Pop! Corn!

What's your favorite movie snack? Many people like to eat popcorn while watching movies, how about you? Did you ever wonder what makes popcorn pop? And why can't you use any old kind of corn?

It's quite simple, actually. Popcorn corn contains a lot of water, and it has an unusually hard shell. When the popcorn kernels are heated, they turn to steam. The steam can't escape at first because of the hard shell. But at some point, the pressure inside the kernel gets so high that the shell bursts, and you get a nice fluffy bit of popcorn! If you looked at popped popcorn under a microscope, you'd see that it has lots and lots of tiny holes - this is a structure that scientists call a foam. What other kinds of foam can you think of?

Just how much water is in popcorn? You could look it up, but it's a lot more fun to do an experiment to find out!

What you'll need:
Bag of microwave popcorn
Microwave oven
Kitchen scale
Calculator

Weigh the popcorn bag before popping, and write down your measurement. If your kitchen scale has metric markings (grams), use those because that's how scientists make measurements. If your scale only has markings in ounces, they will work, too. Pop the popcorn according to the directions. When it is done, carefully take it out of the microwave and open the top slightly. Do you see the steam escaping? Be careful, it's very hot! Wait about one minute, then weigh the bag again. The weight should have gone down because water has been lost.

You can even calculate the % water in the original popcorn:
% Water = 100 x (Weight before popping - Weight after popping)/Weight before popping
What value do you get? You can also compare different bags of the same brand, or different brands, to see whether the % water is always the same.

'Round and 'Round

As mentioned in the previous post, airplanes usually use either jets or propellers to make them move. In this experiment, you'll try out some different propeller designs.

What you'll need:
Several 3" x 5" index cards
Scissors
Straight pin
Pencil with eraser

Cut the index cards into 1" x 5" strips (they don't have to be exact). Push the pin through the center of one of the strips and move the pin around to make the hole slightly larger. The strip should be able to spin around on the pin. Insert the pointy end of the pin into the pencil eraser. Hold the pencil straight up, place the propeller close to your mouth, and blow. What happens? Do the propeller blades turn? Try making some other propeller designs. Fold one or both corners of the strip. Does that make a difference? Does it matter whether both corners are up (or down), or whether one is up and one down? You can cut the other strips into different shapes, or fold and bend them in different ways, to see which design makes the best propeller. You just need to try to blow the same in all cases.

What you'll find is that flat blades don't turn very well. The air just rushes over the blade. But when you put some sort of curve or fold in, the air pushes against it, turning the propeller. The more area there is to push, the easier the blades turn. The air pressure is higher in front of the blade and lower behind. Airplane propellers work on the same principle. They are not flat, but curved and twisted.

Ready, Set, ... Jet!

If you looked at the history of flight links in the last entry, you may have noticed that many of the earliest "flying machines" were actually gliders, which flew without power. Like a modern glider, they needed help getting going. The importance of the Wright Brothers' invention was that their airplane had an engine, and could take off from the ground.

Modern airplanes usually use either propellers or jets to move. Let's talk about jet engines here. Jet engines work by blowing gas out through a small hole called a nozzle. Air is sucked into the front of the engine by a fan. The air is then mixed with fuel and ignited. The hot gas, which is at high pressure, comes rushing out the back, pushing the jet engine (and whatever it is attached to) forward.

You can make a simple jet engine with things you may have around the house.

What you'll need:

Balloon

Tape

Drinking straw

Long piece of string

Blow the balloon up and just hold the empty end closed. Tape the drinking straw to the balloon as shown in the picture. Let the air out of the balloon.

Now find a place to tie the string that lets you stretch it out to its full length. Try a doorknob in a fairly large room. Tie one end of the string, then thread the string through the drinking straw. Stretch the string out, keeping the balloon near your end. Blow the balloon up and hold the end closed again. Make sure that the string is straight, then let go of the balloon. As the air rushes out, the balloon will take off along the string. How far can you make it go?

Wing It!

In the last experiment, you got an idea about how airplane wings work. But airplane wings aren't flat like a piece of paper. In this activity, you'll make a more realistic wing and try out some of your own designs!

What You'll Need:

5" x 8" (12 1/2 cm x 20 cm) index cards, or pieces of paper about the same size

Tape

Scissors

Pencil

Fold the paper and tape it so that the side view looks like black lines in the picture at right. Slide the pencil into the open space so that the wing hangs from the pencil. Hold the pencil near your chin, letting the paper swing freely. Tilt your head down and blow onto both edges of the wing. Air should flow as shown in the picture.

You should notice that the wing lifts slightly. This happens because the air moving over the top of the wing (light blue arrow) has to follow a longer path than the darker blue arrow. This means that the air pressure is lower on the top. This lowers the air pressure (pushing force) on the top. Since the pressure on the bottom is now higher the wing moves up.

Try some different shapes and see whether you can make a better wing! See the pictures below for some ideas. You can also trying cutting holes in the wings. Can you make a wing that lifts up more than the one pictured above?