As I’m about to go off to the Illinois State Fair again, I’m reminded of a nifty experiment that I saw there last year, courtesy of the always entertaining Frank Salter.

Materials: Two plastic cups, one balloon

Experiment:

Start by blowing the balloon up to about ¼ the final size of the balloon. The balloon must be large enough that it will not entirely fit inside the cup opening. Have someone else hold the two plastic cups on each side of the balloon so that the balloon partially fits inside of the cups. Now continue to blow up the balloon while the other person continually holds the cups tightly against the balloon. The cups can be released (don’t pull on the cups!) and will stay attached to the balloon once you have blown the balloon up to about ½ the size that you want it to be. You can continue to blow up the balloon and the cups will stay attached.

What’s happening?

How much space is being taken up in the cup by the balloon when the cups are initially pushed against the balloon? That space is filled with air molecules (nitrogen and oxygen mostly). As the balloon is blown up further, the curvature of the balloon becomes less and less and the balloon takes up less and less space inside the cup. Those air molecules in the cup are now free to spread out over more space. That means that the air pressure inside the cup has lessened and you have created a small vacuum inside the cup and that is why the cup stays adhered to the balloon. Now see what happens as you slowly release the air from the balloon!

by Paul Brandt, Editor

References:

http://www.metrofamilymagazine.com/December-2014/Simple-Science-Experiment-The-Cup-Balloon-Trick/

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