Level: Middle School
Time involvement: 5-20 minutes.
This DIY science toy demonstrates how a puff of air can be sent a further distance if it has the right shape. Constructed out of an oatmeal canister it can project a concentrated puff of air several feet. Although larger sizes of this concept have been constructed by Myth Busters with amazing range and power this small model has a range of only a few feet. It is operated by giving a sharp tap to the flexible diaphragm. A lightweight target such as a styrofoam cup can serve to test the range and accuracy.
Discussion: The difference between this toy and an air hose with a steady stream is the steady airstream encounters significant aerodynamic drag thus limiting its range. A stream of water from a hose will have a long range because the aerodynamic drag is slight compared to the weight (mass) of the water. Blowing smoke rings is an old smokers trick but it is a visible proof of the size and motion of the air puff. Because the ring is formed by exiting through the small hole the air does not shoot out in a stream but more like a donut with the air spinning around the axis of the circumference.
Some engineers might describe this as a circular vortex, not a bad term. In other words, the donut is rolling sort of like a roll of toilet paper being tossed and unrolling as it moves through the air. The inner edge is moving forward and the outer edge nearly standing still. Thus the aerodynamic drag is reduced compared to a steady stream. With much of the energy not being wasted the range (distance) can be amazing. A commercial version, the size of a small wastebasket can have a range of about 30 feet. The air donut/vortex is not a shock wave traveling at the speed of sound. At the 30 foot range the time of travel is about one to two seconds.
Notes: The commercial version is sold under the name “Air Zooka” and is available on-line.