Build a Hot Air Balloon

Hot air balloons are the most elegant way to fly - build one from normal kitchen materials.

What you need

What to Do

What may Happen

The bin liner should gently float up into the air. It may be unstable and fall over - if this is the case adding some tape on the bottom may stabilise it and make it fly for longer.

What is going on?

You wouldn't think it, but air is quite heavy, a cube 1m x 1m x 1m of air weighs about 1kg. So inside a normal pedal bin liner there is probably at least 60-70g of air.

When you heat up air it expands - if you heat it up by 30°C it will expand by about 10%. This means that not all the original air will fit in the bin liner and maybe 6-7g will fall out of the bottom. If the bag only weighs 5g then the combination of the bag and the air inside it is now lighter than the eqivalent amount of cold air.

If you put something less dense than water in water it floats, similarly if you put something less dense than air in air, it also floats, so the balloon floats up towards the sky.

Why is it unstable?

If you look at the bag the heaviest bit is the base (now at the top) where the plastic is all gathered together.

Having the heaviest part of the balloon at the top isn't very stable, this problem can be solved by adding tape to the bottom, making this heavier, keeping everything the right way up.

Written by Dave Ansell