# Science Experiments

## Build a Hot Air Balloon

Sun, 25th Mar 2007

Listen Now    Download as mp3 Part 1,2 from the show The Science of Flight

### What you Need

 A toaster A cheap bin liner - the lighter the better Some card/cardboard - enough to make a tube the size of your bin liner

### What to do

1- Empty the crumbs out of your toaster - they will just smell bad

2- Roll the cardboard into a tube so that the bin liner will fit over it easily, it is longer than the bin liner and will fit over the toaster

3- Put the bin liner on the tube

4- Turn on the toaster

Be careful because the toaster could get hot enough to hurt

It is more impressive if you can find somewhere with a high ceiling to do the experiment in.

### 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.

### Why does it happen?

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.

Dave Ansell