How Things Work - Chapter 6 Demonstrations
Section 6.1 Garden Watering
Demonstration 6.1.1: A Vortex Cannon
Description: A large cylinder with a small circular opening in front has a flexible rubber diaphragm on its back. When the diaphragm is pushed inward rapidly, a vortex of air leaps out of the circular opening and travels across the room. When smoke is introduced inside the cylinder, the vortices appears as giant smoke rings.
Purpose: To show that even air has remarkable and interesting dynamics.
1 5-gallon plastic paint or other liquid container (although even a cardboard box will do)
1 flexible rubber sheet, large enough to seal the open end of the cylinder
1 clamping system to hold the sheet across the open end of the cylinder (or use tape)
elastic bands (optional)
smoke generator (optional)
Procedure: Carefully cut a circular opening about 10 cm in diameter in the center of the plastic container's bottom. Stretch the rubber sheet across the open end of the cylinder and clamp it into place. (We have done this by cutting out the center portion of the container's top and then forcing the top onto the container so that it clamps the rubber sheet in place.) If you now hold the container horizontally and strike the rubber sheet firmly, it will emit vortex rings that travel across the room. We have attached several rubber bands to the rubber sheet in our device, so that these rubber bands pull the sheet toward the circular opening. To create vortex rings, you pull the rubber sheet out of the cylinder and let go. The rubber bands pull the sheet back into the cylinder and a vortex ring emerges from the cylinder. If you fill the vortex cannon with smoke before making vortex rings, they will appears a beautiful smoke rings.
Explanation: As air flows out of the relatively small hole in the vortex cannon, friction with the opening causes the air to form a twisting ring—a vortex ring.
Demonstration 6.1.2: Water Flow through Tubes of Different Diameters
Description: Water flowing out of a wide pipe attached to the bottom of a water container flows much more quickly than water flowing out of a narrow pipe attached to that same container.
Purpose: To show that the diameter of a water pipe dramatically affects the rate at which water flows through that pipe, for a given pressure imbalance.
1 container with a hole in the side, about 1 cm above the bottom (a plastic cup, for example)
2 corks that fit into the hole
1 narrow tube about 10 cm long
1 wider tube about 10 cm long
Procedure: Fit the two tubes into the two corks. Insert the wider tube into the hole in the container and fill the container with water. Water will flow rapidly through the wide tube and the container will drain quickly. Now insert the narrow tube into the container and refill the container. The water will flow much more slowly and the container will drain very slowly.
Explanation: In both cases, there is a specific pressure difference between the elevated pressure inside the container and the atmospheric pressure at the end of the tube. But since the amount of water that can pass through a tube experiencing laminar flow depends on the 4th power of the tube's diameter, the narrower tube carries far less water than the wider tube.
Demonstration 6.1.3: Water Pouring From a Hose, With and Without Your Thumb
Description: Water flowing gently out of a hose quickly fills a container. Water spraying vigorous out of that same hose when its end is nearly blocked by your thumb fills the container much more slowly.
Purpose: To show that slowing down the flow of water in the plumbing by blocking the hose end with your thumb allows the water to retain more of its total energy and thus spray harder out from under thumb than it does when your thumb is not blocking the flow. The rate of water flow, however, is less when your thumb is blocking the flow than when the hose end is unobstructed.
1 water hose connected to a faucet.
1 small transparent container
Procedure: Turn on the faucet slightly so that water pours gently from the end of the hose. Let the hose fill the container part way up. Now pinch off the end of the hose with your thumb so that the water sprays vigorously out of the narrowed opening. Direct this spray into the container. The container will fill much more slowly, despite the high speed of the water.
Explanation: With the hose unobstructed, the water is flowing quickly through the faucet and the hose and is wasting nearly all of its total energy as thermal energy before it pours out of the hose's end. There is so little total energy let in that water that it departs at low speed. But when you pinch off the opening so that the water flow in the plumbing is slowed, the water wastes little of its total energy and can spray hard as it squirts past your thumb. The amount of water flowing out of the narrow opening is much less, however, than it was when your thumb wasn't in the way.
