You'll need a ruler,
a strip of paper as wide as the ruler and 4" long, tape and paper. Tape one edge
of the strip to the ruler so that it lines up with the 1 1/2" mark on the ruler.
Tape the other edge down at the ruler's 5" mark. This should cause a bulge in
the paper. Balance the ruler across the pencil. Then push the ruler a little past
the balance point, so that the paper-wing end wobbles and touches the table. Put
your chin down at the opposite end of the ruler and blow toward the wing. The
paper-wing end of the ruler lifts up. Why? The curve makes the top of the wing
longer than the bottom of the wing. The air on top has to flow faster than the
air on the bottom (under the ruler) to arrive at the back of the wing at the same
time. The fast air flowing over the top pushes down less than the air below pushes
up - so up it goes!
Here's a different
example. Position two books so that they are 4 inches apart. Lay a sheet of paper
across the space between the books. Place the end of a straw just under the edge
of the paper. Blow as hard as you can through the straw. What happens? The air
pressure was equal on all sides before you started blowing. Blowing through the
straw created a stream of fast moving air under the paper. This reduced the upward
pressure on the paper. The air pushing down on the paper is greater than the air
pushing up, so the paper flops down.
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