An Air Weighing Balance - Fun Physics Mechanics Experiments

How can you see that air has weight?

Did you know that air has mass? Because the weight of the atmosphere has been pressing down on you all your life and your body is adapted to it, you don't even notice it! But try traveling to a place at high altitude and you'll quickly notice it's lack.

Even if you are intellectually aware of the mass of air, there are a few experiments like this one (and the "Candle in a Jar") that allow you to directly observe the air's mass

What you need:

• Two balloons.
• A piece of dowel (roughly 30 cm long is ideal, but the exact length is not that important).
• Sticky tape.
• String.
• A ruler or measuring tape long enough to measure the dowel.
• A thumbtack.

What to do

1) Tape one of the uninflated balloons to each end of the dowel.

2) Measure the dowell and find its exact centre, by dividing its length in two.

3) Gently press the tack into this point (press it about halfway in so it's easy to pull it out if you find you need to adjust it).

4) Tie the string to the tack. It should hang neatly balanced from the string. Adjust the position of the tack as required to get the balance right.

5) Once you have the balloons balancing, carefully remove one of them and inflate it. Once it is inflated, tie it off.

6) Tape the balloon back in place. What happens?

What Happens?

Your balance bar, which was nice and level when you compared the two empty balloons will now tilt downwards at the end where you taped the filled balloon. For the bar to tip like this, there must be a greater force pushing downwards on the full balloon.

There are only two forces we need to consider on the two balloons - gravity and bouyancy. Gravity pushes down on the balloons with a force equal to each balloons weight. Bouyancy pushes upwards on each balloon with a force equal to the weight of enough air to fill the volume the balloon occupies. Although much smaller, this bouyancy force is the same sort of bouyancy force as you feel when you float in the ocean (although in that case, you are pushed upwards with a force equal to the weight of water you displace, not air).

So, the empty balloon feels a total force downwards equal (roughly) to the weight of rubber it is made from. Because the balloon is so small when empty, we can ignore the small amount of air inside the empty balloon.

The filled balloon, however, contains a large amount of air, which must be taken into account. The force of gravity on this balloon is larger by an amount equal to the weight of the air inside the balloon, making the filled side heavier. Of course, the filled balloon is a lot bigger, so it displaces a greater volume of air, leading to a larger upwards bouyancy force. As the bouyancy force is equal to the weight of the air displaced by the balloon, you might think that it should exactly cancel out the increased weight. However, because of the "stretchiness" of the balloon rubber, the air inside it is compressed - so that the weight of a liter of air inside the balloon is heavier than a liter of air outside the balloon.

Thus, because air has a significant mass, the filled balloon feels a greater total force than the empty one, and so the balance tips in its direction.