Copyright © by Holt, Rinehart and Winston. All rights reserved.

Bernoulli’s PrincipleHas this ever happened to you? You’ve just turned on the shower.Upon stepping into the water stream, you decide that the waterpressure is not strong enough. You turn the faucet to providemore water, and all of a sudden the bottom edge of the showercurtain starts swirling around your legs. What’s going on? Itmight surprise you that the explanation for this unusual occur-rence also explains how wings help birds and planes fly andhow pitchers throw curve balls.

Fluid Pressure Decreases as Speed Increases The strange reaction of the shower curtain is caused by aproperty of moving fluids that was first described in the eight-eenth century by Daniel Bernoulli (buhr NOO lee), a Swissmathematician. Bernoulli’s principle states that as the speedof a moving fluid increases, its pressure decreases. In the caseof the shower curtain, the faster the water moves, the lesspressure it exerts. This creates an imbalance between the pressure inside the shower curtain and the pressure outside it. Because the pressure outside is now greater than the pressure inside, the shower curtain is pushed toward the water stream.

Science in a Sink You can see Bernoulli’s principle at workin Figure 14. A table-tennis ball is attached to a string andswung gently into a moving stream of water. Instead of beingpushed back out, the ballis actually held in the mov-ing water when the stringis given a tug. Why doesthe ball do that? The wateris moving, so it has a lowerpressure than the sur-rounding air. The higherair pressure then pushesthe ball into the area oflower pressure—the waterstream. Try this at home tosee for yourself!

Figure 14 This ball is pushedby the higher pressure of theair into an area of reducedpressure—the water stream.

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Section

3

Bernoulli’s principleliftthrustdrag

◆ Describe the relationship betweenpressure and fluid speed.

◆ Analyze the roles of lift, thrust,and drag in flight.

◆ Give examples of Bernoulli’sprinciple in real-life situations.

Breathing Bernoulli-Style

1. Hold two pieces of paperby their top edges, one ineach hand, so that theyhang next to one anotherabout 5 cm apart.

2. Blow a steady stream of airbetween the two sheets ofpaper.

3. Record your observations in your ScienceLog. Explainthe results according toBernoulli’s principle.

Copyright © by Holt, Rinehart and Winston. All rights reserved.

It’s a Bird! It’s a Plane! It’s Bernoulli’s Principle!The most common commercial airplane in the skies today isthe Boeing 737 jet. A 737 jet is almost 37 m long and has awingspan of 30 m. Even without passengers, the plane weighs350,000 N. That’s more than 35 times heavier than an aver-age car! How can something so big and heavy get off theground, much less fly 10,000 m into the sky? Wing shapeplays a role in helping these big planes—as well as smallerplanes and even birds—achieve flight, as shown in Figure 15.

According to Bernoulli’s principle, the faster-moving airabove the wing exerts less pressure than the slower-movingair below the wing. The increased pressure that results belowthe wing exerts an upward force. This upward force, knownas lift, pushes the wings (and the rest of the airplane or bird)upward against the downward pull of gravity.

Chapter 7174

Figure 15 Wing Shape Creates Differences in Air Speed

The first successful flight ofan engine-driven heavier-than-air machine occurredin Kitty Hawk, NorthCarolina, in 1903. OrvilleWright was the pilot. Theplane flew only 37 m (aboutthe length of a 737 jet)before landing, and theentire flight lasted only 12 seconds.

The curved top of the wingforces air passing above the wingto travel a longer distance thanthe air passing below the wing.

The air above must speed up toconverge with the air below at thetail end of the wing. Therefore,the air moving above the wingmust move faster than the airbelow it.

As the wing moves through thesky, air passing below the wingtravels in a fairly straight path.

a

b

c

Copyright © by Holt, Rinehart and Winston. All rights reserved.

Thrust and Wing Size Determine Lift The amount of lift created by a plane’s wing is deter-mined in part by the size of the wing and the speed atwhich air travels around the wing. The speed of an airplaneis in large part determined by its thrust—the forward forceproduced by the plane’s engine. In general, a plane with agreater amount of thrust moves faster than a plane with lessthrust. This faster speed means air travels around the wing ata greater speed, which increases lift.

