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How to feel the mass of air






How can you feel the mass of about 1 kilogram of air in your hand? Answer: fill a balloon of 1 m3 with air and shake it. When the balloon is motionless, you will only feel the weight of the plastic of the balloon. But when pushing and pulling the balloon back and forth, you will feel the inertia of the air, like if you were shaking a little balloon full of water. This is a huge experiment for kids but do not let kids alone with the balloon.

One way to make such a balloon is to buy a 20 m3 protection plastic sheet for indoor use. Either 10 micron thick or 20 micron. I prefer the 20 micron sheet because it lasts longer.

Open the sheet on the ground and seize it by its middle:



Hold it high above your head and comb it with your fingers to get the plastic hang neatly:



Push a lightweight tube inside the bottom (orange) and wrap tape around the plastic (green) to hold the whole together:



Cut away the loose parts of the plastic:



Now you can blow air into the balloon by any suitable method; an electric pump, a little computer ventilator, a foot pump for air beds... Use anything suitable to close the tube, like some tape or a cork.

If you'd like a much wider hole at the bottom of the balloon, you can use a wider tube, that you will remove later on. Wrap tape around the tube before you put it inside the plastic but wrap that tape with the glue facing outside. Then push the plastic of the balloon around the tape and wrap some more tape on the outside, like in the example above. Ultimately, remove the tube, leaving only the tape to hold the bottom of the balloon together. You can inflate the balloon by running with it, with the opening forward, or by using a little table ventilator.

Hold the inflated balloon by its closed opening and shake it back and forth with a wide and fast movement. You will see and feel the heavy air bouncing back and forth, unwilling to obey your movements.



If you want to provide this as an attraction for a science fair or such, one problem is that you cannot avoid that the balloon slowly looses air. The losses can become quite fast once the balloon weirs out. A solution I once used is to latch a small computer ventilator to the input tube and have that ventilator permanently connected to a power supply through an electric wire. Only use a low voltage like 9 or 12 Volts, of course. I made the assembly such that the ventilator could be reversed, allowing to deflate the balloon automatically.



Eric Brasseur  -  December 8 2008

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