Demonstration 6.1.4: Speed and Pressure of Air Flowing in a Tube
Description: The pressure of air flowing through a tube changes as the tube's diameter changes—dropping as the tube becomes narrower and rising as the tube becomes wider.
Purpose: To show that air's pressure drops when it speeds up to pass through a narrow channel and that its pressure rises when it slows down to pass through a wide channel.
1 Bernoulli demonstrator—a pipe with a narrow channel near its middle and several pressure monitoring points along its length
Procedure: Attach the compressed air to the Bernoulli demonstrator and allow that air to flow through the tube. Observe that the pressure inside the demonstrator rises at any wide portion of the tube and drops at any narrow portion of the tube. Note also that the pressure in the wide portions before and after a narrow channel are different—the pressure is higher before the narrow channel than after the narrow channel (due to energy loss in the channel).
Explanation: Whenever air flows through a narrow channel, it speeds up to allow the same volume of air to flow through the narrow channel each second as flows through the wider portions of the tube each second. When the air speeds up, its pressure drops so that most of its total energy can become kinetic energy. When the air in the narrow channel then enters the wider portion of the tube, it slows down and its pressure rises as its kinetic energy becomes pressure potential energy. The pressure doesn't reach its original value because the high speed air in the narrow channel loses a substantial amount of its energy through friction with the walls of the narrow channel.
Demonstration 6.1.5: Two Plates Stick to One Another as Air Flows Between Them
Description: Compressed air is allowed to flow out of a hole in the center of a plastic plate. When a second plate is placed a short distance away from the hole, it's blown away from the hole. But when the second plate is touched to the first plate, the two plates are suddenly pressed together by the surrounding air.
Purpose: To show that when air speeds up to flow through the narrow gap between two plates, its pressure drops dramatically and can even fall below atmospheric pressure.
2 flat plastic plates, about 20 cm on a side. One should have a hole drilled in it and a hose attached to the hole. The other should have a metal pin inserted into it.
Procedure: Attach the hose to the compress air and allow the air to flow out of the hole. Show that as the second plate approaches this hole, it's blown away by the rushing air. Now touch the two plates together and allow the pin of the second plate to enter the hole in the first plate. This pin keeps the second plate from sliding off the first plate. The two plates will remain together despite the continued flow of air out of the hole in the first plate. In fact, the two plates will "stick" together!
Explanation: When the air in the hose flows into the narrow gap between the two nearby plates, it speeds up and its pressure drops below atmospheric pressure. The atmospheric pressure on the outside surfaces of the two plates then squeezes the plates together. The spacing between the plates self-regulates so that the pressure between them is just low enough to keep the second plate from moving closer or farther from the first plate. The two plates stay together even when the second plate is hanging below the first plate. In that case, the pressure between the plates drops below atmospheric pressure, providing a pressure imbalance that supports the weight of the second plate.
Demonstration 6.1.6: A Ping Pong Ball Suspended by Air in an Inverted Funnel
Description: An inverted plastic funnel is attached to a compressed air source and a Ping Pong ball is inserted into the wide opening of the funnel. The Ping Pong ball hangs suspended in the funnel as air flows downward around its sides.
Purpose: To show that air's pressure drops as it speeds up to flow through a narrow channel.
1 small plastic funnel
1 Ping Pong ball
Procedure: Use the hose to attach the funnel to the compressed air. Hold the funnel upside down and start the compressed air flowing. Push the Ping Pong ball upward into the wide portion of the funnel. The ball will remain suspended in the funnel.
Explanation: As the air flows through the thin region between the funnel and the upper edge of the Ping Pong ball, its speed increases dramatically. As the air's speed increases, its pressure drops. With low pressure air on part of its upper surface, the ball experiences a net upward pressure force that's sufficient to support it against gravity.
Demonstration 6.1.7: A Paint Sprayer
Description: Compressed air is sent through a narrow channel above a drinking straw. This straw rises from a container of water. The water flows up the straw and into the airstream, creating a mist of atomized water.
Purpose: To show that the air pressure in a narrow channel can drop below atmospheric pressure, even when compressed air is delivered to that channel.
