This Week's Experiment - #289 Air Has Weight
This week's experiment comes from a question sent to me by a homeschooling mom
named Elaine. It is based on a "classic" experiment often seen in textbooks
to show that air has weight. While it starts simple, it takes some twists
along the way. To try it, you will need:
a yardstick or other long, straight stick
some string
four balloons
two paper clips
a bathtub of water
The experiment is supposed to show that air has weight. You first make a
balance by tying a piece of string around the center point of the yardstick
and then tying the other end to something like a shower rod, so that the
yardstick balances. This may take some work, so be patient.
Next, use the paper clips to attach an empty balloon to each end of the
yardstick. If you put them equal distances from the ends of the yardstick,
it should again balance.
Once you get this done, carefully remove one of the balloons and inflate it.
Tie the neck of the balloon, so it will stay inflated and reattach it to the
paper clip. That end of the yardstick now goes down. A balloon full of air
weighs more than an empty balloon. We have just shown that air has weight,
right?
Now lets think about it. Both balloons are the same. Both paper clips are
the same. The only thing that is different is that one is full of air, and
the other is not. But wait a minute. The balloon that is full of air is
also larger. It takes up more space and displaces more of the stuff around
it, which is.....air. That complicates the experiment.
Why is that a complication? Both balloons are surrounded by air, the same
stuff that you put inside. To see this in a different way, you need to make
a water balloon. Carefully, stretch the neck of a balloon over the end of
the faucet in your sink. Just barely turn on the water and carefully inflate
the balloon with water. Don't fill it too full, as it will pop and make a
mess. Pinch the neck below the faucet and remove the balloon. Make sure all
the air is out and then tie a knot in the balloon to keep the water in.
Hold the water balloon in one hand and an empty balloon in the other. Which
is heavier? Easy! The water balloon. The difference is that there is water
inside the balloon and air on the outside. Put both balloons into a tub of
water. Be sure the water is deep enough so that the water balloon will go
all the way under the surface. Under water, hold the water balloon in one
hand and the empty balloon in the other. Be sure that there is no air in the
empty balloon. They both seem to weigh the same, and they do. You have two
balloons with water on the inside and water on the outside. The only thing
you are comparing is the weight of the two balloons.
Now lets go back to our original balloons. Why are the balloons the same
with water and different with air? When you blew up the balloon, your lungs
and the stretchy rubber compressed the air inside. Because it was
compressed, the air inside the balloon is denser than the air outside. You
have more molecules of air in each cubic inch of space. What you have shown
is not that a balloon full of air is heavier than an empty balloon. Instead,
you have shown that a volume of compressed air is heavier than the same
volume of air at normal air pressure.
So, our experiment was based on a misconception, right? It does not show
that air has weight. Or does it? If compressed air weighs more than
noncompressed air, then air must have weight. Compressing a weightless
substance would not make it heavier. So the misconception was in the
explanation, not the experiment. You are not comparing a balloon of air
with an empty one to show that air has weight. Instead, you are comparing
the compressed air on the inside of the balloon with the normal pressure air
on the outside. The empty balloon is just to balance the weight of the
other balloon.
Now, I know what you are thinking. Why is there a difference between the
balloons with air, but not with water? Water is not compressible, so the
water inside the balloon is no denser than the water on the outside.
Speaking of outside, that is a good place to take care of the water balloon
when you are done. Otherwise, you might make a mess in the house. Just
don't toss it at your brother, unless you can run VERY fast.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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