Physics on the Road Handbook
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Hopper Popper
Potential Energy: Make a hopper popper from a racquet ball. Locate a very faint seam line around the circumference of the racquet ball. With a utility knife, make an incision on the seam line then with scissors, cut the ball in half Turn one of the halves inside out and carefully trim all the way around the outer edge. Test your cuts often by dropping the 1/2 racquet ball (turned inside out) on the floor or table. When it springs back higher than your dropping point you have a working hopper popper. If the 1/2 racquet ball will not stay in the inside out configuration, you have trimmed away too much rubber. You must start over with another 1/2 racquet ball.
The Demonstration: Ask, "If I drop this racquet ball from shoulder height, will it bounce back up to my hand?" Ask for their predictions, drop the ball and ask why the ball didn't return to your outstretched hand. If a young audience, have them collectively clap their hands once and hold them together. Energy is lost from the noise. Have the audience rub their hands together. The heat is from friction. Talk about energy loss due to air friction and contact with the floor. Ask, "Is there any ball that will bounce back to my hand?" Have several balls on hand such as a basketball, a golf ball and a super ball and try each one. Show the hopper popper and demonstrate how energy is stored in the hopper popper by turning the racquet ball inside out. Explain that the stored energy allows the hopper popper to bounce higher than the release point.
Action and Reaction
For every action force there is an equal and opposite reaction force. Remove the nozzle from a CO2 fire extinguisher and replace it with a high pressure hose, 4-5 feet long. The CO2 gas escaping from the end of the hose provides the action force. The Demonstration A person holding the hose near the free end feels the reaction force as the hose tries to move in the opposite direction from the escaping CO2 gas. The audience can easily see the hands and arms of the person being pushed in the opposite direction of the escaping CO2 gas. Use volunteers who are big enough to maintain a grip on the hose. CAUTION FIRE EXTINGUISHERS ARE VERY DANGEROUS WHEN USED THIS WAY. IT IS RECOMMENDED THAT YOU TAKE THE FIRE EXTINGUISHER TO A COMPANY THAT MAKES HIGH PRESSURE HOSES AND ADAPT A HOSE TO YOUR FIRE EXTINGUISHER.
CO2 Rocket Cart
To make a fairly low-friction cart, bolt wheels from two skateboards to a 24 inch wide by 48 inch long piece of 3/4 inch plywood. Use a lab stool for a seat and hold the fire extinguisher, with the hose disconnected, firmly between your knees. Make sure the port is facing out before squeezing the handle. A five second burst, from a fully charged 15 pound fire extinguisher, will attain a speed of about 1 m/s across a tile floor. If you only have 30-40 feet of space for a run you should have someone available to catch you.
Inertia and Impulse
Use two 16 oz, returnable, soft drink bottles. Completely fill one bottle with water. You should try to be somewhat selective regarding the bottles that you use. Try to find bottles that sit squarely on the table. A small amount of wobble at the table translates to a large amount of wobble at the top of the system. A good match at the mouths of the bottles also improves the odds of the demonstration working. If you have a large audience, add food coloring to make the demonstration more visible. Fill the other bottle half-full with water. Use paper currency that has seen little circulation. You want a crisp bill with no wrinkles or folds and no dirt and oil from people handling the money so that the surface is slick. A miniature Mag-Light works well to apply a large force for a short duration.
The Demonstration...An object at rest will stay at rest unless a net force acts on it. Place one end of the bill over the mouth of the full bottle and balance the mouth of the full bottle over the mouth of the half-full bottle. The two bottles will stay balanced indefinitely unless someone or something disturbs them. The bill can be removed from between the bottles without disturbing them because of inertia and impulse. The full bottle is placed on top. It has more mass than the half-full bottle and is more resistant to a change in its motion than the half full bottle. Hold the outer edge of the bill in one hand. If you pull slowly on the bill you will pull the top bottle over because you exert a large force over a long period of time To remove the bill without disturbing the top bottle you need a large force over a very short period of time. Still holding the bill, leave a small amount of slack in the bill between the outer edge and the edge held down by the two bottles. Using the mini Mag-Light in your other hand, strike the bill with a swift motion, using lots of wrist action. You should aim for a point below the bill to insure that you strike all of the way through. You will flick the bill from between the two bottles and allow the water in the full upper bottle to flow downward and fill the lower bottle. This demonstration rarely fails for me and always gets an audible reaction and nearly always gets applause. The demonstration will become quite a mess, if you fail to do any one of several things. The system must be in balance with the mouth of the top bottle directly over the mouth of the bottom bottle. The bill must be clean and free of wrinkles or folds. You must keep the hand holding the bill absolutely still. Relax. Use a smooth wrist action with the Mag-Light and follow through by aiming at a point below the bill. On the way to becoming proficient, I broke several bottles. So, think of your own safety as well as the safety of others.
