A Moment of Science

Heavy Air

Here's an odd-seeming question. Does air weigh anything?

balloons

Photo: snlsn (flickr)

When you blow up the balloon, you are increasing its overall density by forcing air inside it.

Here’s an odd-seeming question. Does air weigh anything?

At first this might seem perplexing. On the one hand, air just seems to float there. I mean, to say something is ‘as light as air’ is to say its weightless, isn’t it? However, a strong wind can pull branches down and send things flying, so air isn’t exactly nothing.

Air does, in fact, have weight, and here’s a simple way you can prove it. You’ll need two identical balloons, a string, and a dowel. Attach the uninflated balloons to either end of the dowel. Attach the string to the center of the dowel and then hang it from something. If the balloons are very nearly identical the dowel should hang evenly, without tipping to either side. You now have a basic scale.

Next, inflate one of the balloons by blowing into it. What happens? The side with the inflated balloon hangs lower than the other side. You have demonstrated that air has weight!

This simple demonstration sometimes confuses people. Why does a balloon filled with air fall, when it is only surrounded by air? Why doesn’t it just remain where it is?

The answer lies in density. In a fluid, anything that has a greater density than its surroundings will sink. When you blow up the balloon, you are increasing its overall density by forcing air inside it. That’s because the air inside the balloon is under greater pressure than the air outside. It’s being squeezed tightly by the sides of the balloon. Greater pressure means greater density, so the heavy air inside the balloon will fall.

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