Simple Hovercraft

Wednesday February 10, 2010


Hovercraft transport people and their stuff across ice, grass, swamp, water, and land. Also known as the Air Cushioned Vehicle (ACV), these machines use air to greatly reduce the sliding friction between the bottom of the vehicle (the skirt) and the ground. This is a great example of how lubrication works – most people think of oil as the only way to reduce sliding friction, but gases work well if done right.


In this case, the readily-available air is shoved downward by the pressure inside of balloon. This air flows down through the nozzle and out the bottom, under the CD, lifting it slightly as it goes and creating a thin layer for the CD to float on.


Although this particular hovercraft only has a ‘hovering’ option, I’m sure you can quickly figure out how to add a ‘thruster’ to make it zoom down the table! (Hint – you will need to add a second balloon!)


Here’s what you need:


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  • 7-9″ balloon
  • water bottle with a sport-top (see video for a visual – you can also use the top from liquid dish soap)
  • old CD
  • paper cup (or index card)
  • thumbtack
  • hot glue gun
  • razor with adult help


Download Student Worksheet here.


There’s air surrounding us everywhere, all at the same pressure of 14.7 pounds per square inch (psi). You feel the same force on your skin whether you’re on the ceiling or the floor, under the bed or in the shower. An interesting thing happens when you change a pocket of air pressure – things start to move.


This difference in pressure causes movement that creates winds, tornadoes, airplanes to fly, and the air to rush out of a full balloon. An important thing to remember is that higher pressure always pushes stuff around. While lower pressure does not “pull,” we think of higher pressure as a “push”.


The stretchy balloon has a higher pressure inside than the surrounding air, and the air is allowed to escape out the nozzle which is attached to the water bottle cap through tiny holes (so the whole balloon doesn’t empty out all at once and flip over your hovercraft!) The steady stream of air flows under the CD and creates a cushion of air, raising the whole hovercraft up slightly… which makes the hovercraft easy to slide across a flat table.


Want to make an advanced model Hovercraft using wires, motors, and leftovers from lunch? Then click here.


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Advanced students: Download your Hovercraft Lab here.


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Disappearing Beaker

Wednesday February 3, 2010

We’re going to bend light to make objects disappear. You’ll need two glass containers (one that fits inside the other), and the smaller one MUST be Pyrex. It’s okay if your Pyrex glass has markings on the side. Use cooking oil such as canola oil, olive oil, or others to see which makes yours truly disappear. You can also try mineral oil or Karo syrup, although these tend to be more sensitive to temperature and aren’t as evenly matched with the Pyrex as the first choices mentioned above.


Here’s what you need:


  • two glass containers, one of which MUST be Pyrex glass
  • vegetable oil (cheap canola brand is what we used in the video)
  • sink

Published value for light speed is 299,792,458 m/s = 186,282 miles/second = 670,616,629 mph
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When a beam of light hits a different substance (like glass), the speed of light changes. The color of the light (called the wavelength) can also change. In some cases, the change of wavelength turns into a change in the direction of the beam.


For example, if you stick a pencil is a glass of water and look through the side of the glass, you’ll notice that the pencil appears shifted. The speed of light is slower in the water (140,000 miles per second) than in the air (186,000 miles per second), called optical density, and the result is bent light beams and broken pencils.


You’ll notice that the pencil doesn’t always appear broken. Depending on where your eyeballs are, you can see an intact or broken pencil.


This is a very fine point about refraction: when light enters a new substance (like going from air to water) perpendicular to the surface (looking straight on), refractions do not occur.


However, if you look at the glass at an angle, then depending on your sight angle, you’ll see a different amount of shift in the pencil. Where do you need to look to see the greatest shift in the two halves of the pencil? (Hint: move the pencil back and forth slowly.)


Depending on if the light is going from a lighter to an optically denser material (or vice versa), it will bend different amounts. Glass is optically denser than water, which is denser than air.