Inertia
With Wine Glass and Tablecloth: You can pull a tablecloth from beneath a glass of wine, or colored water, without spilling the liquid. The less friction involved with this demonstration, the more successful you will be. My table is from Maple and it is about 10 inches wide and has legs about 6 inches long. Sand the wood until it is very smooth, then varnish it. This could be done in a shop class, if you do not have the appropriate tools.The tablecloth should be of silk or an equivalent synthetic material and about the same width as your tablecloth. Do not be afraid to fill the glass, the more mass, the more inertia and the greater the chance for success.
The Demonstration: You can pull slowly on the tablecloth and let the audience see that the glass rides along with the tablecloth. Be sure that you don't spill any liquid because that will increase the coefficient of friction and make the demonstration less likely to succeed
Force Per Unit Area
(Bed-of-Nails): Construct from plywood at least 1/2 inch thick. My bed-of-nails is about 30 inches wide and 6 feet long. Hand-hold cutouts were then made about 1/3 of the way in from each end and on both sides of the board. The hand-holds make it much easier for one person to carry the bed-of-nails. From 1/4 inch plywood, cut a cover that will fit over the bed-of-nails. Screw eye screws near each corner and with bungee cords, you can easily and quickly secure the 1/4 inch cover over the nails. I laid out a 1 inch grid on my bed with about a three inch border around the perimeter. I then used a 1/8º drill bit to pre-drill the grid and drove 6 D common nails at every corner on the grid. This resulted in a bed containing several thousand nails. So that clothes will not be ruined by the nails, I provide a lab coat for the victim to wear. I ask for another volunteer to help me lower the victim onto the nails. The victim should make his body stiff and lock his knees and elbows. He is then lowered onto the bed. You need to provide something, a piece of wood for example, to protect the victim's head. Because body weight is distributed over hundreds of nails a person feels little to no discomfort while lying on the bed of nails. In fact, a 150 pound person averages only a few ounces of his weight on one nail. The head is a different matter though. Because the head has so much weight, over 20 pounds, and the shape is rounded, the weight is distributed over just a few nails; therefore, the head should be protected by a wood or even a heavy cardboard "pillow." To add suspense to the demonstration, I add a 50 pound block of wood to the chest of the victim. The block is about 2 feet x 2 feet, constructed of layers of 3/4 inch particle board and has a 12 inch x 12 inch x 1/2 inch aluminum plate bolted to one side. The added weight only adds an ounce or so to the weight that each nail is supporting. The demonstration is taken one step further by introducing a 5 pound sledge hammer. You can give the aluminum plate a healthy blow without hurting the victim. The block absorbs the energy of the blow. I always select a male to lie on the bed-of-nails and a female to wield the sledge hammer. I usually ask the guy if his girlfriend is in the audience and try to get her to volunteer to hit him. Yet another variation is to place a cement block on the wooden block and break the cement block with the sledge hammer. While most impressive, you must shield everyone involved from flying pieces of cement. Your victim should not be younger than an 8th grader and should have some body fat. If you get a kid that is just skin and bones, he will be uncomfortable.
Barrel Crunch An extravagant soda can crunch
The Demonstration: About two gallons of water is poured into a fifty five gallon steel barrel. Increased heat causes molecules to move faster. Heating the water causes the water molecules to move faster. As the molecules move faster, they change from a liquid to a gas. At some point the motion is so violent the molecules start beating against the sides of the barrel. They are also taking up more space in the barrel. Since water is heavier than air, the air is being forced out of the barrel. Eventually, the water vapor will occupy the entire space in the barrel. When a very large amount of steam is coming out of the container, remove or turn off the heating device and quickly screw on the caps before air starts to flow back into the container. The cap of any container can be made more airtight, if you wrap the threads with Teflon tape. Purchase Teflon tape from a hardware store or from the plumbing department of a department store.
The temperature inside the barrel decreases when the heat is removed and the water vapor starts returning to a liquid. A quantity of water requires less space in the container than the same quantity of water vapor. As the amount of water vapor decreases, the volume of the container steadily becomes more under vacuum and the force on the inside walls of the barrel steadily decreases. Because the inside pressure decreases there is less resistance to the force due to atmospheric pressure (which is about 15 pounds per square inch) on the outside of the barrel. When the inside force can no longer resist the outside force, the barrel will crunch inward. The large surface area of the barrel results in a large force on its outside wall. I often cool the barrel in under 5 minutes with liquid nitrogen. If you want added suspense, let the barrel cool naturally. Depending on the thickness of the wall of the barrel, the crunch will occur from five to twenty minutes after removing the heat. With some ingenuity, a barbecue burner could be used to heat the water in the barrel. Another heat source can be obtained from a propane gas dealer who sells high BTU burners for deep-frying vats for under $100. From slotted angle, I built a stand to hold the barrel and the burner. I am fortunate to have a source for inexpensive barrels, $5.00 each, that do not produce an odor after they have been rinsed. Obviously, this demonstration is costly, time consuming to prepare and bulky to transport but it is loud, it creates suspense, it is big and it is very dramatic. I use the barrel at less than half of my presentations because I usually do multiple presentations for one visit. The drawbacks mentioned above are multiplied by the number of presentations.
