Put more than air inside a balloon and the science can get very interesting- perhaps even a little spooky!
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What you'll need
Until we make our own you can watch Steve Spangler's video to help you get started.
When you begin to shake the balloon the penny bounces around inside as you would probably expect to happen, but what might surprise you is how quickly it will begin to roll once it lands on its edge. That's because a round shape like a coin rolls or turns very easily, and once it is rolling (or spinning about an axis like a wheel) it wants to continue spinning in the same direction. In physics we say a shape like this has a high moment of inertia, and this tendency for an object to continue spinning in the same direction (that's just what inertia means for a spinning body) is sometimes called gyroscopic stability. This is what keeps a spinning toy top upright, a gyroscope pointing in the same direction or a spinning bicycle wheel- or in this case your penny) from falling over (check out the videos below).
But it gets better. Because the penny is now rolling inside the round wall of the balloon forces the penny to move in a big circular path, kind of like a planet orbiting the sun. We call this centripetal force, and the circular or rotational motion caused by this force is just another form of spinning or rotational inertia, this time about an axis through the center of the balloon, which is why it will tend to continue moving in the roughly the same circular orbit over and over again. Once it's orbiting inside the balloon if you move your hand back and forth just right and at a rate (or frequency) that exactly matches the penny's orbital frequency you can add more and more energy to its motion, which makes it go faster and faster! When you stop moving your hand the penny keeps rolling (that's its inertia again) until friction eventually slows it down, but since this friction force is relatively small and the penny is moving pretty fast it should keep rolling for a fairly long time.
When you put a hex nut inside the balloon and start shaking the same thing happens. You might think that because the hex nut is not smooth and round like the penny it would not roll, but it turns out that this shape still has a fairly high moment of inertia and therefore does roll easily after all. While the penny rolled around inside the balloon almost silently, the nut makes a screaming sound as it rolls. This is because the corners of the nut bounce as they hit the balloon and cause it to vibrate, or move back and forth very quickly. As the balloon vibrates, that in turn vibrates the air molecules nearby which creates sound waves. The faster the balloon vibrates, the higher the frequency of the sound waves and the higher the pitch will be. As the nut moves faster it also has more energy and makes the balloon move farther with each vibration, which pushes more air and makes the sound louder.
variations and related activities
Try to make the penny orbit in different directions (i.e. vertically, horizontally, etc.). You can also try different shaking motions to start the penny spinning. Once it is spinning fast, turn your whole body and the penny should continue to roll more or less in its original direction, just like a gyroscope.
Try putting 2 pennies, or even 3, inside the same balloon. They might crash into each other at first, but after a few seconds you should see see all the pennies line up perfectly side-by-side and orbit together as one. We're not entirely sure why this works so well, so if you have a theory please let us know in the comment section below.
Once you have your penny rolling really fast, gently toss the balloon into the air and observe what happens as the center of mass of the system changes.
Experiment with different sizes of nuts. Does this change the sound?
Just for fun you can draw a spooky face on your balloon with a marker to make a screaming ghost balloon.
If you want to learn more about the strange things that happen when objects are spinning or rotating check out some of the links below.
references and links to more information
Steve Spangler demonstrates the Screaming Balloon and more:
Spinning bicycle wheel demonstration:
How does a bicycle stay up (disappointing hint- it's not really you doing it!):
Have a question or comment? Let us know at the bottom of the page.