1 drinking straw
1 container of water
Procedure: Use the hose to attached the eyedropper's tube to the compressed air. Insert the straw in the container of water. Start the compressed air flowing and align the eyedropper's nozzle over the top of the straw. Use your fingers to create a moderate seal around the straw and to extend the narrowing at the end of the eyedropper. Be carefully not to direct the airflow down the straw, or you'll get wet. When you have the narrow channel extending all the way across the straw, the pressure in the straw will drop below atmospheric pressure and water will begin to rise up into the airstream. When it reaches the airstream, the water will be atomized into a mist and will spray out into the room.
Explanation: Even though you begin with compressed air at one end of the eyedropper, the pressure of the air flowing out of the eyedropper can drop below atmospheric pressure if its speed become sufficiently high. When this happens, the low pressure can allow atmospheric pressure air to push liquids into the narrow channel containing the fast moving air.
Demonstration 6.1.8: A Water Aspirator Pump
Description: A small gadget is attached to a water faucet and water is sent through it. The pressure in a hose attached to the side of the gadget suddenly drops below atmospheric pressure and begins to suck colored water out of container.
Purpose: To show that water's pressure can drop below atmospheric pressure when it passes through a narrow channel and its speed increases substantially.
1 water aspirator pump
1 container of colored water
Procedure: Attach the water aspirator pump to the water source and attach the hose to the side arm of the pump. Turn on the water flow and immerse the other end of the hose in the container of water. The water will begin to flow up the hose and into the pump.
Explanation: The water flowing through the pump is entering a very narrow channel. As it does, its speed increases dramatically and so does its kinetic energy. To provide this kinetic energy, the water's pressure and pressure potential energy drop precipitously. A small hole in the side of the channel connects to the hose. When the pressure in the channel becomes very low, water flows up the hose toward the channel.
Demonstration 6.1.9: Laminar vs Turbulent Flow - Reynolds Number
Description: A cylindrical stick is drawn slowly through a container of water and leaves no visible wake. But when the stick is drawn quickly through the water, the water swirls behind it.
Purpose: To show the onset of turbulent flow when the Reynolds number exceeds about 2000.
1 cylindrical stick about 1 or 2 cm in diameter
1 container of water (or, better yet, a rheological fluid that makes its motion visible; opalescent liquid soap will work if you avoid making bubbles).
1 overhead projector (optional)
Procedure: First move the stick slowly through the water. Below about 10 cm/s, the Reynolds number will be below 2000 and the flow around it will be laminar. You will see little disturbance in the water. Now move the stick more rapidly—about 50 cm/s. The water will become turbulent behind the stick because the Reynolds number will have reached 5000 or more.
Explanation: The flow around the stick is laminar below a Reynolds number of about 2000 and turbulent above a Reynolds number of about 5000. The faster the stick moves through the water, the higher the Reynolds number and the more likely the flow is to be turbulent.
Follow-up: You can also do this experiment by putting a shallow circular dish of water on a turn-table and lowering the stick into it on a support. The faster you spin the water dish, the faster the water moves past the stick and the more likely it is to exhibit turbulent flow. However, getting the water to spin with the dish isn't so easy. Special rheological fluids are also available that help in flow visualization.
Demonstration 6.1.10: Water Hammer Demonstration Toy
Description: When you shake a water-filled glass object, it emits a sharp ping sound, as though it were struck with a solid object.
Purpose: To show that water can exert a sudden impact on a solid surface.
1 water hammer demonstrator from a scientific supply company
Procedure: Hold the demonstrator vertically, with the long tube end down and the air bubble end on top. Accelerate the demonstrator downward rapidly and then stop abruptly. The water will strike the bottom of the demonstrator and emit a sharp ping sound when it hits.
Explanation: When you accelerate the glass container downward, the water is left behind. It drifts toward the air bubble on top and compresses the air in that bubble. When the glass container stops accelerating downward, the pressure imbalance around the water—high pressure above and almost zero pressure below—propels the water downward until it overtakes the bottom of the container. The water strikes the bottom of the container hard enough to create the ping sound.
Demonstration 6.1.11: Knocking the Bottom out of a Glass Soda Bottle with Water
Description: A root beer bottle full of water is held upright in your hand while you strike its cap with a rubber mallet. The bottom of the bottle drops out with a loud pop, and water and glass drop into a bucket.