USE HEAT RESISTANT GLOVES TO PROTECT YOUR HANDS FROM THE STEAM. AT A PRICE OF ABOUT $10.00 A ªBARREL WRENCHº IS A VERY WISE INVESTMENT BECAUSE YOU CAN QUICKLY GET THE CAPS ON AND GET THEM ON TIGHT. THE PREVIOUS CONTENTS OF SOME BARRELS CAN PRODUCE SOME VERY BAD ODORS AS THE BARREL HEATS UP. IT IS IMPORTANT TO FIND OUT WHAT WAS IN THE BARREL BEFORE BUYING IT AND HAVING BOUGHT IT, RINSE IT OUT THOROUGHLY!
Suction Cups
Bring two suction cups together until their rims barely touch. The inside and outside forces cancel each other so the suction cups can be easily separated. Bring the suction cups together again tightly and squeeze out the air from between them. Now there is no inside force acting against the outside force and the suction cups are "stuck" together. In order to pull the suction cups apart (sliding is cheating) a force greater than 15 times the surface area of the suction cups is required.
Magdeburg Hemispheres
The same demonstration as the suction cups except that they are made of metal and require a vacuum pump to evacuate them. They usually have a larger surface area than the suction cups which means that they can't be pulled apart by young students.
Balloons In A Vacuum
In order to blow up a balloon your lungs must provide a force greater than the force due to atmospheric pressure The balloon gets bigger as you blow in more air because the rubber stretches and the outside force due to atmospheric pressure can not resist the greater force inside the balloon. Inflate several water bomb balloons and place them in the Bell Jar. Ask, "What will happen to the balloons as the air is removed from the Bell Jar?" Most will correctly predict that the balloons will pop; however, there is some suspense involved as they watch the balloons slowly stretch to the breaking point. A competitive aspect can be accomplished by using different colored balloons and having them cheer for their favorite color to hold out the longest before bursting. You can point out the condition of the balloons after bursting in a vacuum. They are like confetti because they have burst over most of their surface. An inflated balloon that is punctured with a pin is ripped apart by the escaping air from that one puncture point and so the remains of the balloon are large pieces. If they release a helium filled balloon into the atmosphere and if conditions allow the balloon to attain sufficient altitude, the balloon that they release will also explode just as the balloons in the Bell Jar explode.
Buzzer and Light In A Vacuum
A buzzer, miniature lamp, switch and battery are electrically connected and housed in a small electronics box. these items can be purchased from an electronics store such as Radio Shack. Fig. 1 is a typical wiring diagram.
The Demonstration: Show the audience that when the switch is closed the buzzer and the lamp are both operating. Place the device under the Bell Jar and observe that the lamp is still lit and the buzzer can still be heard although it is not as loud under the Bell Jar. Evacuate the Bell jar and note that the light is still on but the buzzer can no longer be heard. Ask: "Can anyone hear the buzzer?" Unless you have a pour vacuum, an individual has very acute hearing or is putting you on, there will be no raise of hands.allow a small amount of air into the Bell Jar and do another hearing test. Ask them to raise their hands as soon as they hear the buzzer. Continue, as time allows, until all hands are raised or you have let all the air back into the Bell Jar. If you have exploded the balloons in the Bell Jar, you will have noticed that the exploding balloons can be heard. There are two reasons that you can hear this sound. One, is that the glass and the aluminum platform provide a medium that will pass sound waves. Two, there is a brief period of time when the air that was inside the balloons is free inside the Bell Jar and this air can also allow sound to reach the glass and the aluminum platform.
Marshmallow Astronaut
Using 1/8" brass or aluminum rod and 1/2" x 1/2" brass or aluminum flat construct a person-like form as Fig 2. Push marshmallows onto the arms legs and head, Kraft Jet-Puffed are excellent. The astronaut legs can then be pushed into holes in a metal base to give stability.
The Demonstration: Ask, "What is a marshmallow made of?" If a young audience, many will respond, "marshmallow." The answer you are looking for is air and sugar. Ask your audience to predict the condition of the marshmallow astronaut after the air has been removed. Evacuate the Bell Jar and note that after all of the air has been removed the marshmallows shrink slightly from their maximum size. The sugar molecules are elastic and once the outward force of the escaping air has stopped the springy sugar molecules will contract slightly. With the Bell Jar still evacuated and the pump still operating, ask the audience to predict the condition of the marshmallows after letting the air back into the Bell Jar. The air does not reenter the Bell Jar fast enough to enter the marshmallows. The force of about 15 pounds is sufficient to shrivel the marshmallow.