Here’s a chart:


Vacuum 1.0000
Air 1.0003
Ice 1.3100
Water 1.3333
Pyrex 1.4740
Cooking Oil 1.4740
Diamond 2.4170


This means if you place a Pyrex container inside a beaker of vegetable oil, it will disappear. This also works for some mineral oils and Karo syrup. Note however that the optical densities of liquids vary with temperature and concentration, and manufacturers are not perfectly consistent when they whip up a batch of this stuff, so some adjustments are needed.


Not only can you change the shape of objects by bending light (broken or whole), but you can also change the size. Magnifying lenses, telescopes, and microscopes use this idea to make objects appear different sizes.


Questions to Ask


  • Does the temperature of the oil matter?
  • What other kinds of oil work? Blends of oils?
  • Does it work with mineral oil or Karo syrup?
  • Is there a viewing angle that makes the inside container visible?
  • Which type of lighting makes the container more invisible?
  • Can we see light waves?

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Bouncy Ball

Sunday January 10, 2010

This is one of those ‘chemistry magic show’ type of experiments to wow your friends and family. Here’s the scoop: you take a cup of clear liquid, add it to another cup of clear liquid, stir for ten seconds, and you’ll see a color change, a state change from liquid to solid, and you can pull a rubber-like bouncy ball right out of the cup.


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If you have trouble locating the ingredients, you can order them online here:


  • Sodium Silicate (from Unit 3) MSDS
  • Ethyl Alcohol (check your pharmacy) MSDS
  • Disposable cups (at least two – and don’t use your kitchen glassware, as you’ll never get it clean again)
  • Popsicle sticks (again, use something disposable to stir with)


Download Student Worksheet & Exercises


1. In one cup, measure four tablespoons of sodium silicate solution (it should be a liquid). Sodium silicate can be irritating to the skin for some people, so wear rubber gloves when doing this experiment!


2. Measure 1 tablespoon of ethyl alcohol into a second cup. Ethyl alcohol is extremely flammable—cap it and keep out of reach when not in use.


3. Pour the alcohol into the sodium silicate solution and stir with a Popsicle stick.


4. You’ll see a color change (clear to milky-white) and a state change (liquid to a solid clump.


5. Using gloves, gather up the polymer ball and firmly squeeze it in your hands.


6. Compress it into the shape you want—is it a sphere, or do you prefer a dodecahedron?


7. Bounce it!


8. Be patient when squeezing the compound together. If it breaks apart and crumbles, gather up the pieces and firmly press together.


Store your bouncy ball in a Ziploc bag!


What’s Going On?

Silicones are water repellent, so you’ll find that food dye doesn’t color your bouncy ball. You’ll find silicone in greases, oils, hydraulic fluids, and electrical insulators.


The sodium silicate is a long polymer chain of alternating silicon and oxygen atoms. When ethanol (ethyl alcohol) is added, it bridges and connects the polymer chains together by cross-linking them.


Think of a rope ladder—the wooden rungs are the cross-linking agents (the ethanol) and the two ropes are the polymer chains (sodium silicate).


Safety information for Sodium Silicate: MSDS.


Questions to Ask


1. Before the reaction, what was the sodium silicate like? Was it a solid, liquid, or gas? What color was it? Was it slippery, grainy, viscous, etc.?


2. What was the ethanol like before the reaction?


3. How is the product (the bouncy ball) different from the two chemicals in the beginning?


4. Was the bouncy ball  the only molecule that was formed?


5.  Was this reaction a physical or chemical change?


Did you know? Silly putty is actually a mixture of silicone and chalk!


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Flying Contraptions

Friday January 1, 2010

Mathematically speaking, this particular flying object shouldn’t be able to fly.  What do you think about that?


Why can this thing fly? It doesn’t even LOOK like a plane! When I teach at the university, this is the plane that mathematically isn’t supposed to be able to fly! There are endless variations to this project—you can change the number of loops and the size of loops, you can tape two of these together, or you can make a whole pyramid of them. Just be sure to have fun!



