Wednesday, 6 June 2012

walking, running and even stopping on the water

‘This,’ said Nicholas, when I clicked on the photo…..


‘is something which stands on water.’

‘It can walk fast on the water,’ added Aalian.

‘It can walk and run and stop on the water,’ Joseph told us.

‘And it doesn’t sink,’ said Bianca.

Melchior remembered.  ‘We saw it at Chateau de Penthes.’

And we did.  There it was, this pond skater (for that is what it is) walking and running and even stopping on the water.  How so?  How does it do that?  Why doesn’t it sink?

Joseph thinks he knows.  ‘I think it’s because it goes so fast,’ he suggests.

But what about when it stops?

‘Because maybe it is so light.  It’s not heavy.’ This from Yousouf.

And yes, they are light.

Bianca has yet another theory.  ‘Maybe because its legs are a bit sticky?’ she wonders.

‘Maybe,’ says Melchior, ‘it’s just the way he is,’ he sighs.

Time I think for a little bit of physics.  If I was to pour some water…..

into a bowl and then carefully drop in a paperclip, what do you think would happen?  Would the paperclip stay on the surface of the water or go down to the bottom?


Most of us thought that the paperclip would sink - and so it did.  We tried it a couple of times, just to make sure.

But you can make a paperclip stay on the top using something called surface tension.  First you need to float a piece of tissue paper carefully on the surface of the water and then carefully lay the paperclip on that. 


Next, using a pencil…..


carefully poke the tissue so that it sinks; the paperclip should (fingers crossed!) stay on the surface.


Well, you can imagine the excitement as one after another, everyone managed to get a paperclip to sit on the surface of a bowl of water.  But how is it possible?

It’s a bit complicated but there is a sort of stretchy ‘skin’ on the surface of the water.  And paperclips as well as pond skaters can use this stretchy ‘skin’ to help them stay there rather than sinking to the bottom.

Look again at the pond skater.


See those little dents where his feet touch the water?  That’s the ‘skin’ doing a bit of stretching and keeping him safe from drowning!

And if you look at this photo of a pond skater…..


you will see more evidence of the stretchiness of water.  See those drops on the lily pad? 

PS You might also spot that the pond skater has the remnants of a tasty meal held between his front legs.  Some time, remind me to tell you how he will have caught it.  And be very glad that you don’t live in a pond!

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