Car brakes are a
good example. In Figure 6, a driver’s foot exerts pressure on a
cylinder of liquid. This pressure is transmitted to all parts of the
liquid-filled brake system. The liquid moves the brake pads. The
pads press against the wheels, and friction stops the car. The
force is multiplied because the pistons that push the brake pads
are larger than the piston that is pushed by the brake pedal.
Aircraft Chart Display two or
three photographs or models of
different types of aircraft, such
as a glider, a jet, a biplane, or
even an airship. Ask students to
select two of the aircraft and to
make a chart that compares and
contrasts the aircraft in terms of
lift, drag, thrust, and gravity.
l Verbal
Answer to Reading Check
Airplanes can reduce turbulence by changing
the shape or area of the wings.
196
How do airplanes reduce turbulence?
Chapter 7 • Forces in Fluids
Answers to Section Review
Figure 6 Because of Pascal’s principle, the
touch of a foot can stop tons of moving metal.
1 When the driver pushes the brake pedal, a small piston
exerts pressure on the fluid inside the brake system.
2 The change in
pressure is transmitted to the large
pistons that push
on the brake pads.
Review
Using Key Terms
For each pair of terms, explain how
the meanings of the terms differ.
1. Bernoulli’s principle and Pascal’s
principle
Summary
principle
• Bernoulli’s
states that fluid pressure
•
decreases as the speed
of the fluid increases.
Wing shape allows airplanes to take advantage
of Bernoulli’s principle to
achieve flight.
on an airplane is
• Lift
determined by wing size
and thrust.
Drag opposes motion
through fluids.
•
principle states
• Pascal’s
that a change in pressure in an enclosed fluid
is transmitted equally to
all parts of the fluid.
2. thrust and drag
Understanding Key Ideas
3. The shape of an airplane’s wing
helps it gain
a. drag.
b. lift.
c. thrust.
d. turbulence.
9. Making Inferences When you
squeeze a balloon, where is
the pressure inside the balloon
increased the most? Explain.
Interpreting Graphics
10. Look at the image below. When
the space through which a fluid
flows becomes narrow, fluid
speed increases. Using this
information, explain how the
two boats could collide.
4. What is the relationship
between pressure and fluid
speed?
5. What is Pascal’s principle?
6. What force opposes motion
through a fluid? How does this
force affect lift?
7. How do thrust and lift help an
airplane achieve flight?
Critical Thinking
8. Applying Concepts Air moving
around a speeding race car can
create lift. Upside-down wings,
or spoilers, are mounted on the
rear of race cars. Use Bernoulli’s
principle to explain how spoilers
reduce the danger of accidents.
For a variety of links related to this
chapter, go to www.scilinks.org
Topic: Bernoulli’s Principle
SciLinks code: HSM0143
CHAPTER RESOURCES
Chapter Resource File
CRF
• Section Quiz g
• Section Review g
• Vocabulary and Section Summary g
• Reinforcement Worksheet b
• Critical Thinking a
1. Bernoulli’s principle states
that the pressure in a fluid
decreases as the fluid’s velocity
increases. Pascal’s principle
states that a fluid in an enclosed
container exerts pressure
equally in all directions.
2. Thrust is the pushing or
pulling force exerted by the
engine of an airplane that
moves the airplane forward.
Drag is a force that opposes
motion in a fluid.
3. b
4. As fluid speed increases,
the pressure exerted by the
fluid decreases.
5. Pascal’s principle states
that an enclosed fluid exerts
pressure equally in all
directions.
6. Drag is a force that opposes
motion through a fluid. Lift is
often reduced when turbulence
causes drag.
7. Lift helps an airplane achieve
flight by pushing the airplane
up. Thrust helps an airplane
achieve flight by causing the
airplane to move faster through
the air. The faster speed means
that air travels faster around
the wings, which increases lift.
8. Sample answer: Air traveling
around the spoiler produces
a downward force. This downward force pushes down on
the rear of the car and helps
keep the rear wheels of the
cars in contact with the road.
The cars travel more safely
because the rear wheels stay
in contact with the road.
9. The pressure inside the balloon increases equally in all
directions. Squeezing a balloon
demonstrates Pascal’s
principle.
10. As the fluid speed between the
boats increases, the fluid pressure decreases. The pressure
on the outer sides of the boats
then becomes greater than the
pressure between them. This
increased pressure from the
outside can push the boats
together, causing them to
collide.
Section 3 • Fluids and Motion 197