Purpose: To demonstrate the effects of water hammer.
1 glass root beer bottle, filled to the base of the neck with water and sealed on top with either the original cap or with plastic wrap and a rubber band
1 rubber mallet
1 bucket or trash receptacle
Procedure: Hold the root beer bottle upright in one hand, gripping it around the body of the bottle, and strike the top of the bottle firmly with the mallet. A gentle hit will cause the bottle to emit a loud ping sound. A strong hit will knock the bottom out of the bottle. Hold the bottle over the bucket, so that the broken glass and water have somewhere to go.
Explanation: When you strike the top of the bottle, the glass container accelerates downward very suddenly. The water, which is not directly attached to the container, remains essentially in place and enters the neck of the bottle as the bottle shifts downward. Since there is air already in the neck of the bottle, that air becomes compressed. When the bottle stops accelerating downward, the elevated pressure in the neck of the bottle and the near absence of pressure at the bottom of the bottle cause the water to accelerate downward, toward the bottom of the bottle. When the water reaches the bottom of the bottle, the pressure at the bottom surges upward and the enormous force on the bottom of the bottle exceeds its breaking strength. The bottom of the bottle tears away for the sides and the water pours out into the bucket.
Section 6.2 Balls and Air
Demonstration 6.2.1: Suspending Dust with Viscous Drag
Description: You release dust into the air and it drifts around with the air currents
Purpose: To show that viscous drag forces can dramatically slow the fall of tiny particles.
Dust (chalk erasers, for example).
Procedure: Release the dust into the air and watch as it drifts about with the air currents. Note that the dust is tiny particles of solids and is not being supported significantly by buoyant forces. It is experiencing severe viscous drag forces and is effectively unable to fall.
Explanation: Because air's viscosity is small, it's not easy to get viscous effects to dominate inertial effects with an ordinary ball. That's why you're using dust: the dust is so tiny that air's viscosity is able to keep the flow orderly all the way around each dust particle -- no turbulence. In the absence of turbulence, the only drag force the dust can experience is viscous drag.
Demonstration 6.2.2: Suspending a Ball in an Airstream with Pressure Drag
Description: A Ping Pong ball or a beach ball remains suspended in a jet of air emerging from a pipe.
Purpose: To show that air's pressure changes as its speed changes.
1 Ping Pong ball (or a beach ball)
compressed air (or a leaf blower for the beach ball)
Procedure: Attach the hose to the compressed air and direct a stream of air upward into the room. Carefully lower the Ping Pong ball into the airstream and it will remain suspended above the hose opening indefinitely.
Explanation: The ball is pushed upward by pressure drag (a topic discussed in Section 4.3). What keeps the ball stable near the center of the airstream is Bernoulli's effect. Whenever the ball drifts away from the center of the airstream, the airflow on the side of the ball nearest the center of the airstream becomes stronger than anywhere else. While the air pressure in the unperturbed airstream is atmospheric, the ball's presence can change that pressure. Since this airstream must speed up as it flows around the sides of the ball, effectively passing through a narrow channel at the sides of the ball, its pressure there drops. Since this pressure drop is strongest on the side of the ball nearest the center of the airstream, the ball experiences a net pressure force toward the center of the airstream.
Demonstration 6.2.3: Throwing a Balloon - Pressure Drag
Description: You throw a balloon forward and it comes to a stop almost immediately.
Purpose: To show the slowing effects of pressure drag.
1 inflated balloon
Procedure: Throw the balloon forward and observe how quickly it slows to a stop (and begins descending slowly to the floor).
Explanation: The air flow around the balloon becomes turbulent at any significant speed. The pressure in front of the balloon rises above atmospheric pressure, the pressure at the sides of the balloon drops below atmospheric pressure, and the pressure behind the balloon begins to rise above atmospheric pressure. However the air flow separates from the back of the balloon shortly after rounding the sides of the balloon, leaving a large turbulent air wake behind the balloon. Since the air pressure behind the balloon doesn't rise very high, the high pressure in front of the balloon is unbalanced and the balloon experiences the slowing force of pressure drag.