You can understand the relationship between wing size,thrust, and speed by thinking about a jet plane, like the onein Figure 16. This plane is able to fly with a relatively smallwing size because its engine creates an enormous amount ofthrust. This thrust pushes the plane through the sky at tremen-dous speeds. Therefore, the jet generates sufficient lift with smallwings by moving very quickly through the air. Smaller wingskeep a plane’s weight low, which also contributes to speed.

Compared with the jet, a glider, like the one in Figure 17,has a large wing area. A glider is an engineless plane that ridesrising air currents to stay in flight. Without engines,gliders produce no thrust and move more slowlythan many other kinds of planes. Thus, a glidermust have large wings to create the lift necessaryto keep it in the air.

Bernoulli’s Principle Is for the Birds Birds don’t haveengines, of course, so they must flap their wings to

push themselves through the air. The hawk shownat left uses its large wing size to fly with a mini-

mum of effort. By extending its large wings totheir full length and gliding on wind currents, ahawk can achieve enough lift to stay in the air

while flapping only occasionally. Smallerbirds must flap their wings more often to

stay in the air.

Forces in Fluids 175

Figure 17 The wings of thisglider are very large in order tomaximize the amount of liftachieved.

Soaring science! See how wingshape affects the flight of your

own airplane on page 661of the LabBook.

Figure 16 The engine of this jetcreates a great deal of thrust, so the wings don’t have to be very big.

Self-CheckDoes air travel faster or slowerover the top of a wing? (Seepage 724 to check your answer.)

Drag Opposes Motion in Fluids Have you ever walked into a strong wind and noticed that thewind seemed to slow you down? Fluids exert a force that

opposes motion. The force that opposes or restricts motionin a fluid is called drag. In a strong wind, air “drags” on

your clothes and body, making it difficult for you tomove forward. Drag forces in flight work against theforward motion of a plane or bird and are usuallycaused by an irregular flow of air around the wings.An irregular or unpredictable flow of fluids is knownas turbulence.

Lift is often reduced when turbulence causesdrag. At faster speeds, drag can become a seriousproblem, so airplanes are equipped with ways toreduce turbulence as much as possible when in flight. For example, flaps like those shown inFigure 18 can be used to change the shape or areaof a wing, thereby reducing drag and increasing lift.

Similarly, birds can adjust their wing feathers inresponse to turbulence to achieve greater lift.

Chapter 7176

Lift and Spoilers

At high speeds, air moving around the body ofthis race car could lift the car just as it lifts aplane’s wing. This could cause the wheels to lose contactwith the ground, sending the car out of control. To prevent thissituation, an upside-down wing, or spoiler, is mounted on the rearof the car. How do spoilers help reduce the danger of accidents?

Figure 18 During flight, the pilot ofthis airplane can adjust these flapsto help increase lift.

Copyright © by Holt, Rinehart and Winston. All rights reserved.

Copyright © by Holt, Rinehart and Winston. All rights reserved.

Wings Are Not Always Required You don’t have to look up at a bird or a plane flying throughthe sky to see Bernoulli’s principle in your world. In fact,you’ve already learned how Bernoulli’s principle can affectsuch things as shower curtains and race cars. Any time fluidsare moving, Bernoulli’s principle is at work. In Figure 19, youcan see how Bernoulli’s principle can mean the differencebetween a home run and a strike during a baseball game.

Forces in Fluids 177

1. Does fluid pressure increase or decrease as fluidspeed increases?

2. Explain how wing shape can contribute to liftduring flight.

3. What force opposes motion through a fluid?

4. Interpreting Graphics When the space throughwhich a fluid flows becomes narrow, fluid speedincreases. Explain how this could lead to a col-lision for the two boats shown at right.

Figure 19 A pitcher can take advantage of Bernoulli’s principle toproduce a confusing curveball that is difficult for the batter to hit.

Direction of airflow

Direction of spin

Bernoulli’s principle at play—read how Frisbees® wereinvented on page 182.

Air speed on the left side of the ball is decreasedbecause air being dragged around the ball moves inthe opposite direction of the airflow. This results in aregion of increased pressure on the left side of the ball.

Air speed on the right side of the ball is increasedbecause air being dragged around the ball moves in thesame direction as the airflow. This results in a region ofdecreased pressure on the right side of the ball.

Because air pressure on the leftside is greater than that on theright side, the ball is pushedtoward the right in a curved path.

a

b c

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