Straw Oboe

This is a noisy, amusing demonstration of the physics of music. It can take a bit of practice to get exactly right, but it's well worth the effort.

You will need:

straws (need to be straight – cut off the bendy bits if there are any)
scissors

What to do:

Flatten one end of the straw ~2cm from the end to the tip.
Make two cuts in the now flattened end of the straw, to form a triangular tip.
Insert the triangular tip of the straw into your mouth and blow hard. You should hear a loud 'buzzing' sound.
While blowing on the straw oboe, get a volunteer to cut the straw shorter, ~1cm at a time. With each cut you will hear the pitch of the oboe sound go up.

What's going on?

The flattened triangular tip acts like the reed found in most wind instruments. Blowing on the reed causes the straw to vibrate. A standing wave pattern is created along the length of the straw, which we hear as sound. As you shorten the straw you shorten the wavelength of the standing wave pattern and therefore increase the pitch of the note.

Note:

It can take some practice to get the right sound – if it doesn't work straight away then slowly move the straw in and out of your mouth whilst still blowing until you hear the sound. Definitely a good demonstration to practice before performing it in front of an audience!

Did You Know?

As long ago as the fifth century BC Pythagoras and his followers were experimenting with standing waves and calculating the values of their harmonics. Another way to set up a standing wave is to blow across the top of a drinks bottle. In this case the note gets deeper as you drink the drink (sorry, tune the instrument).

Egg-straordinary physics tricks

You will need:

raw egg
hard boiled egg

What to do:

Place the two eggs on a flat surface and set them both spinning.
Gently and briefly place your finger on the top centre of each egg.
Notice that the hard boiled egg is much easier to spin, but it stays still when you take your finger off. In contrast, the raw egg is difficult to start spinning but will keep spinning when you take your finger off.

What’s happening?

Momentum is the key to this demonstration. A raw egg is filled with a liquid, whereas a hard boiled egg is effectively a solid. Firstly consider what happens when you stop the eggs: When you gently place your finger on the top, you stop the outer shell of both eggs from moving. Since the hard boiled egg is solid, all of the egg stops moving, and so the egg remains stationary when you remove your finger. However, the liquid inside the raw egg will keep spinning even though the outside shell is stationary. The drag of that liquid on the shell will start the raw egg spinning again. Similarly, a hard boiled egg is easier to spin since the entire egg starts spinning at the same time, whereas in the soft boiled egg only the outer shell is spinning at first, and gradually the liquid insides begin to spin as they are dragged around by the shell.

Tips for Success

Don’t set your eggs spinning too hard or they may roll off the table. Make sure you start them spinning at approximately the same rates or your audience may think you are trying to fool them! As if!

Another egg-sample of physics at work:

Eggs are traditionally thought of as being very fragile, but in fact the physics behind their shape is astounding.

You will need:

raw egg
plastic bag or glove (for the unconfident!)

What to do:

Challenge audience members to break the egg just by squeezing it. Let them wrap the egg in a plastic bag or wear a glove if they're worried… Believe it or not, it can't be done! Shift it Mr Doppler!

What’s happening?

The shape of an egg is actually one of the strongest designs possible. The curved structure means that applying pressure to any particular area actually spreads the force out over the entire egg. So just squeezing it won't cause it to break. Of course applying a very sharp force to one point WILL cause it to break – which is why we usually tap the egg on the side of a bowl to break it when cooking.

Tips for Success

Ask your volunteers to remove any rings etc. before trying this trick – the sharp uneven force from such metal objects can cause the egg to break.

Check your eggs for hairline fractures before attempting this trick – if there is any existing damage to the egg it won't work.

Did You Know?

The ornate and intricate arched doorways and ceilings in many old buildings aren’t just there for their aesthetic qualities. Arches are in fact one of the strongest building structures. In effect, every brick or piece of masonry within the arch is falling on all the others, distributing the weight evenly over the structure.

What happens when you add mentos to coke?

Caution: this rocket can fly up to 3-4m. Adult supervision required.

You will need:

Film canister with lid (clear film canisters where the lid presses inside work the best)
Baking powder
Vinegar
Teaspoon
Toilet paper

What to do:

Take the lid off the film canister.
Measure three teaspoons of vinegar into the canister.
Take a sheet of toilet paper and place it on top of the canister.
Press it down to form a small well. Don’t press it down so far that it touches the vinegar! The aim is to keep the two ingredients apart.
Place a teaspoonful of baking powder in the well.
Press the lid on tightly and trim the excess toilet paper from the edges.
Do not turn it over yet and definitely DO NOT SHAKE IT! You don’t want it to go off and hit you in the eye.
Find a clear, safe area outdoors. It is not a good idea to do it where a slumbering dog or nervous cat is in the vicinity. Come to think of it make sure grandma or granddad are not snoozing in the garden either.
Turn the canister over and place it lid-down on the ground then retreat to a safe distance i.e. at least 2m away.
Brace yourself. It will take about 20 seconds or so to launch. The advantage of the clear canister is that you can see the mixture bubbling up. This means that you don’t have to get impatient and approach it just as it shoots up.
Whe-hay! Thar she blows! If you’re lucky it will go over the shed much to the pleasure of any watching kids.

What’s going on?

Vinegar and baking powder react together to form carbon dioxide gas. The gas builds up until it forces the lid off the film canister. The gas pushes downwards which in turn (thanks to Newton and his Third Law) causes the canister to be forced upwards.

Are these forces balanced in a year 8 tug-of-war?