Demonstration 6.2.2: The Decreased Pressure Drag of a Golf Ball
Description: Two balls of equal diameters and weights hang from long strings in the airstream leaving a fan. One ball is smooth and the other is dimpled—a golf ball. The smooth ball is deflected outward farther by the airstream than the smooth ball. (Note that this demonstration is hard to do convincingly because the balls tend to dither about in the uneven airstream.)
Purpose: To show that the pressure drag experienced by a dimpled golf ball is less than that experienced by a smooth ball of equal size and weight.
1 golf ball
1 very smooth ball with the same diameter and weight as a golf ball
1 tall supporting arm
2 strings, approximately 2 meters long. Woven thread that doesn't untwist is helpful.
1 powerful fan
Procedure: Use the strings to suspend the two balls from the support. Attaching the strings to the balls with tiny screws works best. Let the balls come to rest and mark their starting positions with a line on the table or floor. Now expose both balls to the strong airstream from the fan. They will both swing outward away from the onrushing air. But the smooth ball will swing outward farther, reflecting its greater pressure drag.
Explanation: The dimples on the golf ball delay the flow separation from its rear surface and reduce its pressure drag. As a result, it's pushed on less strongly by the wind from the fan and swings outward less far.
Demonstration 6.2.5: A Spinning Foam Ball or Beach Ball Curves in Flight
Description: A Styrofoam ball or beachball curves in flight when you throw it with spin.
Purpose: To show that a spinning ball experiences a lift force that causes it to curve in flight and to show that lift forces aren't always in the upward direction.
1 Styrofoam ball or beachball (or any low-mass but large ball)
1 paddle with a surface that will grip the ball well (optional)
Procedure: Throw the ball forward with as much spin as you can manage. If you have a paddle, you can whip the ball forward while allowing the ball to roll along the paddle's surface and acquire a huge spin. The ball should curve in flight toward the side that's heading back toward you as the ball spins. With some practice, you can make the ball curve in different directions by adjust its axis of rotation. Discuss the fact that the lift force that causes these curves isn't always upward. It can even be downward!
Explanation: As the ball spins, it experiences both the Magnus force and the wake deflection force, which both push it toward the side that's heading back to the pitcher. Because of the Styrofoam ball or beachball's low mass, it accelerates easily and curves substantially. A more massive ball, such as a baseball, won't curve as dramatically.
Demonstration 6.2.6: A Badminton Birdie Flies Bumper First
Description: A badminton birdie supported at its center of mass on a string flies bumper first as you swing it around in a circle.
Purpose: To show that a birdie has dynamic stability because its center of aerodynamic pressure (its center of drag) is located in its feathers. The feathers naturally drift behind its center of mass.
1 badminton birdie
Procedure: Attach the string to the birdie near its center of mass (near its bumper). When the string is properly positioned, the birdie will remain level when you support it with the string. Now swing the birdie around in a circle overhead. It will always fly bumper first.
Explanation: As the birdie flies through the air, the air slows the feathers more than the bumper and the feathers drift to the rear of the moving object. Whenever the feathers begin to drift forward, the birdie experiences an aerodynamic torque that turns its feathers back to the rear.
Demonstration 6.2.7: An Arrow Always Flies Point First
Description: An arrow, supported by a string at its center of mass, flies point first as it's swung around in a circle.
Purpose: To show that an arrow has dynamic stability because its center of aerodynamic pressure (its center of drag) is located in its feathers. These feathers naturally drift behind its center of mass.
1 target arrow (we found that enlarging the feathers with sheets of thin cardboard improves this demonstration)
1 target arrow without any feathers
Procedure: Attach the string to the arrow's center of mass with the help of the tape. Now swing the arrow around in a circle overhead. It will always fly point first (if it doesn't, increase the size of the feathers). Now create a similar arrow, but without any feathers at all. When you swing it around your head, it will fly with any end forward—it has no dynamic stability.
Explanation: As the arrow flies through the air, the air exerts a torque on its about its center of mass whenever the feathers begin to drift forward. This torque always returns the feathers to the rear of the flying object.
Demonstration 6.2.8: The Flight of a Frisbee
Description: When you throw a Frisbee across the room, the airflow around its upper and lower surfaces creates a lift force that supports it against its weight.
