ScienceMakers Toolkit Manual - The History Makers

Use a small piece of tape to make a handle that you can pinch with
your fi ngers on the center of the membrane. Fold the tape so that it
sticks to itself, but leave the ends free to stick to the membrane.
Now you can pull and push
on the membrane. Pull back
slowly and push the membrane quickly while aiming the hole towards
a target. Anything fl exible makes a good target: your friend’s hair,
posters, cats, candles (be careful with fi re and hot wax, ask an adult to
assist you).
What’s Going On?
When the membrane thrusts a burst of air out of the hole, it displaces the surrounding air out side. The surrounding
air swirls around in the shape of a twisting doughnut. The twisting doughnut will carry the original
burst of air as far as 50 feet away.
The physics of moving air is called fl uid dynamics. It may seem strange to call air a fl uid, but the motion of
gases and liquids is very similar. When a fl uid twists or swirls, we call it a vortex. Again, note the advantage
of using the fancy word vortex over the word swirl. Swirl is used when talking about the physics of ice cream.
Can you think of where you might observe a vortex at home? Look in the bathtub, sink, or toilet. Now, our cannon
generates very special vortices that differ from those found in the bathroom. They twist in the shape of a
doughnut and the mathematical name for this is called a toroid. Hence, our cannon generates toroidal vortices.
Vortices are amazing phenomena because the motion of the air is stable and doesn’t require any outside intervention
to keep it going. For instance, try blowing a burst of air with your mouth. This is an unstable burst of air
that disperses by the time it reaches an arms length. A toroidal vortex, on the other hand, will travel all the way
across the room. Believe it or not, the mathematics of vortices is exciting too! You will fi nd that out when you
get to college. When you get there, be sure to check the box labeled “PHYSICS” as your major.
Try This!
If you have a fog machine, then you can observe the shape of the toroidal vortex created by your air cannon.
You can also put some fl our inside a bucket and shake it up to simulate fog. If dry ice is available, that will
work too.
See if you can use your toroidal vortex generator as smell cannon. Not all science has to be done with the eyes.
Spray some perfume into the hole and see if you can send someone fragrant vortex. You can also try sending
blasts of hot air or cold air.
Experiment with holes of different sizes. Will a smaller hole affect the velocity of the vortex? How small and
how big can the hole be?
Experiment with holes of different shapes. Can you make a triangular fog vortex or a square?
What happens when two vortices crash into each other? Try it with a friend.
See this experiment online at: http://www.physicscentral.com/experiment/physicsathome/cannon.cfm
153
Physics