It’s actually a bit complicated to explain how this thing flies when “mathematically” it isn’t supposed to, but here goes: there are FOUR forces at work with your flying machine. Gravity is always pulling it down, but air pressure keeps it up (called lift). The way real airplane wings generate lift is by having a curved surface on the top which decreases the air pressure, and since higher pressure pushes, the wing generates lift by moving through the air. (If this idea doesn’t make sense, be sure to watch this video first!)


Ok, but what about a flat wing?


If you drop a regular sheet of paper, it flutters to the ground. If you wad it up first, you’ll find it falls much faster. The air under the falling paper needs to get out of the way as gravity pulls the paper, which is a lot easier when the paper is wadded into a ball.


For a flat wing (like on a paper airplane) to glide through the air, it needs to be balanced between gravity and the air resistance holding it up. In order for a glider to fly, the center of pressure needs to be behind the center of gravity (learn more about center of pressure and center of gravity in the third video below). By adding paper clips to your paper airplane, you move the center of gravity and center of pressure around to find the perfect balance.


When designing airplanes, engineers pay attention to details, such as the position of two important points: the center of gravity and the center of pressure (also called the center of lift). On an airplane, if the center of gravity and center of pressure points are reversed, the aircraft’s flight is unstable and it will somersault into chaos. The same is true for rockets and missiles!


Let’s find the center of gravity on your airplane. Grab your flying machine and sharpened pencil. You can find the ‘center of gravity’ by balancing your airplane on the tip of a pencil. Label this point “CG” for Center of Gravity.


Materials:


  • sheet of paper
  • hair dryer
  • pencil with a sharp tip

We’re going to make a paper airplane first, and then do a couple of wind tunnel tests on it.


For the project, all you need is a sheet of paper and five minutes… this is one my favorite fliers that we make with our students!



Find the Center of Pressure (CP) by doing the opposite: Using a blow-dryer set to low-heat so you don’t scorch your airplane, blast a jet of air up toward the ceiling. Put your airplane in the air jet and, using a pencil tip on the top side of your plane, find the point at which the airplane balances while in the airstream. Label this point “CP” for Center of Pressure. (Which one is closest to the nose?)



Besides paying attention to the CG and CP points, aeronautical engineers need to figure out the static and dynamic stability of an airplane, which is a complicated way of determining whether it will fly straight or oscillate out of control during flight. Think of a real airplane and pretend you’ve got one balanced on your finger. Where does it balance? Airplanes typically balance around the wings (the CG point). Ever wonder why the engines are at the front of small airplanes? The engine is the heaviest part of the plane, and engineers use this weight for balance, because the tail (elevator) is actually an upside-down wing that pushes the tail section down during flight.


When we use math to add up the forces (the pull of gravity would be the weight, for example), it works out that there isn’t enough lift generated by thrust to overcome the weight and drag. When I say, “mathematically speaking…” I mean that the numbers don’t work out quite right. When this happens in science for real scientists, it usually means that they don’t fully understand something yet. There are a number of ‘unsolved’ mysteries still in science.. maybe you’ll be able to help us figure them out?


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Pop Rockets

Friday January 1, 2010

Rockets shoot skyward with massive amounts of thrust, produced by chemical reaction or air pressure. Scientists create the thrust force by shoving a lot of gas (either air itself, or the gas left over from the combustion of a propellant) out small exit nozzles.


According to the universal laws of motion, for every action, there is equal and opposite reaction. If flames shoot out of the rocket downwards, the rocket itself will soar upwards. It’s the same thing if you blow up a balloon and let it go—the air inside the balloon goes to the left, and the balloon zips off to the right (at least, initially, until the balloon neck turns into a thrust-vectored nozzle, but don’t be concerned about that just now).


A rocket has a few parts different from an airplane. One of the main differences is the absence of wings. Rockets utilize fins, which help steer the rocket, while airplanes use wings to generate lift. Rocket fins are more like the rudder of an airplane than the wings.


