Monday 6 August 2018

5 Minute STEM Activity 8: The Banger

The Butterfly Effect suggests that when a butterfly flaps its wings in the Amazon jungle, the ripples around the world might cause a roof to blow off in China, an apple to fall from a tree in the UK, or a Russian woman's hat might blow away. We see the butterflies' wings flap, but we can't see the waves of pressure it causes. In this activity we can hear them though...

Bangers and Sound Waves 

A sheet of A4 paper

1. Fold the paper in half lengthways

2. Unfold the paper
3. Fold in all four corners to meet the centre line

5. Fold in half so you can only see the plain outside

6. Fold in half the other way to make a crease

7. Unfold the last fold

8. Fold both sides into the middle line

9. Fold in half backwards

10. Hold your banger by the 'tails' and flap it as quickly as you can

Getting the 'flapping' right is something you'll get better at with practise. The best place for this is outdoors where you won't annoy anyone. Hold the banger in your hand as if you are throwing a dart.  A quick wrist movement is as helpful for making a big bang as the arm movement.

The reason we hear a bang is because as we flap, the air is squashed inside the banger until there's too much pressure and the side pops out. The sudden movement when it pops out compresses the air outside the banger and forces it to squeeze up together really quickly. What we hear is the sound of that wave of squashed air rippling out.

You can see and hear a very similar effect when you drop a stone into water and ripples spread out around it...

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