Bernoulli's principle helps explain how an airplane's wing makes the plane fly.
Bernoulli's principle describes the flow of fluids as well as how changes in speed can lead to changes in pressure. For instance, the shape of an airplane's wing causes air to travel over the top of the wing faster than under the wing, which makes the air pressure above the wing less than the pressure beneath the wing. Several activities can help students see this principle in action.
The Cardboard That Won't Fall
To do this experiment you need an empty thread spool, a thumbtack and a piece of cardboard. Stick the thumbtack through the center of the cardboard and then place the thumbtack through the bottom of the thread spool. This will keep the cardboard from moving from side to side. Blow through the top of the spool and let the cardboard go. As long as you keep blowing air through the spool, the cardboard won't fall. This is because by blowing air you have increased the air speed and thus reduced the air pressure above the cardboard.
Plastic Cups
You'll need two plastic drinking cups, two lengths of string each about one foot long, tape and a table. Attach a string to the bottom of each cup with tape, then tape the cups to the edge of the table by the strings so that they hang at the same height from the table about two inches apart. Blow air into the space between the two cups. The cups will swing together because you have decreased the air pressure between them.
The Candle
You will need a lighted candle and a large can. Place the candle behind the can. Then have a student blow on the center of the can to try to blow out the candle. Even though the can blocks the student's breath, the candle will extinguish because the air travels around the can from either side. This demonstrates that when a fluid (in this case, air) is separated by an object into two streams, its natural tendency is to join back into one stream on the other side of the barrier, just as air behaves when flowing over the wing of an airplane.
The Sheet of Paper
Take two books that are the same height and place them about 4 1/2 inches apart, then place a sheet of paper on top of the books. Using a straw, blow air gently just above the paper. Then do the same just beneath the paper. The paper will lift or drop toward whichever side has the flowing air because the flow of air has caused the air pressure to drop.