Shaving Cream In a Vacuum
I use a 250 ml narrow-mouth reagent bottle, the least expensive shaving cream available and food coloring. To facilitate getting the shaving cream into the bottle I affix shrinkable tubing about three inches long to the spout of the shaving cream can. The shrink tubing allows you to get the shaving cream to the bottom of the bottle and allow air to escape from the bottle around the shrink tubing.
The Demonstration: Ask, "What is shaving cream made of?" The response should be water and soap. Fill the bottle with shaving cream to the bottom of the neck then pour about 1 ml of food coloring over the shaving cream. Place the bottle under the Bell Jar and start the vacuum pump. The shaving cream begins to spew out of the bottle in a continuous column. The food coloring causes the column to look like Stripe Toothpaste. As the column piles up on itself the whole thing reminds you of intestines which gets quite a response from the audience. The column of shaving cream increases in diameter according to the time it has been out of the reagent bottle. It takes about one minute to completely fill the Bell Jar with this disgusting looking mess. Ask, "What will happen when the air is let back into the Bell Jar?" The result is such a mess that if this demonstration is used, it should be the last demonstration using the vacuum system! I usually use red food coloring. Before the Christmas Holiday I also use green food coloring for a festive look.
Pencil Shoot
A short burst of CO2 gas from a fire extinguisher provides enough force to accelerate a dowel rod along a length of brass tubing to a velocity sufficient to allow the dowel rod to penetrate a piece of plywood that is 1/2 inch thick. I use a 4 foot length of brass tubing with a 3/8 inch O. D. x .063 inch wall, mounted to a 2 x 4. At one end of the 2 x 4, cut a notch at least 1 inch wide x 1 inch long x 1/2 inch deep to allow access for a compression nut. The brass tube is mounted to the 2 x 4 using six 5/8 inch copper, tubing straps. The 2 x 4 extends about 1/4 inch beyond the brass tube at both ends so that the ends of the tube will be protected. At the notched end of the 2 x 4, slide on a 3/8 inch compression nut with the thread end pointing out. Slide a 3/8 inch ferrule next to the compression nut. A 3/8 inch compression body is now butted against the end of the tube. Slide the ferrule and compression nut to the compression body and screw the nut onto the body while still butting the body against the tube end. Unscrew the compression nut and mark the position of the ferrule. Soft solder the compression nut to the tube at this position.
Warning: This is a potentially lethal demonstration!
All high pressure fittings must be properly connected!
Use only high pressure hose!
I use a 15 pound fire extinguisher to supply the needed force for the pencil shoot; however, a ten pound fire extinguisher also works well. You should take your fire extinguisher and the 3/8 inch compression body to a commercial business to have a high pressure hose adapted to the fire extinguisher and to the 3/8 inch compression body. You can use a quick disconnect coupler to connect the high pressure hose to the fire extinguisher.Because all standard size pencils are not equal, I have discontinued using pencils. I now use 1/4 inch dowel rod cut into lengths ranging from about 6 1/2 to 7 1/2 inches. I compensate for the excess clearance between the dowel rod and the inside wall of the brass tube by wrapping electrical tape around one end of the dowel rod. I try to achieve a close fit with the tape barely touching the wall of the brass tube. The length of tape that works for me is 2 9/16 inches. The remaining component of the demonstration is to clamp the plywood in front of the brass tube and a shield to protect you and your audience from flying wood splinters. I mounted a small, standard drill press vise, to a piece of aluminum sheet 3/16 inch thick. The aluminum sheet also serves as the base for an acrylic box which will collect all of the splinters. I use 1/4 inch acrylic sheet for the remaining sides of the box. A full fire extinguisher will actually drive the dowel rod into the back of my box cracking the plastic. To prevent this, I mounted a 3/16 inch aluminum plate to the back of the box. The front of the box needs an opening large enough to allow the plywood piece to be clamped in the vise and then removed with the dowel rod sticking through it. I cut the 1/2 inch plywood into 5 inch x 7 inch pieces. I have found that by coupling the 2 x 4 to the box I don't need to clamp either component to the table. I couple the two together by using two, 2 1/2 inch, 1/4 x 20 bolts as studs through the box base plate and lowering the 2 x 4 onto the studs through two holes drilled in the end of the 2 x 4. I believe that it is important to have the vise mounted so that the dowel rod has begun to enter the plywood before it has completely cleared the tube. It is important that the dowel rod hit the plywood straight on. Having part of the dowel rod confined in the tube on impact helps with the alignment.