Purpose: To show that the airflow around an object can exert enough lift force on it to support its weight.
Procedure: Explain how the airflow around the Frisbee will develop during the throw. That flow will initially involve air moving at equal speeds above and below the Frisbee. However, the initial pattern of flow is unstable because it involves air flow up and around the trailing edge of the Frisbee. A few moments into the throw, this unstable airflow will blow away from the trailing edge of the Frisbee as a vortex, leaving a new pattern of airflow around the Frisbee. In this new pattern of airflow, the air moving over the top of the Frisbee will travel faster than the air moving under the bottom of the Frisbee. Since this faster moving air has more kinetic energy, it must have less pressure and pressure potential energy. The pressure above the Frisbee is thus less than the pressure below it and there is a net upward pressure force on the Frisbee. Having explained this airflow, throw the Frisbee across the room and observe how it hangs in the air. It descends remarkably slowly and may even rise at first because the magnitude of its upward lift force can equal or exceed the magnitude of its weight.
Explanation: The Frisbee is an airfoil that experiences upward lift when the speed of the air flowing over its top exceeds the speed of the air flowing under its bottom.
Follow-up: Perform a similar analysis for an Aerobee, an even more efficient flying disk. Because of its thin profile, the aerobee experiences less pressure drag and flies farther than the Frisbee.
Section 6.3 Airplanes
Demonstration 6.3.1: Blowing Air Across a Sheet of Paper
Description: You hold one edge of a sheet of paper so that it forms an arc in front of you. When you blow across the top of this arc, the paper rises.
Purpose: To demonstrate the upward lift force that appears when a stream of air speeds up as it flows over a convex surface.
1 sheet of paper
Procedure: Hold one edge of the sheet of paper so that it arcs slightly upward at first and then drapes downward on the end farthest from your fingers. Bring the edge that you're holding close to your lips and blow air across the bump in the sheet. The paper will experience an upward lift force and will rise.
Explanation: As the air leaves your lips, its pressure drops to atmospheric pressure. In passing through your lips, it has converted most of its pressure potential energy into kinetic energy. When it then encounters the bump in the sheet of paper, it speeds up still further—in effect, it's going through a narrow channel with only one curved wall: the bump in the sheet. As the air speeds up, its pressure drops below atmospheric pressure. With atmospheric pressure below the sheet and less than atmospheric pressure above it, the paper experiences an upward lift force and rises.
Demonstration 6.3.2: Throwing a Toy Glider
Description: A large toy plane glides through the air after being thrown forward.
Purpose: To show the upward lift force that's obtained by the wings of an airplane.
1 toy glider (e.g., a large Styrofoam glider or a small balsa wooden glider)
Procedure: Hold the airplane in your hand and discuss how the air will flow around its wings as you throw it forward. Point out that the initial airflow will involve air moving at equal speeds above and below the wings—creating no lift. However, this initial pattern of flow will have air from under the wings turning upward around the trailing edges of the wings. This pattern of airflow is unstable and will blow away from the trailing edges during the throw. It will form a vortex of swirling air behind the plane and will leave a new pattern of airflow around the wings. In this new pattern, the air flowing over the wings will travel faster than the air flowing under the wings and the wings will experience lift. Having discussed how the wings develops an upward lift force, throw the plane forward and watch the lift in action.
Explanation: The plane's weight is at least partially balanced by the upward lift force created by its wings. The wings develop a lower air pressure above them than below them and thus experience an upward pressure force—an upward lift force. The glider must lose height in order to maintain airspeed, so it inevitably lands (or crashes).
Follow-up: Hold the plane upside down and then throw it forward. It still flies because the wings still develop an upward lift force. However, now it's their angles of attack rather than their asymmetric curved shape that creates this lift. As before, the air flows faster above the inverted wings than below them and it experiences an upward pressure force.
Demonstration 6.3.3: Steering a Toy Glider
Description: By adjusting the surfaces of a toy glider, you can cause its flight to become curved.
Purpose: To show how a plane's surfaces can exert torques on the plane that curve its flight.
