Big Bag of Air
• Bernoulli Bag
• Writing materials
• A friend (or parent or teacher) to help you
• Critical thinking skills: Make predictions and test the hypotheses you come up with!
• Laws of physics: You will see a complex physical phenomenon in a real, hands-on manner and increase your understanding of the natural world.
1 Tie a knot in one end of your bag before beginning.
2 Ask your helper to hold the end with the knot while you hold the other end. Stand far enough apart from each other that the bag is fully extended, but do not stretch it. How many breaths do you think it will take to fully inflate the bag? Write down your guesses.
3 Take a deep breath, hold the bag’s opening around your mouth, and blow as forcefully as you can! If it takes multiple breaths to inflate the bag completely, be sure you hold the opening shut in between them to keep air from escaping.
4 If the bag is not completely inflated after 5 breaths, hold the opening closed and slide your hand down the length of the bag so the air is pushed all the way to the knot. Do you want to change your guess from Step 2? If so, write it down!
5 Continue blowing into the bag until it is completely inflated. How many breaths did it take you? Was it more or less than you had guessed?
6 Now we’ll try doing it another way. Open the bag back up and let all the air out, flattening the bag as you do.
7 Hold the end of the bag open and stand about 10 inches away from it. Take a deep breath and blow into the bag. What happened? Why?
In 1738, a scientist named Daniel Bernoulli discovered that as the speed of a noncompressible fluid increases, its pressure decreases, and vice versa. In the case of the bag, when you blew into the opening from a distance, you created a stream of fast-moving (and low-pressure) air surrounded by air of relatively high pressure. This pressure imbalance caused a large amount of the air outside the bag opening to be pulled inside, inflating the bag very quickly. When you held the bag closer around your mouth, there was no additional air surrounding the fast-moving air you were blowing, so inflating the bag took much longer.
What does Bernoulli’s principle have to do with model aviation? It has to do with the way airplane’s wings are designed! If you look at them closely from the side, wings usually have a curved upper surface and a flat lower surface. This shape is called an “airfoil.” As the wing moves forward, some of the air flows over the upper surface and some flows under. Because the curved upper surface is a greater distance, the air flowing over it moves faster, with lower pressure. The pressure imbalance between the upper and lower surfaces produces a strong force that lifts the wing – and the airplane – into the air!
• NEVER seal a bag completely around both your mouth and nose.
• Deep breaths will help you with this experiment, but don’t blow so hard that you give yourself a headache. If you feel like you need to take breaks between steps, you should.
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