The Demonstration: I announce that I am going to drive the dowel rod through the piece of plywood. First, I try to do this by hand, with a stabbing motion. I point out that I can't grip the dowel rod tight enough to keep it from sliding through my hand and that I can't develop enough arm speed to actually drive the dowel rod through the plywood. With the help of the vise to hold the plywood and the fire extinguisher to provide the necessary force to accelerate the dowel rod, the dowel rod really can be driven through the plywood. An estimate of the speed of the dowel rod upon impact with the plywood is 200-300 miles per hour. I invite the audience to join me in a countdown to the discharge of the fire extinguisher. There is a technique to discharging the fire extinguisher. I use a sharp chop with the heel of my hand to provide a short burst of CO2. Removing the plywood and dowel rod usually takes less than ten seconds. I lift the 2 x 4 off of the studs and set it aside, unscrew the vise and retrieve the plywood. On a rare occasion, the dowel rod will stop before it completely clears the tube. When this happens, I unclamp the plywood and lift the 2 x 4 and the plywood at the same time. I then pull the 2 x 4 and tube back from the dowel rod and then remove the plywood from the shield. I then call attention to the condition of the dowel rod and the plywood. There is a clean entry hole but where the dowel rod exits the plywood it has driven the fibers of the plywood ahead of it causing splinters and even large pieces of the plywood angling out or even missing. If the dowel rod stops before hitting the plate at the back of the shield, the penetrating end of the dowel rod will be undamaged. I explain to the audience that the dowel rod strikes the plywood squarely, with a very large force.over a short period of time. Since the impact is felt by the dowel rod along its entire length, the wood is able to withstand the stresses from impact. I then talk about the wind speed in the funnel cloud of a tornado. These wind speeds have been measured as high as 300 miles per hour. This is fast enough to drive straw and sticks into trees or buildings. I then let them pass the plywood and dowel rod around while I continue with the program.
Needle In A Balloon
A balloon can be impaled completely on a long needle without bursting. From 1/8 inch stainless steel rod, cut a length about 15 inches long. On a grinder, grind one end of the rod to a very sharp point. Spray the "needle" with WD 40 before the audience arrives. I buy large round balloons, 130 x 75 mm, catalog #18040k, from Central Scientific Co., 3300 CENCO Parkway, Franklin Park, IL 60131-1364.
The Demonstration: Inflate a balloon to 6-7 inches. Insert the oil-coated needle into the near the mouth where the rubber is thickest. Push the needle through to the opposite end where the rubber is thickest and out of the balloon. the balloon will not "pop" because the rubber is thick enough to keep from ripping. You now have a balloon shish-ke-bab. A magician can accomplish this "trick" by first putting a piece of clear tape on the side of the balloon and pushing the needle through the tape. The tape reinforces the thin rubber side so that the balloon can't rip apart. This leads directly to the surface tension of the WD 40.
Surface Tension, Oil
While you are still displaying the balloon shish-ke bab tell the audience that you can remove the needle and the balloon will remain inflated for a short period of time. Remove the needle and the balloon should remain inflated. The oil that was on the needle covers the opening made by the needle. Surface tension of the oil will create a weak seal so that you can gently press on the sides of the balloon. Bring the point of the needle to the side of the balloon and pop the balloon. This is a good demonstration for bringing a bit of mystery and magic into your dialogue. You can lead the audience on and then tell them what is really going on.
Surface Tension, Water
Use a clear, one gallon jug of the type that apple cider is bottled in. Or, buy a jug from a chemical outlet that comes with a clear plastic coating. Measure the inside diameter of the mouth of the jug. On a piece of 1/8 inch acrylic sheet, scribe the circumference of the jug mouth I.D and drill a 3/8 inch hole at the center. Using a jig saw, cut a circle around the hole that you have drilled so that you have a disk of a slightly larger diameter than the inside diameter of the mouth of the jug. You now have an acrylic disk with a hole in the center. Using a grinding tool, grind the disk around the circumference until it can be press fitted into the mouth. The fit should be so snug that it can only be removed by prying it out with a key or small screwdriver.
The Demonstration: Before the audience arrives, fill the jug to the top with water, press the disk into the mouth and then screw on the cap. Have an empty bucket nearby. To begin the demonstration, let the audience see the capped, jug of water. Ask, "If I remove the cap and turn the jug upside down over the bucket, will the water come out fast or slow?" The majority of students have had personal experience that the water will come out slowly. Remove the cap and invert the jug over the bucket. Save for an occasional drip, no water comes out of the jug. To confuse them about the possibility of a plug, push a pencil through the hole in the acrylic. The pencil floating in the jug will convince them that there is no obstruction. Admit that you have tricked them by removing the plug but show the plug with the hole. Explain that surface tension, the ability of liquid molecules to cling together, is great enough at the small hole to keep gravity from pulling the water out and air from entering the jug through the small hole.
Vortex in a Liquid
After you have talked about surface tension you still have a jug full of water. Ask again," If I turn the jug upside down, will the water come out fast or slow?" Many will change their answer to fast. Remind them of their initial reasoning. Tell them that there is a way to get the water out of the jug that is fairly fast. Invert the jug and hold it still, over the bucket. After a few seconds, swirl the jug around until you have started the vortex, then stop swirling and let them observe the tornado.
