Hands-on Science Carnival 2010 Activity Stations: General Physics

Centripetal Force and Gyroscopic Stability: Penny Racers and Screaming Balloons

[Shopping List: pennies; hex nuts; 9" balloons (preferably transparent)]

Penny Racers

1. Place a penny inside a latex balloon, blow it up and tie it.
2. Hold the balloon like a bowling ball and shake it gently in a circular motion
3. The penny should soon begin to spin around on its edge in circles inside the balloon
4. See how fast you can make the penny spin
5. Stop moving the balloon and see how long the penny continues to spin inside

Screaming Balloons

1. Place a hex nut inside a latex balloon, blow it up and tie it.
2. Hold the balloon like a bowling ball and shake it gently in a circular motion
3. The hex nut should soon also begin to spin around on its edge in circles inside the balloon, but this time it will make a loud screaming sound as it spins. The faster it goes, the louder and higher pitched the sound.

What's Happening: When you shake the balloon the penny or nut begins to bounce around and will soon find itself spinning along its edge simply due to random interactions, but once it does, because of the shape of the penny (or nut) the spinning motion produced is very stable and it will tend to keep spinning, especially if you continue to "pump" it at the right frequency (much like pushing someone on a swing). This is called gyroscopic stability, and its what keeps a gyroscope pointing the same direction, a top spinning, or a rolling bicycle wheel from falling over. The round walls of the balloon also forces the penny or nut to roll in a circle about the center of the balloon. This is called centripetal force, and it will keep the penny rolling until friction eventually slows it down, but since the frictional force is very small and the penny is rolling very fast, it can keep going for a very long time. If it's spinning horizontally, as it slows it will roll in smaller and smaller diameter circles and move down the side of the balloon.

While the penny rolls 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, it in turn vibrates the air molecules nearby which creates sound waves. The faster the balloon vibrates, the faster the frequency of the sound waves and the higher the pitch will be. As the nut moves faster it also has more momentum and makes the balloon move farther with each vibration, which pushes more air and makes the sound louder.

Variations: Try to make the penny spin in different directions. 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 2 pennies, or even 3 inside the same balloon. They might crash into each other at first, but you should find it quite easy to make all the pennies line up side-by-side and spin together. With a penny spinning fast, gently toss the balloon into the air and observe what happens as the center of mass of the system changes.