Another difference is the how rockets get their speed. Airplanes generate thrust from a rotating blade, whereas rockets get their movement by squeezing down a high-energy gaseous flow and squeezing it out a tiny exit hole.


If you’ve ever used a garden hose, you already know how to make the water stream out faster by placing your thumb over the end of the hose. You’re decreasing the amount of area the water has to exit the hose, but there’s still the same amount of water flowing out, so the water compensates by increasing its velocity. This is the secret to converging rocket nozzles—squeeze the flow down and out a small exit hole to increase velocity.


There comes a point, however, when you can’t get any more speed out of the gas, no matter how much you squeeze it down. This is called “choking” the flow. When you get to this point, the gas is traveling at the speed of sound (around 700 mph, or Mach 1). Scientists found that if they gradually un-squeeze the flow in this choked state, the flow speed actually continues to increase. This is how we get rockets to move at supersonic speeds or above Mach 1.


f18The image shown here is a real picture of an aircraft as it breaks the sound barrier. This aircraft is passing the speed at which sounds travel. The white cloud you see in the photo is related to the shock waves that are forming around the craft as it moves into supersonic speeds. Because the aircraft is moving through air, which is a gas, the gas can compress and results in a shock wave.


You can think of a shock wave as big pressure front. In this photo, the pressure is condensing water vapor in the air, hence the cloud. There are lots of things on earth that break the sound barrier – bullets and bullwhips, for example. The loud crack from a whip is the tip zipping faster than the speed of sound.


The rockets we’re about to build get their thrust by generating enough pressure and releasing that pressure very quickly. You will generate pressure both by pumping and by chemical reaction, which generates gaseous products. Let’s get started!


For this experiment, you will need:


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  • film canister or other plastic container with a tight-fitting lid (like a mini-M&M container)
  • alka-seltzer or generic effervescent tablets
  • water
  • outside area for launching

The record for these rockets is 28′ high. What do you think about that? Note – you can use anything that uses a chemical reaction… what about baking soda and vinegar? Baking powder? Lemon juice?



Important question: Does more water, tablets, or air space give you a higher flight?


Variations: Add foam fins and a foam nose (try a hobby or craft shop), hot glued into place. Foam doesn’t mind getting wet, but paper does. Put the fins on at an angle and watch the seltzer rocket spin as it flies skyward. You can also tip the rocket on its side and add wheels for a rocket car, stack rockets, for a multi-staging project, or strap three rockets together with tape and launch them at the same time! You can also try different containers using corks instead of lids.



More Variations: What other chemicals do you have around that also produces a gas during the chemical reaction? Chalk and vinegar, baking soda, baking powder, hydrogen peroxide, isopropyl alcohol, lemon juice, orange juice, and so on.


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Advanced students: Download your Pop Rockets Lab here.


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Microwaving Soap

Friday October 2, 2009

soapWhen you warm up leftovers, have you ever wondered why the microwave heats the food and not the plate? (Well, some plates, anyway.) It has to do with the way microwaves work.


Microwaves generate high energy electromagnetic waves that when aimed at water molecules, makes these molecules get super-excited and start bouncing around a lot.


We see this happen when we heat water in a pot on the stove. When you add energy to the pot (by turning on the stove), the water molecules start vibrating and moving around faster and faster the more heat you add. Eventually, when the pot of water boils, the top layer of molecules are so excited they vibrate free and float up as steam.


When you add more energy to the water molecule, either by using your stove top or your nearest microwave,  you cause those water molecules to vibrate faster. We detect these faster vibrations by measuring an increase in the temperature of the water molecules (or in the food containing water). Which is why it’s dangerous to heat anything not containing water in your microwave, as there’s nowhere for that energy to go, since the electromagnetic radiation is tuned to excite water molecules.


