## Standing waves on a spring

Posted: October 29, 2014 in Institute of Physics
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You might have seen/done this one before, but the teachers I was showing it to found it quite useful, so I thought I would post it on talkphysics. The nodes and anti-nodes are a lot clearer when the amplitude of the oscialltions is higher, but I was told the signal generator broke the previous vibrator when set too high, so I was a bit cautious with this one.

There are several teaching and learning points with this demo. For example, you could get the students to calculate the speed of the wave along the spring, as we know the frequency from the signal generator and can measure the wavelength (for example by measuring the length of the stretched spring with a ruler).

Once we know the speed of the wave, we could ask the students to predict the next frequency in which a standing wave will form.

What else would you use this demonstration for? What other questions would/could you ask?

## A nice Pressure in Liquids question

Posted: December 6, 2013 in Institute of Physics, Thoughts and ideas
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I first was introduced to this really nice question by Neil Atkin (@natkin) and ever since I have tried to find a good way of showing it. So, look at the question and the explanation that I think is correct, as far as I can tell (but please point out any faults in my reasoning). Then, check out the simple demo I used to show this.

“If I am on a boat in a pond and I hold a 10 kg rock in my hand, what will happen to the level of the water if I drop the rock inside the pond? Will the water level increase, stay the same, or be lower?”

It’s all to do with Archimede’s Principle that states that the upward buoyant force exerted on a body immersed in a fluid is equal to the weight of the fluid displaced by the body.

This should help us think about this problem. In fact, if the boat is floating it means that the weight of the water displaced by the rock when it is inside the boat must be the same as the weight of the rock itself. That is because the upthrust balances the weight of the boat, myself and the rock, or the boat would sink. So the rock displaces a volume equivalent to the space occupied by 10kg of water, i.e. 10 litres.

When I throw the rock inside the pond the water displaced by the rock is only the volume of the rock itself, which is most likely not 10 litres, but much less. So, the level of the water in the pond decreases!

I took these two photos before and after to convince you of this (the measuring cylinder we used in another attempt was to big to appreciate the difference). Click on either photo to enlarge them and see them in Gallery view.

This blog post is about a great demonstration I got from David Featonby from the Institute of Physics and a little adaptation I thought about after seeing his version. He tipped us all up on a really cool vacuum container to store coffee which you can find here. Then, he put a teddy bear shaped marshmallow in, closed the lid and sucked the air out. The effect is really dramatic as you see the teddy bear mallow increasing in size and become “huge”. My boys think it is a Humongous Mallow (only avid Ben 10 fans will understand the reference).

Anyway, the advantage of using this container instead of a normal empty bottle of wine is that you can put much bigger things inside and that’s where I thought of this additional demo to do with this device. There are various ways in which you could introduce the demo. For example, you could watch a bit of a scientifically questionable Sci-fi film and ask your kids “Is it possible?” The clip I have in mind is Star Wars: The Empire Strikes Back, when Han Solo hides the Millenium Falcon inside an Asteroid cave (which turns to be a giant worm’s belly). In that part of the film the crew goes out of the spaceship wearing their normal clothes and just a tiny breathing mask, as if 🙂

The point here is that if Han Solo and friends were on an asteroid in outer space, they would be surrounded by almost perfect vacuum and their internal body pressure would cause them to pop as soon as they get out of the ship! So, what would your students think? Would they believe it is possible?  Would they think they would just freeze?

At this point you can show your coffee saver and put a tea cake inside. Then, ask the kids what they think would happen if we could take most of the air out!

And see their faces as this happens!

You can now go back and ask the same question from the Star Wars clip. Is it possible? What would happen to Han Solo and his crew?

Please, see my next blog post as there is an interesting update pointed out by Ashley Kent (@AshleyKent) that suggests that your body wouldn’t actually blow up in space! So, the demo is becoming even more interesting and pedagogically really valuable, as we have the opportunity to really confuse our kids up to then let them construct a new and stronger meaning. This is active Constructivism and Metacognitive Conflict brought to its extremes 😉

Thank you Ashley for pointing this out!