posted on 18 Dec 2013 by guy
last changed 3 Jun 2015
Your vote (click to rate)
ages: 9 to 99 yrs
budget: $6.00 to $20.00
prep time: 5 to 10 min
class time: 10 to 30 min
Bernoulli's principle states that a region of fast moving fluid exerts less pressure on its surroundings than a region of slow moving fluid. This principle explains a clever trick for blowing up a long thin balloon in a single breath. Stage a friendly competition in class and use the exercise to focus a lesson on pressure, fluid flow, and Bernoulli's principle. Buy a long plastic balloon, or make your own out of diaper genie plastic following our instructions.
For more ideas on Bernoulli's principle, check out our summary curriculum on Bernoulli Effects.
Steve Spangler demonstrating how to blow up a windbag most efficiently.
The Steve Spangler video above demonstrates the quickest method for blowing up a long thin balloon. With one breath, Steve clearly puts much more air into the balloon than his lungs can possibly hold. The explanation behind the trick involves an application of Bernoulli's principle.
Bernoulli's principle, published by Daniel Bernoulli in 1738, states that a region of fast moving fluid exerts less pressure on its surroundings than a region of slow moving fluid. In the video, air flowing into the mouth of the balloon creates a low pressure region, which draws extra room air into the air stream, thereby helping to fill the balloon quickly.
Figure 1: Air flowing through the throat encounters a restriction at the open of the mouth and speeds up as it passes through the lips. The situation parallels the case of air flow through a Venturi tube, where the air that passes through the narrow portion of the tube exerts less pressure on its surroundings than still air.
how does this effect work?
As air moves through the throat and past the lips, the air stream is constricted through the narrow opening of the lips (figure 1). In this regard, the mouth acts like a Venturi tube , which causes the air to speed up through the constriction. As the air speeds up, it creates a low pressure region according to the Bernoulli principle. (See our lesson on Bernoulli's principle and the Venturi tube for more explanation.) In a nutshell, the thermal velocity of air molecules that pass through the lips are more likely to be headed in the direction of the air flow, and less likely to have velocity components perpendicular to the air flow. As a consequence, those air molecules are less likely to push towards the outside of the air stream, and the pressure is reduced in that direction. Outside room air is drawn into the region of low pressure, and is carried along with the stream to help fill the balloon.
technique and tips
In order to make the trick work, the mouth must be close to, but not right up against, the opening of the balloon. The region of lowest pressure is next to the mouth, but there must be enough space between the mouth and the balloon for the room air to enter. With practice, you should be able to put 5 to 10 lungfuls of air into the balloon with each breath.
Any non-elastic balloon can be used for the demonstration. We like to make our own out of Diaper Genie refills (see equipment link above) following Mr. Spangler's suggestion. Cut a two to three meter length of plastic and tie off one end.
This demonstration is best preceded by a discussion on air pressure. Talk with students about the cause of air pressure (air molecules hitting a surface) and why it is higher at sea level than on top of a mountain.
To introduce the Bernoulli principle, try blowing across a piece of paper to make it levitate, as in our lesson on Bernoulli's principle and the Venturi tube. Perhaps discuss the role of the Bernoulli principle in providing lift to an airplane wing.
When introducing the demo on the Bernoulli balloon, make a competition out of it. Have an unsuspecting victim blow one breath into a balloon in order to measure the volume of the lungs. Make sure the victim puts the opening of the balloon right next to his mouth when he blows. Then have the victim stand back to back with someone who knows the trick. Let them race to blow up two identical balloons. The victim won't be able to see why he loses the race, and the audience can be called on to provide the explanation.