1 toy glider
1 piece of thin cardboard
Procedure: Crease the cardboard and bend it to form about a 135° angle. Use the tape to brace the angle by running a strip of tape through the air from one surface to the other. Now tape the angle to the top of the right wing tip, with the angled flap of cardboard leaning upward and toward the rear of the plane. When you throw the glider this time, the right wing tip will experience less upward lift than before and the plane will tip so that its right wing is lower than its left wing. As it flies, the plane will now curve toward the right.
Next, tape the small surface of the paper angle to the top of the elevator surface of the tail (either side of the horizontal tail winglet), again with its angled flap leaning upward and toward the rear. When you now throw the plane, the elevator will experience less upward lift than before and the plane will tip nose upward. It may even stall in flight!—if it does, discuss the resulting loss of lift and onset of severe drag.
Finally, tape the small surface of the paper angle to the right side of the tail rudder (the vertical tail winglet), with the angled flap leaning rightward and toward the rear. The rudder will now experience a leftward horizontal lift. When you throw the plane this time, it will rotate horizontally and will slip sideways through the air.
In each situation, you can discuss the effect of the paper angle on the lift forces, the resulting torque on the plane, the plane's change in orientation, the altered aerodynamic forces that accompany this changed orientation, and how these altered aerodynamic forces affect the plane's trajectory.
Explanation: The paper angle spoils the symmetry of the forces on the plane's surfaces and exposes the plane to aerodynamic torques. When the plane tips toward the right or left, its overall lift stops being vertical and it accelerates toward the right or left respectively. When the plane tips nose high or nose low, its wings' angles of attack change and so do the lift forces they experience. When the plane turns sideways in its flight through the air, it doesn't fly very well and can lose lift in one or both of its wings, initiating a tailspin.
Demonstration 6.3.4: A Wind-Up Airplane
Description: A toy balsa airplane with a rubber band motor pulls itself through the air and flies around the room.
Purpose: To show how a propeller can push a plane forward.
1 toy balsa airplane with a rubber band motor
Procedure: Observe the shape and motion of the plane's propeller and discuss how it's essentially a rotating wing. Point out that the lift force this rotating wing experiences is in the forward direction and is renamed "thrust" as a result. Now wind up the propeller, noting that you are doing work on it as you wind it, and release the airplane. The propeller will turn and pull the airplane through the air.
Explanation: The air flowing over the propeller blades speeds up as it flows over the forward surfaces of the blades. Because it converts pressure potential energy into kinetic energy, this air experiences a drop in pressure. With higher pressure behind it than in front of it, the propeller experiences a net forward pressure force—a thrust force. The propeller pulls the plane forward through the air.
Demonstration 6.3.5: A Fan on a Cart
Description: A powerful fan propels a small cart across the room.
Purpose: To show how a propeller can push a plane forward.
1 powerful fan
1 cart with very low friction wheels
Procedure: Put the fan on the cart and turn it on. With the fan aligned to push air along the path that the cart can roll, the air will accelerate in one direction and the cart will accelerate in the other.
Explanation: The air flowing over the fan blades speeds up as it flows over the forward surfaces of the blades and the pressure in front of the blades drops below atmospheric pressure. Air also slows down as it flows over the rearward surfaces of the blades and the pressure behind the blades rises above atmospheric pressure. This imbalance in pressures creates a forward pressure force on the fan that propels it and the cart forward. The air accelerates backward, from the higher pressure behind the blades to the atmospheric pressure behind the cart.
Demonstration 6.3.6: A Toy Helicopter
Description: You launch a toy helicopter and it flies around the room.
Purpose: To show that when a propeller spins about a vertical axis, its thrust can be directed upward and it can support its own weight.
1 toy helicopter or an equivalent spinning-blade toy
Procedure: First observe that the rotating blades of the helicopter are actually rotating wings that obtain lift in the upward direction as they turn through the air. Then launch the helicopter and watch it lift itself into the air. Note that the blades slow down as it rises, as must occur in order to conserve energy.
Explanation: As the blades of the helicopter turn through the air, the air speeds up to flow over them and slows down to flow under them. With the air pressure lower above the blades than beneath them, the blades experience an upward lift force that initial raises the toy helicopter into the air and then slows its descent.