Magic Sand

What you need:

Magic sand
Ordinary sand (Hey ho it’s off to the beach we go!)
Teaspoon
Water
Small glasses
Washing up liquid

What you do:

Half fill each glass with water.
Add a couple of teaspoons of ordinary sand to the first glass.
Stir it up and bring a spoonful to the surface. It appears wet. The ordinary sand sinks to the bottom of the pot.
Add a couple of teaspoons of Magic Sand to the second glass.
Some of the Magic Sand sinks to the bottom of the pot. A small amount floats on the surface. Stir it up and bring a spoonful to the surface. It appears dry. The sand that has sunk appears silvery as if it has a coating around it.
See how the sand can be sculpted underwater to form different shapes?
Now add a few drops of washing up liquid to this glass. Stir it up. Can the Magic Sand be sculpted now? What has happened to the silvery layer? If you lift up a spoonful of the sand is it still dry? No it’s not!
Now look at what happens with vegetable oil.
Add two teaspoons of Magic Sand to another empty clean glass. Now add two teaspoons of vegetable oil. See how the vegetable oil soaks into the Magic Sand? Now you can lift out a spoonful of the sand and it no longer appears dry.

What’s going on?

Ordinary sand grains are hydrophilic (water-loving). This is because the surface of ordinary sand contains hydroxyl (OH) groups which are attracted to the OH groups in water molecules.

Magic Sand has been treated with a chemical called trimethylhydroxysilane. After this treatment the surface of the sand contains CH3 groups instead of OH groups. This makes the Magic Sand hydrophobic (water-hating). The Magic Sand repels water and so a layer of air is trapped around the Magic Sand as it sinks. This forms a bubble around the Magic Sand and the silvery layer is caused by the curved surface of the bubble.

Although Magic Sand hates water it loves oil (oleophilic) and so the vegetable oil is absorbed easily and the sand is ‘wet’.

Washing up liquid is a detergent which means that it contains molecules with both a water-hating and a water-loving end. When it is added to the Magic Sand in water it attaches to the water-hating groups on the surface of the sand. The water-loving end is attracted to the water molecules and so the Magic Sand is able to be wetted.

Magic Sand was originally developed as a way to trap oil spilled from oil tankers near the shore. It was developed by chemists at Cabot Corp. with the idea that it could be used to cleanse water of oily contamination. When sprinkled on an oil slick, magic sand attaches to the oil, adds weight, and sinks.

For more experiments with Magic Sand

Magic eh?

Bags of polymer fun

What you need:

A plastic carrier bag.
A pair of scissors.

What you do:

Use the scissors to cut a nick in the carrier bag, between the handles. Try to tear the bag from top to bottom - it's very easy, right?
Now cut a nick in the side, and try to tear it from right to left (or vice-versa). Hard, eh?

What's going on:

Carrier bags are usually made out of polythene, which is a polymer - long chains of carbon atoms. The chains mostly line up, and 'cross- link' to each other.
However, those cross-links are much weaker than the immensely strong bonds along the chains. Thus, while it's easy to separate the chains (tear down the bag), it's harder to split them in two (tear across the bag).
Why, then, are the chains arranged vertically in the bag? Because the chains are much stronger along their length, so the bag can carry a heavier load without stretching.
Most of the time, that works very well. But the designers of carrier bags are nevertheless making a trade-off, since even a small tear in the bag can cause a run, separating the chains. And we all know what happens then.

Build a weather vane

A change in wind direction often indicates an imminent change in the weather. Be prepared for sudden change by making this weather vane. Then sit smugly under your umbrella as everyone else races off the beach in the downpour.

You will need:

a long pin e.g. a map pin
scissors
ruler
glue stick
thin, coloured card
drinking straw
2 pencils with eraser
compass

What to do:

Make a sandcastle. Alternatively if you’re not on the beach you will need to anchor an upturned yoghurt pot firmly.
Make a hole in the centre by inserting the pencil, sharp end first. Make sure that it is firmly in place.
With another pencil and a ruler, draw two large triangles and four small ones on the coloured card. Then cut out the shapes.
Place the small triangles on the base (of the sandcastle or yoghurt pot) as if they were the points of a compass. You may need to weigh them down (pebbles are very handy if you’re at the beach) or stick them to the bottom of the yoghurt pot.
Cut short slits in each end of the straw and insert one large triangle in each end to make an arrow-shaped "vane." Both points of the triangles should be pointing in the same direction.

Push the pin through the centre of the straw and into the eraser on the pencil sticking out of the sandcastle or pot. Make sure the vane swings round easily.
Watch the vane swing in the wind.
Finally, use your compass to determine East, West, North and South, and then label the small triangles accordingly. Now you can tell which direction the weather vane is pointing.
Sit with your back to the wind before eating your picnic!

What's going on?

Weather vanes are one of the oldest of all weather instruments; working by swinging around in the wind to show which direction it is blowing from. Traditionally, weather vanes had a religious importance and appeared in the form of weathercocks on church roofs as early as the 9th Century AD. The head of the cockerel would point into the wind, indicating the direction the wind was blowing from.

The direction in which the vane points indicates the direction from which the wind is blowing. For instance, in a westerly wind, the vane points "West."

The air is nearly always in motion, and this is felt as wind. Two factors are necessary to specify wind, its speed and direction. The direction of wind is expressed as the point of the compass from where the wind is blowing. Air moving from the north-east to the south-west is called a north-east wind. It may also be expressed in degrees from true north. A north-east wind would be 45°. A south-west wind would be 235°.

For more interesting facts about the wind
http://www.weatherwizkids.com/wind1.htm and
http://www.rcn27.dial.pipex.com/cloudsrus/wind.html

For more details of how to measure the wind
http://www.rcn27.dial.pipex.com/cloudsrus/measurewind.html

And if that has whetted your appetite for the weather then checkout
http://www.weather-climate.org.uk/05.php