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Hold a piece of paper between your thumb
and forefinger, as shown. Now blow over
the paper. What happens?
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Try this activity with a friend. Blow up
two balloons and tie each one to a
string. Hold the balloons a few inches
apart and try to blow them together. Can
you do it? What happens? Try different
ways of blowing on the balloons to see
what happens. Hint: Squirt a little water into the balloons before you blow them up. This will help steady them.
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Hold a Ping-Pong ball over a flexible
straw, as shown. As you blow into the
straw, let go of the ball. What happens?
Play around with holding the straw in
different ways. For example, can you
tilt the straw and still keep the ball
in the air? Hint: You can use any lightweight ball or a small balloon, but you may need to blow harder.
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Stick a pin through the middle of a card
from below. Place the spool over the
pin. Hold the card and pin in place with
one hand; hold the spool with the other.
Blow through the spool and let go of the
pin and card. What happens?
Hint: Make sure the spool has only one
main hole. If others exist - even pin
holes - tape them closed.
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So What's a Bernoulli?In the early 1700s, a Swiss mathematician named Daniel Bernoulli discovered that when flowing air or water changed its speed, its pressure also changed. As you do these activities, can you figure out how the pressure changes? How does this help airplanes stay in the air? |
Fluids, such as air and water, change speed as they flow between and around objects. To see how this happens, build this tiny stream channel. Tape pencils on a cookie sheet so that they make a channel that starts out wide, then narrows. Drape the pencils and cookie sheet with plastic wrap_this creates a water-proof channel. Now barely tilt the cookie sheet against the sink and slowly pour soapy water into the channel. Does the speed of the water change? How? When? Hint: You may want to add small scraps of paper or Styrofoam to the water to help your observations.
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| (1) Air is pretty pushy stuff. It never pulls or sucks; it pushes. Air is pushing on you right now from every direction. We're so used to air being around us that we often don't notice it. This constant push of air is called air pressure. It allows us to breathe_not a bad thing! But what was happening in these activities? Why did the balloons come together when you blew between them? Why did the paper lift up when you blew over it? Why did the ball or card stay in the air? Air must be pushing these things, but how? | (2) Even before you blew at the balloons, they were surrounded by air pressure. If you tried blowing between them, you disturbed this push in a very special way. How? Think about this: Either the air between has stopped pushing as hard or the air on the outer sides is pushing harder. Which do you think happened? Which air did you disturb - the air between the balloons or the air on the outer sides of the balloons? |
| (3) Can you figure out what happened with
the paper? Now you know that the paper
was surrounded by air pressure; how did
you change the air when you blew over
the paper? Remember, air can't suck up
anything, but it can push. Did you
change the push of air on the top or the
bottom of the paper? OK, enough questions! Here's what was going on: In both the balloon and paper activities, air lost pressure and stopped pushing as hard. This happened because you blew the air, and it had to squeeze between or around the objects. |
(4) As it squeezed through, it sped up, lost pressure, and stopped pushing as hard. That's what happened when you did the spool activity, too. As air squeezed between the spool and card, it moved faster, and its pressure dropped. The air pressure below the card didn't change; it continued to push as hard as it always does. It held the card in place even though you were blowing hard. Now think about the activity with the Ping-Pong ball and straw. What was happening there? |
| (1) Now that you know about push and lift, can you see how this might relate to airplanes? If we can make air speed up over a wing, the pressure of the air over the wing will drop. The higher-pressure air below the wing will then push the airplane up. How would you shape a wing so that the air will move more quickly over the top than under the bottom? Squeeze the Stream shows what happens when a fluid is forced to flow from a wide space through a narrower channel. | (2) In order to squeeze through a narrower space, something must either compress (think of pulling a sponge through a bottle neck), or it must speed up. Freely flowing water does not compress easily. Instead, it speeds up as the channel narrows. Water also speeds up as it moves around an object, such as a rock in the river. Air is a fluid, and at relatively slow speeds it behaves like water when it moves through a narrow channel or around an object - it speeds up. As you saw with the other activities, when air moves faster, its pressure drops and it pushes less. |
| (3) When an airplane flies, it pushes air out of the way. That air must go somewhere - so it squeezes between the wings and the surrounding air. The wings are shaped and tilted so that the air moving over the top has less room than the air moving below the wings. Because it has less room, the air moving over the top must speed up more. At the same time, the air loses pressure and push. The air moving below the wing doesn't speed up as much and so maintains more of its pressure; this higher pressure pushes the wing - and the plane - up. | (4) An airplane wing affects moving air much like a rock in a stream affects the moving water. Remember that the space around the wing is already jammed full with air, so there's no empty space for the air to move into. As the oncoming air hits the wing and moves either over or under it, it speeds up and squeezes between the wing and the surrounding air. |
| Many books state that air speeds up over a wing because it has further to travel than air moving under the wing. This implies that air separates at the front of the wing (point A) and must rejoin behind the wing (point B), but this isn't true. Air moving over the top of a wing speeds up so much that it arrives at point B sooner than air that travels beneath the wing. | 
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