September 21, 2020

Do Try This at Home episode 2: Waterproof hanky

A total success! Hello welcome to Do TryThis at Home brought to you by the Institute of Physics.

We're making thesevideos because we want parents and carers all the way around the UK andIreland to be able to get their kids really excited and curious about theworld around them – even if you can't leave the house.

My name is Imogen and you might be thinking “Imogen what are you doing inyour bathroom?” The reason is this is not my bathroom.

I'm staying with my in-lawsat the moment and this trick has got the potential to go a bit wrongso I'm presenting from somewhere nice and waterproof.

This trick is called thewaterproof hanky and I love it.

The first thing you'regoing to need is a glass.

The next thing you're going to need is a hanky.

Now Idon't actually have a hanky so I'm using one of these cleaning cloths.

Don'tuse a paper hanky or kitchen roll – it will not work.

I speak from experience.

The next thing you're going to need is a plate and finally you're going to need asource of water.

The first step is to prove to your family that your hankyis not waterproof at all.

So what you want to do is just drape your hankynice and loosely over the top of your glass and then pour water through it.

You're aiming to get your glass half full.

The next step is to show to yourfamily that the hanky can be made waterproof.

So what you need to do is now stretch it over the mouth of your glass.

If you're doing this trick with yourfamily you might want to get slightly smaller glasses so they can get a reallygood kind of grip on the material around the outside.

Then put your plateon top and you're going to aim to flip your glass hanky/plate combination inone really quick motion so that it's completely upside-down.

You should beable to take it away and the water does not leak out.

Once you've practiced this afew times bring in the help of a volunteer.

Sam! Can I borrow you for a second?What are we doing? Take your glass and your plate put it over the top and, remembering tokeep that cloth pulled nice and tight, flip the whole thing over your totallywilling volunteer's head and then take away the plate.

Ta dah! What's going on here andhow can you explain it to your family? If you look at your hanky really closelythere are loads of teeny tiny holes in the cloth.

This lets the water travelthrough it and air go through it as well.

Now get your hanky nice and wet.

Canyou blow through it? It's actually much much harder and that is because we havewater blocking up all of those tiny tiny holes, so for the air to go throughit's got to push the water out of the way and that's actually pretty difficult.

How can water block the holes I hear you cry! Have a look at this picture.

Thesurface of the water looks like it's covered in a stretchy skin almost likethe surface of a balloon.

This effect is called surface tension and that surfacetension is strong enough to be able to hold up my pond skater so that itdoesn't sink it's also a really cool and then you could try it with your familywith this paper clip.

With a glass of water that's completely full the surfacetension over the top is strong enough to be able to hold up something that shouldsink like a paperclip.

And as soon as we break that surface tension my paperclipsinks to the bottom.

Surface tension is a bit like a balloon stretched over a hole.

With a big hole then my surface is easy to push and with a small hole it's muchharder to push.

Now with a glass of water without a hanky when I flip it over thesurface tension is nowhere near strong enough.

It breaks easily across the large mouth of the glass and the water pours out.

When I add the hanky water soaks into the hanky and blocks the holes in mycloth because the holes are smaller the surface tension skin across each hole isstronger and actually it's strong enough be able to keep the water inside of myglass.

Which is why my hanky becomes waterproof, your family stays dry and youstay friends with your family.

If you're not totally sure about science don'tworry.

Investigate with your family, maybe look it up with your family if youneed to and have fun along the way.

Have a lovely day and see you next time! It dripped all over my head!.

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