Lockdown Challenge: Going out with a bang

We’ve had a lot of fun with the Lockdown Challenge for the past few months. Largely blessed with good weather the majority of these challenges will have been accessible to most, and if the schedule was a bit heavy for some there is now a back catalogue of ideas to inspire your family in the future. The majority of these ideas (all still available on this website), although designed for engineers at home with their restless offspring during the lockdown, are equally appropriate for projects at schools, youth clubs or any environment where engineers can lend their expertise and enthusiasm to introduce inquisitive children (and they are all inquisitive) to the joys and possibilities of engineering.

Appropriately, we are going to go ‘over and out’ with a ‘big bang’. First up this week, Crispin Andrews shows how you could encourage a budding radio engineer to set up their own radio station. Following that, we have Neil Downie’s final experiment and, true to form, he is finishing with a Big Bang. 

The IET has a host of resources, which adults can use to engage children with the world of STEM.

Lockdown Challenge #31 – How to make your own radio station

My Christmas-party expert friend Ebenezer says" “Bangs from Balloons? Bah, Humbug! Balloons don’t go bang, they go pop.” Not so. With a little effort, you can prove Scrooge wrong and make a really Big Balloon Bang. And what about an even bigger bang with the Boyle’s Law Banger, which you can make out of an old bike pump? But first, more bang from balloons... 

Do you have ear-defenders? If you don’t, do all your Big Bangs outside. Now find some balloons; the stronger and bigger the better. Take a balloon and inflate it until it’s full and somewhat pear-shaped. Now pop it with a pin. Nice pop? A little bang, maybe? You can do better. Much, much better.

Now take another balloon and simply inflate it. Use a pump, a balloon pump, or, even better, put the balloon on a small piece of plastic tube, use duct tape to keep it from coming off and use a bike track pump or foot pump. A valve from an old inner tube can make the connection to the plastic tube. Keep on inflating the balloon... and inflating it...  and inflating it... until... BANG! It’s gone. With luck, that was much, MUCH louder.

Try testing bang loudness with a phone sound app; make the measurements fair by exploding at the same distance. And take a look at the exploded balloon pieces. The pic shows a pin-exploded balloon and a pressure exploded balloon – and the YouTubes give insight into why the shapes are different. 

Here is a pin popping a balloon:

Here is a balloon popped by pressure by Dr Yan Wong, from a BBC programme rather appropriately called 'Bang Goes the Theory':

What next? Bigger balloon? Even better, try putting one balloon inside another similar one. Inflate the double-balloon, now the pin... with luck, that will be a nice big bang. Try exploding a double-balloon by pure pressure. Prepare for a serious bang.

Boyle’s Law says that when you squeeze air down in volume X times then the pressure will go up X times. Squeeze the gas to a tenth, and it will go to ten times the pressure and... BANG!

The picture below shows the stages in making a Boyle’s Law Banger, from a bike pump at the bottom to Banger at the top. You’ll need an old ‘frame’ bike pump, a soda bottle, glue, and 40-50mm-wide ‘Sellotape’ type tape – the thinnest grade. First saw off the tyre-filling end; make sure that the piston goes nearly to the end of the tube.

Now saw the end off a 1 litre soda bottle, 30mm of the bottle neck past the screw thread part, a total of ~50mm. Sandpaper the pump end if it is shiny/slippery and maybe coat with cyano glue. Similarly the inside of the soda bottleneck. Glue this onto the end of the bike pump with hot-melt glue. Make a really good job of it and, for extra insurance, wrap a couple of turns of duct-tape around the joint.

Make as large hole as you can in the bottle cap. Now ‘arm’ your Boyle’s Law Banger by putting sticky tape over the end, then screw the cap firmly on over it.

With ear-defenders on, prepare to pump the bike pump just once, but pushing very hard. With luck, the volume will go down, the pressure will go up and... BANG!  

If you find it hard to get to the necessary force, try putting something soft and grippy like Blutack on the pump handle, place the handle on the ground and hold the barrel and push it vertically down onto the handle.

The energy in compressed air is a bit more than atmospheric pressure × volume of air in the pump. That will typically work out at 10 Joules. The crack crosses the tape at the speed of sound in plastic, maybe 2,000ms^-1. With a 20mm crack, that gives 10 microseconds crack time. Power is energy/time = 10J / 10^-5s = 106W; so your Boyle’s Law Banger emits a megawatt of power.

Challenges

Can you make a triple balloon and explode it?

Does a balloon make differently shaped pieces when pierced at the top rather than the side?

Can you get a video of the Boyle’s Banger exploding in slow motion? Some phones record slo-mo.

And finally, what about an underwater bang? It won’t sound as loud, but it is powerful; look at what it did to the washing up bowl below.

If you liked this, you will find lots more fun science stuff in my books like 'The Ultimate Book of Saturday Science' from Princeton University, and for lots of other things (and a free copy of the 'Exploding Disk Cannons' book), visit www.saturdayscience.org

Many thanks to Crispin Andrews and Neil Downie for their contributions to this column.