![]() ![]() The victim won't be able to see why he loses the race, and the audience can be called on to provide the explanation. Let them race to blow up two identical balloons. Then have the victim stand back to back with someone who knows the trick. Make sure the victim puts the opening of the balloon right next to his mouth when he blows. Have an unsuspecting victim blow one breath into a balloon in order to measure the volume of the lungs. When introducing the demo on the Bernoulli balloon, make a competition out of it. Perhaps discuss the role of the Bernoulli principle in providing lift to an airplane wing. Bernoulli’s equation states that for an incompressible, frictionless fluid, the following sum is constant: P+ (1/2)v2+ghconstant, where P is the absolute pressure, is the fluid density, v is the velocity of the fluid, h is the height above some reference point, and g is the acceleration due to gravity. 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. 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. This demonstration is best preceded by a discussion on air pressure. Cut a two to three meter length of plastic and tie off one end. We like to make our own out of Diaper Genie refills (see equipment link above) following Mr. With practice, you should be able to put 5 to 10 lungfuls of air into the balloon with each breath.Īny non-elastic balloon can be used for the demonstration. 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. ![]() In order to make the trick work, the mouth must be close to, but not right up against, the opening of the balloon. Outside room air is drawn into the region of low pressure, and is carried along with the stream to help fill the balloon. 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. (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 the air speeds up, it creates a low pressure region according to the Bernoulli principle. In this regard, the mouth acts like a Venturi tube, which causes the air to speed up through the constriction. how does this effect work?Īs air moves through the throat and past the lips, the air stream is constricted through the narrow opening of the lips (figure 1). 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. 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.
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