To explain this to younger kids (who might confuse radio waves with sounds waves) you might try this:


There’s light everywhere, some of which you can see (like rainbows) and others that you can’t see (like the infrared beam coming from your TV remote, or the UV rays from the sun that give you a sunburn). The microwave shoots invisible light beams at your food that are tuned to heat up the water molecule.


The microwave radiation emitted by the microwave oven can also excite other polarized molecules in addition to the water molecule, which is why some plates also get hot. The soap in this experiment below will show you how a bar of Ivory soap contains air, and that air contains water vapor which will get heated by the microwave radiation and expand. Are you ready?


Here’s what you need:


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  • bar of Ivory soap
  • microwave (not a new or expensive one)
  • plate (optional)

The following experiment is a quick example of this principle using a naked bar of Ivory soap. The trick is to use Ivory, which contains an unusually high amount of air. Since air contains water moisture, Ivory also has water hidden inside the bar of soap. The microwave will excite the water molecules and your kids will never look at the soap the same way again.


 
Download Student Worksheet & Exercises


Toss a naked bar of Ivory soap onto a glass or ceramic plate and stick it into the microwave (don’t use a new or expensive microwave!) on HIGH for 2-3 minutes. Watch intently and remove when it reaches a “maximum”. Be careful when you touch it after taking it out of the microwave oven – it may still hold steam inside. You can still use the soap and the microwave after this experiment!


Note: Scientists refer to ‘light’ as the visible part of the electromagnetic spectrum, where radio and microwaves are lower energy and frequency than light (and the height of the wave can be the size of a football field). Gamma rays and x-rays are higher energy and frequency than light (these tend to pass through mirrors rather than bounce off them. More on that in Unit 9.)


Exercises


  1.  What is it in your food (and the soap) that is actually heated by the microwave?
  2.  How does a microwave heat things?
  3.  Touch the soap after it has been allowed to cool for a few minutes and record your observations.

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Plasma Grape

Friday October 2, 2009

CAUTION!! Be careful with this!! This experiment uses a knife AND a microwave, so you’re playing with things that slice and gets things hot. If you’re not careful you could cut yourself or burn yourself. Please use care!


We’re going to create the fourth state of matter in your microwave using food.  Note – this is NOT the kind of plasma doctors talk about that’s associated with blood.  These are two entirely different things that just happen to have the same name.  It’s like the word ‘trunk’, which could be either the storage compartment of a car or an elephant’s nose.  Make sense?


Plasma is what happens when you add enough energy (often in the form of raising the temperature) to a gas so that the electrons break free and start zinging around on their own.  Since electrons have a negative charge, having a bunch of free-riding electrons causes the gas to become electrically charged.  This gives some cool properties to the gas.  Anytime you have charged particles (like naked electrons) off on their own, they are referred to by scientists as ions.  Hopefully this makes the dry textbook definition make more sense now (“Plasma is an ionized gas.”)


So here’s what you need:


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  • microwave (not a new or expensive one)
  • a grape
  • a knife with adult help

 
Download Student Worksheet & Exercises


1. Carefully cut the grape almost in half. You want to leave a bit of skin connecting the two halves.


2. Open the grape like a book. In other words, so that the two halves are next to one another still attached by the skin.


3. Put the grape into the microwave with the outside part of the grape facing down and the inside part facing up.


4. Close the door and set the microwave for ten seconds. You may want to dim the lights in the room.


s2You should see a bluish or yellowish light coming from the middle section of the grape. This is plasma! Be careful not to overcook the grape. It will smoke and stink if you let it overcook. Also, make sure the grape has time to cool before taking it out of the microwave.


Other places you can find plasma include neon signs, fluorescent lights, plasma globes, and small traces of it are found in a flame.


Note: This experiment creates a momentary, high-amp short-circuit in the oven, a lot like shorting your stereo with low-resistance speakers. It’s not good to operate a microwave for long periods with little to nothing in them.  This is why we only do it for a few seconds. While this normally isn’t a problem in most microwaves, don’t do this experiment with an expensive microwave or one that’s had consistent problems, as this might push it over the edge.


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