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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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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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.

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

  • Sodium Silicate (from Unit 3)
  • Ethyl Alcohol (check your pharmacy)
  • 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!


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!

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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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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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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