Vortex Generator
The vortex ring (smoke ring) produces a low pressure inside it. The stability of vortices are explained by Bernoulli's Principle.
The Demonstration: Ask for a volunteer. Have the volunteer hold a candle while you light it. I use a V-Jaw buret clamp to hold the candle. The volunteer holds the clamp stem and keeps hot wax off their fingers. I tell the audience that I will attempt to blow out the candle from 20 feet away. I fail of course. Make sure there is plenty of room between the candle and the front row students then ask the class to try to blow out the candle at the count of three. The person holding the candle does not participate. Even though their combined effort moves a large volume of air, they can't blow the candle out because the air spreads out too much when it comes out of their mouths. The vortex generator produces a large volume of air but because of the round hole the burst of air is contained in a unique shape. This unique shape is called a vortex. It is able to stay in this shape for a fairly long distance. It has plenty of force to blow out the candle from across the room. Try to blow out the candle without the liquid nitrogen. If you can't get lined up, pour the LN2 into the pan so you can see where the rings are going. Once the candle is out, you can direct the vortices at the students. The cart that I use to haul my equipment to and from the van enables me to make my vortex generator mobile. I push the cart the full length of the audience, blasting them with vortices as I go. I try to include at least one teacher, if I can. All airplane wings produce vortices. The larger the wing, the larger the vortices. Pilots of small aircraft must maintain a safe distance from large airplanes because the vortices produced by a 747, for example, can cause a single engine airplane to crash. Most large airports have runways that run parallel but since these vortices can create dangerous turbulence for even large aircraft, the runways are offset.
Gravity Demonstration
(For younger audiences) Hold a regulation basketball and a miniature at the same height. Ask which will hit the floor first if released at the same time.
Energy Transfer
Use regulation basketball, miniature basketball, softball, Styrofoam ball. Drop each ball and note how high they bounce. With the miniature basketball resting on top of the regulation size basketball. Ask, "Will the miniature basketball bounce higher, the same height or lower if it bounces off of the regulation basketball?" Also ask, "How high will the regulation basketball bounce?" Energy is transferred to the miniature ball causing it to bounce much higher while energy is lost to the regulation basketball which doesn't appear to bounce at all. Do the same routine with the lighter balls.on top of the regulation ball. Lighter balls bounce higher. Also note that the regulation ball starts bouncing higher as lighter balls are used.
Electrostatics
When electrostatics demonstrations work they produce more squeals and laughter than any other demonstration that I do. When electrostatics demonstrations don't work they are as exciting as watching grass grow. In the midwest, we can confidently do certain electrostatics demonstrations about nine months out of the year. When the humidity gets too high the electrostatics stay in the van. If you do back to back shows, the humidity can increase by the hour so what works the first hour can bomb the second hour. My electrostatics kit consists of a Van de Graaff generator, a small shorting sphere with a wire lead to ground, 10 aluminum pans of the size that Pot Pies come in, a 12º X 12º piece of mirror tile and a large Styrofoam container that is used for shipping bottles of acid. This Styrofoam container may now be obsolete but you can use a product called DOW BOARD. Purchase DOW BOARD from a lumber company in a 2' X 8' sheet. Cut the sheet into 2' squares until you have a stack about one foot high and epoxy them together. Cut a 2' square piece of 1/4º plywood and epoxy it to one end of the stack of DOW BOARD. You now have an insulated platform for people to stand on. The plywood end needs to be up so that shoes don't gouge the DOW BOARD. I also use a dedicated outlet strip with a metal screw extended from the case so that it is easy to connect my ground wire.
