Hands-on Science Carnival 2010 Activity Stations: Pressure & Buoyancy

Airflow and Lift: Ping-Pong Ball and Straw Bernoulli

[Shopping List: straws (the kind that bend at one end); ping-pong balls; shop-vac or blow dryer with tube to concentrate flow; small funnel]

1. Bend the straw into a 90-degree turn.
2. Point the short end up and blow a steady stream of air into the long end.
3. Hold the ping-pong ball about 1/2" above the end of the straw as you continue to blow a steady stream (not too hard) of air. You should be able to levitate the ball directly above the end of the straw as long as you are blowing air.
4. With practice you can levitate the ball at various heights by controlling the rate of airflow, from almost touching the straw to several inches above the straw. The ball will probably spin as it floats.
5. Once you can levitate the ball easily, wiggle the straw slowly from left to right and notice that the ball wiggles but remains leviating in the air stream, it doesn't fall off to the side. In fact, if you stop blowing and try to simply balance the ball on the end of the straw, you will likely find that it is actually easier to keep the ball in place above the straw while you are blowing on it and it's floating.
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6. Now the real trick, while the ball is steadily levitating at a comfortable height (and plenty of breath left), tip the end of the straw at an angle (the easiest way is to tip your head forward a little). The ball should continue to levitate, but now it is no longer directly above the end of the straw, but in front of the straw. This will take some practice, but you should be able to levitate the ball even while it is 3" or more from the end of the straw in the horizontal direction.
7. For another amazing trick, blow steadily through a small funnel like a trumpet (it's best if it's not too much bigger than the ping-pong ball), then while you are blowing, place the ball in inside the funnel. Instead of blowing it away as you might expect, the ball is actually sucked into the funnel and will stay there until you stop blowing. In fact, you can even point the funnel straight down and still keep the ball inside the funnel as long as you are blowing- not sucking- air. Blow as hard as you like, the ball will not come out!

For kids who have trouble blowing air long enough to perform these tricks, you can demonstrate them easily using a blow dryer (tape a short toilet paper tube or an inverted funnel on the end to confine and direct the airflow) or a small shop-vac (tape a small funnel to the end for Step 7).

What's Happening: If we blow air on a ping-pong ball it will move. The moving air molecules provide a force that can easily push the light weight ball in the same direction. We also know that gravity will pull the ball down (actually towards the center of the Earth) if we let go of it. So it's probably not too surprising that if we blow air through the straw straight up, pointed directly at the center of the round ball, that we can balance the force of gravity pulling down with the force of our air pressure pushing up, and float the ball in mid air. What may be more surprising is that it is so easy to keep it there, not only in the vertical direction, but horizontally as well.

First let's talk about the vertical direction. Over a few inches, the force of gravity remains constant, but the force of the air coming out of our straw varies. The stronger be blow, the stronger the force and the faster and farther it will push the ball. Even if we blow air at a steady rate, the force will always be strongest right at the end of the straw, and get weaker farther away. If you hold the ping-pong ball right at the end of the straw and blow, you will shoot it much farther than if you hold the ball several inches from the end and blow just as hard. Now imagine that we point the straw up, hold a ball just above the end of the straw and start blowing at a steady rate. The air pressure will push the ball up, but as it rises higher the force of our air pressure weakens until gravity becomes stronger and the ball starts to fall back down, where the air pressure is stronger and starts to push it back up again. The ball will eventually float at the height where the two forces are balanced. We call this a stable equilibrium, because if the air pressure pushes the ball a little too high then it moves farther away from the straw, where the force of the air is weaker, gravity stronger, and the ball falls back down again. On the other hand, if the ball falls a little too low it gets closer to the end of the straw where the force of the air is stronger than gravity, which pushes it back up until the two forces are balanced once again.

The horizontal stability is a little more complicated. For that we need Bernoulli's Law, which states that the faster an air stream is moving, the lower the air pressure in that stream. If we point our straw straight up and blow air at the center of the ping-pong ball, the ball obstructs the air stream, but it will flow smoothly around the ball on all sides. If we move the ball a little to the left however, the right side of the ball now presents less of an obstruction to the air stream, and the air will flow faster on the right side of the ball than it does on the left side. Bernoulli's Law says that faster air means lower pressure, so the air pressure is now lower on the right side of the ball and higher on the left side of the ball. Since air will move from regions of high pressure to regions of lower pressure, a force is produced which pushes on anything in the way. In the atmosphere we call this wind, in our case the force pushes our ping-pong ball from left to right, back into the center of the air stream. And if it overshoots the center a little the same thing happens again on the other side. Once again we have a stable equilibrium that holds the ball in the center of our straw air stream.

When we tip the straw at an angle while blowing, we might expect that the ball would fall. The force of gravity pulls straight down while the force of the blowing air is pointed at some angle away from vertical, so the airflow should push the ball farther away from the straw horizontally where the force weakens and gravity pulls it down. Bernoulli's Law is once again at work here. If we look closely at the ball while it is levitating at an angle from the straw, we will see that the straw is not pointing at the center of the ball, but rather slightly above the center. As gravity pulls the ball straight down, the airflow from the straw is less obstructed over the top side of the ball than on the bottom side, creating lower pressure above the ball and higher pressure below the ball. If the air is blowing fast enough, this pressure difference will be string enough to produce a force pushing up and slightly towards the straw, balancing both gravity and the horizontal component of the airflow which tries to push the ball away, resulting in a perfectly levitated ball even at an angle from the straw.

Finally, Bernoulli's Law is also what holds the ping-pong ball inside the funnel even when you are blowing air out the funnel. The shape of the funnel is very important. Think about when you played with a garden hose by holding your thumb over then end. By restricting the opening you increased the water pressure and made it squirt out much faster. When you place a ball in the funnel while air is blowing out, the same thing happens. Air can only escape through a narrow gap around the edge, so it flows much faster through this gap between the ball and the funnel, creating a region of lower pressure behind the ball, even though the air would seem to be pushing it away. The force produced by this low pressure is strong enough to hold the ball in place even when the funnel is pointed down and gravity is trying to help pull the ball out. In fact the harder you blow, the stronger will be the force pulling the ball in!