Bernoulli s Principle

Transcription

1 Bernoulli s principle lift thrust drag Section 3 Describe the relationship between pressure and fluid speed. Analyze the roles of lift, thrust, and drag in flight. Give examples of Bernoulli s principle in real-life situations. Breathing Bernoulli-Style 1. Hold two pieces of paper by their top edges, one in each hand, so that they hang next to one another about 5 cm apart. 2. Blow a steady stream of air between the two sheets of paper. 3. Record your observations in your ScienceLog. Explain the results according to Bernoulli s principle. Bernoulli s Principle Has this ever happened to you? You ve just turned on the shower. Upon stepping into the water stream, you decide that the water pressure is not strong enough. You turn the faucet to provide more water, and all of a sudden the bottom edge of the shower curtain starts swirling around your legs. What s going on? It might surprise you that the explanation for this unusual occurrence also explains how wings help birds and planes fly and how pitchers throw curve balls. Fluid Pressure Decreases as Speed Increases The strange reaction of the shower curtain is caused by a property of moving fluids that was first described in the eighteenth century by Daniel Bernoulli (buhr NOO lee), a Swiss mathematician. Bernoulli s principle states that as the speed of a moving fluid increases, its pressure decreases. In the case of the shower curtain, the faster the water moves, the less pressure 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 work in Figure 14. A table-tennis ball is attached to a string and swung gently into a moving stream of water. Instead of being pushed back out, the ball is actually held in the moving water when the string is given a tug. Why does the ball do that? The water is moving, so it has a lower pressure than the surrounding air. The higher air pressure then pushes the ball into the area of lower pressure the water stream. Try this at home to see for yourself! Figure 14 This ball is pushed by the higher pressure of the air into an area of reduced pressure the water stream. 173

2 The first successful flight of an engine-driven heavierthan-air machine occurred in Kitty Hawk, North Carolina, in 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 seconds. It s a Bird! It s a Plane! It s Bernoulli s Principle! The most common commercial airplane in the skies today is the Boeing 737 jet. A 737 jet is almost 37 m long and has a wingspan of 30 m. Even without passengers, the plane weighs 350,000 N. That s more than 35 times heavier than an average car! How can something so big and heavy get off the ground, much less fly 10,000 m into the sky? Wing shape plays a role in helping these big planes as well as smaller planes and even birds achieve flight, as shown in Figure 15. According to Bernoulli s principle, the faster-moving air above the wing exerts less pressure than the slower-moving air below the wing. The increased pressure that results 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. Figure 15 Wing Shape Creates Differences in Air Speed a The curved top of the wing forces air passing above the wing to travel a longer distance than the air passing below the wing. c The air above must speed up to converge with the air below at the tail end of the wing. Therefore, the air moving above the wing must move faster than the air below it. b As the wing moves through the sky, air passing below the wing travels in a fairly straight path. 174 Chapter 7

3 Thrust and Wing Size Determine Lift The amount of lift created by a plane s wing is determined in part by the size of the wing and the speed at which air travels around the wing. The speed of an airplane is in large part determined by its thrust the forward force produced by the plane s engine. In general, a plane with a greater amount of thrust moves faster than a plane with less thrust. This faster speed means air travels around the wing at a greater speed, which increases lift. You can understand the relationship between wing size, thrust, and speed by thinking about a jet plane, like the one in Figure 16. This plane is able to fly with a relatively small wing size because its engine creates an enormous amount of thrust. This thrust pushes the plane through the sky at tremendous speeds. Therefore, the jet generates sufficient lift with small wings by moving very quickly through the air. Smaller wings keep 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 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 necessary to keep it in the air. Self-Check Does air travel faster or slower over the top of a wing? (See page 724 to check your answer.) Figure 16 The engine of this jet creates a great deal of thrust, so the wings don t have to be very big. Figure 17 The wings of this glider are very large in order to maximize the amount of lift achieved. Bernoulli s Principle Is for the Birds Birds don t have engines, of course, so they must flap their wings to push themselves through the air. The hawk shown at left uses its large wing size to fly with a minimum of effort. By extending its large wings to their full length and gliding on wind currents, a hawk can achieve enough lift to stay in the air while flapping only occasionally. Smaller birds must flap their wings more often to stay in the air. Soaring science! See how wing shape affects the flight of your own airplane on page 661 of the LabBook. Forces in Fluids 175

4 Lift and Spoilers At high speeds, air moving around the body of this race car could lift the car just as it lifts a plane s wing. This could cause the wheels to lose contact with the ground, sending the car out of control. To prevent this situation, an upside-down wing, or spoiler, is mounted on the rear of the car. How do spoilers help reduce the danger of accidents? Drag Opposes Motion in Fluids Have you ever walked into a strong wind and noticed that the wind seemed to slow you down? Fluids exert a force that opposes motion. The force that opposes or restricts motion in a fluid is called drag. In a strong wind, air drags on your clothes and body, making it difficult for you to move forward. Drag forces in flight work against the forward motion of a plane or bird and are usually caused by an irregular flow of air around the wings. An irregular or unpredictable flow of fluids is known as turbulence. Lift is often reduced when turbulence causes drag. At faster speeds, drag can become a serious problem, so airplanes are equipped with ways to reduce turbulence as much as possible when in flight. For example, flaps like those shown in Figure 18 can be used to change the shape or area of a wing, thereby reducing drag and increasing lift. Similarly, birds can adjust their wing feathers in response to turbulence to achieve greater lift. Figure 18 During flight, the pilot of this airplane can adjust these flaps to help increase lift. 176 Chapter 7

5 Wings Are Not Always Required 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. In fact, you ve already learned how Bernoulli s principle can affect such things as shower curtains and race cars. Any time fluids are moving, Bernoulli s principle is at work. In Figure 19, you can see how Bernoulli s principle can mean the difference between a home run and a strike during a baseball game. Bernoulli s principle at play read how Frisbees were invented on page 182. Figure 19 A pitcher can take advantage of Bernoulli s principle to produce a confusing curveball that is difficult for the batter to hit. a Air speed on the left side of the ball is decreased because air being dragged around the ball moves in the opposite direction of the airflow. This results in a region of increased pressure on the left side of the ball. Direction of airflow Direction of spin b Air speed on the right side of the ball is increased because air being dragged around the ball moves in the same direction as the airflow. This results in a region of decreased pressure on the right side of the ball. c Because air pressure on the left side is greater than that on the right side, the ball is pushed toward the right in a curved path. REVIEW 1. Does fluid pressure increase or decrease as fluid speed increases? 2. Explain how wing shape can contribute to lift during flight. 3. What force opposes motion through a fluid? 4. Interpreting Graphics When the space through which a fluid flows becomes narrow, fluid speed increases. Explain how this could lead to a collision for the two boats shown at right. Forces in Fluids 177