The Demonstration: I explain that friction between our feet and the floor causes excess electrons to build up on our bodies. When we come near an object like a door knob or another person those excess electrons try to jump from us to that object. If they are able to jump from us in great quantity, we feel a shock. I then point out the Van de Graaff and explain that electrons are picked off of the rubber belt and collect on the sphere. When I bring the shorting sphere with its connected ground wire close to the Van de Graaff, electrons swarm to the area of the Van de Graaff where the shorting sphere is approaching and jump to the shorting sphere when it has come close enough. When I am setting up the show I use the shorting sphere to judge the effectiveness of electrostatics for the day. If I can't draw at least a two inch arc, I set electrostatics aside for the day. After using the shorting sphere during the presentation, I bring my hand close to the Van de Graaff knuckles first. Since there are fewer nerve endings on your knuckles than you do on your finger tips, you don't feel the shock as keenly. You get lots of gasps from the audience when they see at least a two inch spark striking your skin and you are talking in your normal voice. You do have to be ready for.an occasional arc from your toe to the floor. I explain that electrostatic charges are like magnets. Just as like poles of a magnet repel so do like electrostatic charges repel. I stack the aluminum pans on the Van de Graaff and as the charge builds up the top pan flies away then the next and the next etc. The next step is volunteer time. I usually scan the audience as they are entering for boys and girls whose hair is most likely to stand up. I always ask for volunteers and hope my first choices raise their hands. Most of the time I pick out three girls and one boy. My victims stand on the insulated platform with one hand on the Van de Graaff. When their hair stands out,(I often ask them to shake their head until the strands of hair separate) and the rest of the audience is screaming their heads off I let my victim look in the mirror. I save the boy until last and ask him if he is brave enough to get shocked. As a rule he will say yes. When his hair is standing up I explain that I am about to discharge him. I ask the audience to keep watching his hair. When I touch the Van de Graaff with the shorting sphere he is discharged and his hair falls flat.I hold the shorting sphere to the Van de Graaff as I explain what happened. I also ask him if he can handle another shock. If he can, I remove the shorting sphere. Since his hair has already had some charge it stands up immediately. If my victim is initially reluctant to be shocked, I remind him that he saw me hold my hand up to the Van de Graaff. Most of the time they will consent to receiving a shock. If he is still negative, I excuse him. If time allows I ask for any volunteer who will consent to being shocked. I like to get a teacher to volunteer but many times they have applied hair spray to their hair. Hair spray makes the hair too stiff to work. The image of the perfect volunteer is a boy or girl with shoulder length blond hair that is very fine. You would expect that it would be futile to use a minority person with braided hair. I have been in schools where I had no other choice. I was amazed at how well those braids stood up. Now I try to include a minority when I ask for volunteers.
Laser-Light Show
The major components of this demonstration are the following: a mini MagLight, a 1/4 mwatt He/Ne laser, a boom-box with a CD player, an external speaker, (which is optional), a lab-stand to hold the laser, a piece of plastic mirror tile about 1 square centimeter in area and an outlet strip. I also borrow two dirty chalkboard erasers from the school that I am visiting.I plug the LASER and the boom box into the outlet strip and turn their power switches on.Power to my components is controlled by the switch on the outlet strip which is very convenient. My boom-box has external input/output jacks so I am able to use an inexpensive speaker purchased from Radio Shack. I like the external speaker because it is convenient for me to duct tape a thin neoprene rubber sheet to the speaker and epoxy a mirror to the neoprene. In the absence of the external speaker it may be necessary to epoxy the mirror directly to a speaker cone.of the boom box. A LASER pointer will work fine. They now sell for as little as $50.00. A battery operated LASER pointer would probably eliminate the outlet strip.
It is necessary for the room to be fairly dark for the patterns to be seen easily. Most presentations are held in either a gym or a cafeteria. Cafeterias usually have lots of windows and most schools have blinds or window shades. Gymnasiums now have gas vapor lighting which takes several minutes to come up once they have been turned off. I do the LASER Light Show last and I have the person operating the light switch turn them back on about half way through the song. Many schools have key operated light switches so you must make sure someone with a key is present.
I usually begin this demonstration by saying that light enters our eyes either by reflection or by direct transmission. I mention that objects are seen because light has been reflected from that object into the eyes. I then turn on the mini MagLight and say that the light from the bulb is being transmitted directly into their eyes. The eye holds on to an image for about 1/30 th of a second. While I am still directing the beam of the MagLight toward the audience I start moving it in a circular motion. (I have asked for the lights to be turned out at this time.) As I increase the speed the beam starts to develop a tail. If I can move the light fast enough to complete the circle in 1/30 th of a second, the image of the bulb looks like a circular pattern of light because the eyes have held on to the image of the bulb at every point on its path.. Having said this, I turn on the outlet strip. I explain that the red spot on the reflective surface, (usually a screen or a wall) is light from a LASER beam that is being reflected from the reflective surface into their eyes. That intense beam is traveling from the demonstration table to the reflective surface; however, there are not enough impurities in the air to reflect the beam. At this time I clap the erasers together along the path of the beam and a nice red beam of light appears. On rare occasions erasers are not available so I blow into a beaker containing LN2 and the beam is briefly reflected by the fog. Chalk dust is far more effective and nearly always gets an audible response. Following the chalk dust, I manually jiggle the speaker so that the red spot is seen by the audience to be a vertical line. I explain the setup at this time. The LASER is aimed at a mirror which reflects the beam to the reflective surface and then to their eyes. The mirror is attached to a speaker. The speaker cone will vibrate in many different directions according to the frequencies that activate it. As the speaker moves so does the mirror move which causes the LASER beam to be transmitted to different places on the reflective surface. High frequencies do not cause the speaker to vibrate enough for visible displacement of the red spot. As the frequencies become lower, there is more speaker vibration and more displacement of the red spot. A pure tone will cause the speaker to vibrate in an elliptical or even circular pattern translating to an elliptical or circular pattern of the red spot. Several instruments together, including the drums, create very complicated but symmetrical speaker vibrations translating to very complicated but symmetrical red patterns of LASER light. I prefer to use musical selections having a synthesizer as the dominant instrument. I use track one, (Toccata) from the CD Fresh Aire III by Mannheim Steamroller. This selection features synthesizer solos with a range of frequencies that cause no visible movement of the red spot to red circles. I also get, in a variety of positions; ellipses, figure 8's and saddles. A ªbeatº is present through a significant part of the track so that many different complicated patterns can be seen. I doubt if this CD would be on the top 10 list of any youth but it seems to be well accepted. I have had students like it well enough to ask about it. While the music is playing, I usually remind the students that the patterns that they are seeing is simply a red spot going faster than their eyes can follow. However, if a camera with a fast speed lens were to be used to take a picture of any pattern, the lens could be set to open and close fast enough to capture the red spot at some point in the pattern. The lens of a video camera is always ªopenº so the playback of a taped LASER Light Show would reveal the same images that the audience saw live.
