ScienceMakers Toolkit Manual - The History Makers

Earth Science
nozzle before the balloon falls? What causes the balloon to stay in the stream of moving air?
Finally, try placing a table tennis ball in the air stream created by the hair dryer. Does it fl oat? If your hair dryer
is powerful enough, the ball will fl oat in the stream just like the balloon. Can you place both the balloon and the
table tennis ball in the air stream at the same time? Which object must be placed on top for them to both remain
fl oating? Why is this the case?
When you fi rst let the balloon go, it sinks because it is denser than the surrounding air. (Why do balloons fi lled
with helium rise?) However, the balloon is just a little denser than air, so you were able to keep it from falling
by blowing on the bottom of the balloon, either by mouth or with the hair dryer. The column of moving air
pushes on the bottom of the balloon, forcing it to rise. The effect is the same in either case, but the hair dryer
doesn’t run out of breath.
But what happened when you tilted the nozzle of the hair dryer? If the balloon were simply held up by the column
of air, it would have fallen down when you tilted the nozzle. Instead, the balloon followed the column of
air when it moved—so something else must be happening.
The reason the balloon stays in the moving stream of air has to do with Bernoulli’s Principle. Bernoulli’s Principle
says that the pressure decreases inside a stream of fl owing air. When the balloon begins to move out of this
low pressure stream, the higher pressure of the air in the room pushes it back into the moving stream. When you
tilt the nozzle, the pressure in the room is still high enough to push the balloon back into the air stream, even if
the moving air stream is at an angle. Eventually, at a large tilt, the force of gravity will become greater than the
force of the air pressure holding the balloon in the stream, and the balloon will fall.
Let’s consider an alternate explanation: could it be that the balloon is held up by the heat of the warm air from
the hair dryer? This is a reasonable guess, since we know that hot air balloons fl oat. In fact, hot air balloons
have been around since 1783. Hot air balloons are able to fl oat high in the air, because when the air inside a hot
air balloon is heated, it expands, making it less dense than the surrounding air. This causes the balloon to fl oat
up to a higher altitude, where the air is less dense.
How can you test this explanation? If you can, turn off the heat on the hair dryer, and repeat the experiment. Are
you able to keep the balloon afl oat without the use of hot air? Actually, when you think about it, the heat from
the warm dryer makes the air outside the balloon warmer than the air in the balloon. So this explanation isn’t
correct.
Wind Tube
For this part of the experiment, you will need a WindTube from www.teachersource.com (also available as a
Bernoulli Bag from www.arborsci.com).
Stretch out the WindTube, and have someone else hold the closed end while you hold the open end. How many
breaths does it take you to infl ate the WindTube?
If you infl ated the WindTube like most people infl ate a balloon or paper bag, then it probably took a lot of
breaths. If you use Bernoulli’s Principle however, you can infl ate the bag with a single breath!
Get all of the air out of the WindTube, and again have someone hold the closed end while you hold the open
end. Hold the WindTube about a foot from your face, and hold the open end wide open. Blow one big, long
breath (like you are blowing out birthday candles) straight down the middle of the WindTube and close the open
end off when you are done. Did it work? How were you able to fi ll the WindTube with only one breath?
When you blow one breath straight down the middle, your moving breath creates a stream of low-pressure air,
which pulls air from the room into the bag with it. Have you ever been on the side of the road when a big bus or
truck sped by? You probably felt the rush of air that the vehicle was pulling along with it. Your breath acts in the
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