Bernoulli s Principle at Work

Transcription

1 Diagram of demonstration Denise Winkler and Kim Brown July 25, 2003 Bernoulli s Principle at Work *Airflow should be straight on the edge of the airfoil. pivot rod airflow counter weight support rod airfoil base Materials: 2 dowel rods (3/4 inch support rod and 3/8 inch pivot rod) wooden base, wood screw, washer, nail and assorted drill bits string and counter weights airfoil (use any thick paper: we used manila folders cut into varying sizes such as 5 cm x 15 cm, 5 cm x 20 cm, taped on the end to produce a flat side and a curved side) hair dryer Materials can be found at home or a local hardware store for less than $10. The stand and airfoils can be produced within one hour. The Concept: Bernoulli s principle states that the pressure exerted by a moving stream of fluid is less than the pressure of the surrounding fluid. (Kahan 2002) This principle applies to objects in air as well as pure liquids or gases. Different shaped objects can cause air to move at varying speeds above and below them. As long as the air above moves faster, pressure from below pushes the object upward. This difference in pressure creates an upward force on an object, called lift. Missouri Science Frameworks: Science 5-8: IV B 1 and Scientific Inquiry 1A Science 9-12: IV A 1a, IV B 2a, and IV C 1a. These frameworks address net force causing a change in motion, experimental design (middle school), and motion as shown by vectors, equations and graphs. The Misconceptions: Often students obtain knowledge that is not necessarily

2 correct. With the concept of air and pressure as related to Bernoulli s law some students think: strength and pressure are interrelated, i.e. flowing stream = fastest= most powerful gas is not a fluid, only liquids are fluids air and other gases do not have material properties such as mass and weight gases rise or float only wind, not still air, has pressure pressure is greater in a downward direction air or vacuums have the ability to suck moving air only has pressure in the direction it is moving; no movement= no pressure cold wind = high speed; warm wind = slow and gentle Sources: Diver, R., Squires, A., Rushworth, P., & Wood-Robinson, V. (1994). Making sense of secondary science: Research into children s ideas. London, England: Routledge. Kahan, P, Science Explorer: Motion, Forces, and Energy. Upper Saddle River, New Jersey: Prentice-Hall. The Demonstration: The apparatus is set up with an airfoil on the right side and a counterweight on the left side. Rubber bumpers are used as guides. A blow dryer supplies a constant air stream that is aimed straight at the front of the airfoil. The airfoil should be placed with the curved surface on the top. As the air flows over the airfoil, that side of the apparatus will lift up. The air must travel farther over the top of the airfoil, so it travels faster. This creates an area of low pressure, so the unbalanced pressure from the bottom causes the airfoil to lift. If the airfoil is accidentally placed upside down, the motion will be down instead of up. This could be done as a discrepant event to check for student understanding. Different lengths of airfoils will be shown as an inquiry activity. For a Gee Whiz follow-up, place two glasses 2-3 cm apart on the table and tape or hold them in place. Place a hardboiled or plastic egg in one, and ask the students to use Bernoulli s law to move the egg to the other jar without touching it. The trick is to blow at an angle behind the egg, but toward the second glass. The air pressure there will cause the egg to hop out and the lower pressure airflow will place it in the second glass. A stronger force can move the egg farther. The glasses can be adjusted or other objects can be used as an inquiry activity. Blow in this direction to reduce air pressure in the cup and cause the egg to move!

3 How does doing this demonstration provide an example of the scientific concept? This demonstration shows that airflow causes the mass of the airfoil to lift up. Flowing fluids (liquid or gas) create less pressure. When the air travels over the curved surface of the airfoil it travels faster, farther, and at less pressure. An unbalanced force on the airfoil causes it to lift. The same principles apply for the jumping egg. The air is traveling over the slightly curved surface of the glass, which changes its pressure. What predictions will students make? Students will predict that nothing will happen with the airfoil, or that the airfoil arm will drop towards the table. Most will not relate the motion to a change in pressure in the fluid (air) and an unbalanced force. Students will predict the egg will sit in the glass and not move, or it will jump out and land on the table. Exhibitor s Role: Due to the use of electricity, it would be safer for a person to maintain a position near the hallway exhibit. An instructional diagram will accompany the exhibit. Those interested may participate, starting with a brief discussion predicting the outcome. Participants may use the hair dryer at varying angles and at various levels to experiment with the airfoil. An airfoil that is put on the pole upside down will cause the arm to dip instead of raise. Once finished the docent may provide an explanation of the demonstration. If the egg and glasses are placed in the hall, the glasses should be taped in place and plastic eggs should be used. Teacher Roles: Middle School and High School: This demonstration could be done during a unit on air pressure or when discussing unbalanced forces. The lever arms could be used alone to discuss unbalanced forces. The teacher will demonstrate Bernoulli s law by displaying the apparatus. Students should sketch the apparatus in their notebooks/journals. Before initiating airflow, the teacher should generate predicting questions to stimulate students background knowledge. For example, What do you think will happen if I applied airflow to this balanced apparatus? This question is what they think, so all answers should be considered as long as they expand or elaborate on their prediction. Then as the discussion settles, the teacher will actually apply airflow straight at the airfoil. Students will be asked to write down their observations. The teacher will then proceed with the lesson on Bernoulli. After background information is shared, students can experiment with the position of the airfoil, the size of the contact surface at the front, or the length of the airfoil as an inquiry activity. A question and answer session will close the activity. The teacher could ask what would happen if the airfoil was turned over (curved surface on bottom) and air was applied. Follow-up activities such as the egg with the glasses could be done for assessment (see others in the Outcomes section). In closing, students could be challenged to look for situations in real life where Bernoulli s principle affects their surroundings. Student Roles: Middle School: The students will be asked to keep a journal of drawings, predictions, notes, and explanations for Bernoulli s law. Students will also be expected