If the kids have been rowdy throughout the show, you can be sure that rowdiness will increase when the lights go out. If this is the case, I will try to do most of the explanation with the lights on and the music not yet playing and hope they remember some of what I have said during the light show. Once the music starts, the kids usually settle down and become somewhat mesmerized by what they are seeing.
Liquid Nitrogen Demonstrations
The first difficulty in doing liquid nitrogen demonstrations could be getting the liquid nitrogen. If you do not have access through a university, you may need to go to a veterinarian, a hospital or possibly a factory. Once you have the liquid nitrogen you may have a hard time keeping it because it boils away so quickly. As with electrostatics, there are many demonstrations that can be done with liquid nitrogen.
You can be sure that someone in the audience will have difficulty conceptualizing something that boils at -195 degrees Centigrade. I have developed a routine that seems to greatly reduce this conceptual problem. I begin the demonstration by establishing which gases are in each breath of air that we inhale. If time allows, I let them try to name the gases that we breathe. I get some interesting answers. I hold up a clear wine glass filled with water. I use water to illustrate the three states, solids liquids and gasses. Since the melting temperature and boiling temperature of H2O are events common to the majority of the planet's population, ice, water and steam are experienced by most of us at a very young age. I explain that all of the elements will behave like water but at temperatures that are not common to most of us. For instance, there are steel mills in towns and cities that I visit so it is not difficult for most of my audience to identify with molten iron. Gaseous iron atoms then are conceptually within reach for many audiences when I mention the temperatures present on the sun.
After this bit of discussion on water and iron I return to nitrogen. Most of my presentations are given in an environment of about 70F degrees. I use that temperature as the temperature of the air they are breathing. The air they are breathing can be turned into liquids and solids if it is made to be cold enough. In fact, someone did make the nitrogen in my dewar cold enough to turn it into a liquid. It had to be cooled to more than 300 degrees below zero to go below it's boiling point where it changes into a liquid. The temperature difference between the nitrogen they are breathing at a room temperature of 70 degrees and the liquid nitrogen in the dewar at more than -300 degrees is 370 degrees. Most of us know what would happen if we pour water into a container that has been heated to 370 degrees. We can expect the same thing to happen when liquid nitrogen is poured into a beaker at room temperature. The audience, often including adults, see the boiling action in the beaker and relate to it as being hot as they understand hot. It just doesn't make sense for something that cold to boil. At this point in the presentation I warn the audience of the danger of liquid nitrogen. I warn them that in the brief time that it takes for the nerves in the finger to send and receive a message, ªFinger to brain. This is to cold for further contact!º ªBrain to finger.Disengage immediately!º the finger could have received permanent damage from frostbite. The objects that I immerse in liquid nitrogen are a balloon, Tygon tubing and a racquet ball. Be very careful about shattering these materials because the flying pieces of frozen rubber and plastic can cause injury. I don't use fruit or flowers except for special occasions. Broken bananas are very messy when they thaw. Flowers also leave a mess and they are expensive. I sometimes use an LN2 cannon. This is a brass tube about 12ºL x 2ºOD x 1/8º Wall that I hard soldered to a brass base 4º x 4º x 3/16º. I pour a small amount of LN2 into the cannon and jam a cork into the tube with the heel of my hand. The nitrogen boils and the expansion is great enough to send the cork a distance of about 80 feet and a height of about 15 feet at the optimum firing angle. In the 1970's someone in our university stores bought thousands of corks for less than a dime apiece. Today those corks are about $2.00 apiece so I am rationing my supply of corks. I finish the LN2 demonstrations with a bang. Using the same LN2 cannon, I fill the tube with LN2 and slip the mouth of a large balloon over the brass tube. The nitrogen gas boils into the balloon quickly at first, then more slowly as the brass cools and the boiling slows. The audience knows that the report will be quite loud but as they watch the balloon expand to an unexpected size they don't know when the bang will occur. It often takes more than one minute so the suspense mounts quite nicely and there are always screams of fright as well as delight.
If you have any questions regarding Physics on the Road please contact Roger Boyce boyce@physics.purdue.edu