4 to discuss with other students and the teacher how it is that airplanes are able to fly. High School: Students will apply their prior knowledge of pressure and other properties of gases to predict the motion of the airfoil. A free-body diagram of all the forces on the airfoil will be drawn. Students will be expected to use Bernoulli s principle to explain lift, and apply the same principle to several other situations (see Outcomes below). Students are likely to say that the hair dryer came up from underneath and pushed the airfoil up. With the hanging objects, students are most likely to say that the objects will move apart when you blow air between them. Most middle school students will not believe that the air traveling over the curved surface is traveling faster and has less pressure. Some may say that faster flowing air has less density so the airfoil rises. Outcomes: The ultimate outcome is for the airfoil to extend upwards as the airflow is applied over the airfoil from the front. Most students may think when airflow is applies from this angle, the force of the air would cause the airfoil to go down. Inquiry can be done by changing the width, length, or material of the airfoil, or by changing the air source. Assessment: In order to better understand this concept other examples or demonstrations may be used. One example is that of suspending two empty soda cans, balls or apples a few centimeters apart. Apply airflow through the space between the objects. Students will tend to think that the objects will move farther apart. Actually, the two move closer together due to the moving air between them reducing the pressure. The greater pressure exists on the outsides of the objects and therefore causes them to move inward. The diagram below shows this simple experiment. two cans 2-3cm apart support rod base Students can be asked how this situation relates to why a passing semi-truck seems to pull your car toward it as it passes, or why the windows sometimes blow out of buildings when wind blows down the narrow spaces between two buildings. Another application of this is when the water from your shower causes the air pressure around you to drop, the pressure outside the curtains is unbalanced, and the curtains are pushed in by the unbalanced force and stick to your legs. For more assessment, several other situations that require knowledge of Bernoulli s law can be presented to the students. How well they predict and/or explain the situation will give feedback to the teacher about their understanding of Bernoulli s law.

5 A) Cut a piece of paper 3 cm wide and as long as a piece of notebook paper. Fold over one end slightly. Hold it near your chin and blow across the paper. When air is blown, the students will expect the paper to move down. Instead, the air travels quickly down the paper and the slower moving air below the paper pushes it up. When the student stops blowing, the paper falls. An alternative is to bend a 3x5 notecard in half and set it on the table like a pyramid. Blowing underneath forces the card to the table instead of up as the students expect. B) Gently blow air toward a candle and observe the flame s movement. Then a 10 x 15 cm card can be held in front of a burning candle. Blow behind the card, and the candle flame will bend towards the card. Place a large jar in front of the candle, and blow behind the jar. The candle flame bends away from the jar and may even go out. (Blowing against the card produces turbulence and lower air pressure. The flame is drawn to the lower pressure. With the jar, the air moves smoothly around the jar. Discuss vehicles with square backs that create drag.) C) This question can be given to middle school students: Billy Bob is interested in racing. His uncle gives him advice on making his car faster. Billy Bob s uncle tells him to use a spoiler that is flat on the top and curved underneath. How is this spoiler design going to make Billy Bob s car faster? How does this relate to Bernoulli s principle? (Answer: By designing the spoiler as given, the air moving over the flat top pushes downward with more pressure. This pressure allows the car to stay closer to the ground and gives the tires better traction. This concept relates to Bernoulli in that the air moving over the curved surface of the spoiler travels farther, moves faster and exerts less pressure.) D) Students can be asked to design a demonstration that shows Bernoulli s properties, or to design an inquiry experiment that tests one variable and holds all others constant. On a high school level, the topic of flight could be a branch activity. Some calculations may be done with an upper level course. Other Sources: Lien, T. (1987). Invitations to Science Inquiry. USA: Science Inquiry Enterprises. Hewitt, P. (1999). Conceptual Physics. Menlo Park, California: Addison Wesley Longman, Inc.