eksperiment phsics

Balloon powered boat

How fast can your boat travel? Follow these instructions to make your own balloon powered boat and learn about Newton's third law of motion.

• What you need • What to do

Turn a margarine container into a balloon-powered jet boat? It's easy.

What you need

To do this activity you will need to gather:

• a margarine container • a balloon • a straw • a rubber band • plasticine • scissors • something that will pierce the margarine container to make a hole big enough to fit a straw

through.

What to do

1. Find a clean, rectangular, margarine container and carefully make a hole in the centre of one of the shorter sides about 1 cm from the bottom.

2. Cut a straw in half and insert one end into the neck of a balloon. Fix the balloon firmly to the straw with a rubber band.

3. Push the straw through the hole in the marg container and seal it in place with plasticine. Weigh the back of the marg container with more plasticine in the centre. Blow the balloon up through the straw and pinch the end to keep the air inside.

4. Put the boat in the water, let go - and away she goes.

This is an example of Newton's third law of motion - every action has an opposite and equal reaction. The air rushing out of the straw is the action, and the equal reaction is the push against the boat in the opposite direction.

Can you improve the design of your boat?

Shot put 

Get your fill of water-balloons and science fun! Illustration by: Ed Radclyffe Follow these instructions for the shot put activity and learn how physics and sport go hand in hand.

• What you need • Competition • Sport 'n' science • Secret scientific advantage

The physics of the physical has a long history. Understanding how the body works allows us to develop techniques and equipment that helps athletes improve their performance. So, why not use innovation and know-how to add scientific advantage to your own backyard sport?

What you need

To do this activity you will need the following:

• water balloons • water • measuring tape • outdoor space.

Competition

Fill your water balloons with water until they are about the size of tennis balls. These will be your ‘shot’. Holding the shot to your neck, push it up into the air by extending your arm. Measure how far away it lands, using the puddle of water from the impact as a marker. The furthest throw wins.

Sport 'n' science

The path your shot takes as it sails through the air is a lovely curved arch that stretches from your hand to the ground. This path, called a trajectory, is ‘parabolic’ - an arched trajectory.

When you push your shot into the air, it wants to keep moving in that direction - upwards and onwards. Gravity, as usual, has to get in on the action. Gravity accelerates your shot downwards,

and it's mostly the combination of your throwing power and gravity that creates the curved path of the shot as it falls to the ground.

Secret scientific advantage

With practice, you can create momentum before you even let go of the shot. Start by facing backwards and then spin around as you throw, using your body like a spring. In this case, you get momentum from your body weight and the speed of the direction you move it in - and the momentum can move from you to the shot. By using body weight and leg strength in your throw, your shot will have more force.

By Beth Askham

Balloon surfing

Let's go surfing now... Follow these instructions to surf on balloons and learn about the difference between force and pressure.

• What you need • What to do • What's happening

Who needs a beach when you can surf on balloons? Try this activity at home or at school.

What you need

To do this activity you will need:

• an adult assistant • balloons • an upside-down desk or some other flat-bottomed object that can survive you standing on it • a carpeted floor • a table, pole or wall you can use to help yourself balance.

What to do

1. Check that there is nothing sharp on the desk or floor that could damage a balloon. 2. Half-inflate four balloons and tie them off. 3. Place the balloons under the corners of the desk. 4. Have your assistant hold the desk still. They shouldn't try to take the weight, just help keep

it balanced. Make sure they do not put any of their fingers under the desk - they might get squashed!

5. Carefully step up onto the desk. You can use another table or a pole to help you balance as you climb up. Unless something sharp bursts them, the balloons should be able to support your weight.

If there are other people around, you could try testing to see how many people the balloons can support.

What's happening

You should find the balloons can support the combined weight of you and the desk. You may be able to fit several people onto the desk before the balloons burst. My personal record is 27 seven-year olds. I used four balloons.

The reason we can stand on the balloons like this is that although we apply a fair bit of force to them, we don't apply a lot of pressure.

When we think of pressure, we often think of a force. To a scientist, pressure is a force divided by the area it is spread out over. The greater the area, the lower the pressure.

The shape of a balloon is determined by pressure. The air inside the balloon feels pressure from the balloon rubber squeezing in. The air outside the balloon is also squeezing in. For a balloon to stay inflated, the air inside a balloon must push out. The shape and size of a balloon is where the pressure inwards and the pressure outwards are equal.

If you look closely at a balloon as you put it under the desk, you will see that it flattens out where it touches the desk. As it flattens out, the area of the balloon that is touching the desk increases, so the pressure of the desk on the balloon goes down.

When the balloon flattens out enough, the pressure of the air inside the balloon equals the pressure of the desk on the balloon and the balloon can support the weight of the desk. The same thing happens where the balloon touches the floor.

When you stand on the balloons, they stretch and flatten out even more, until the pressure is equal again.

There is a limit to how much the rubber in the balloons can stretch. If you add enough weight to the desk, the rubber will eventually stretch to its limit. Once this happens, adding any more weight will increase the pressure inside the balloon. This will start to tear the rubber.

Once a tear starts, the air rushing out will tear the balloon even faster, and the balloon bursts. This is why we only half-inflated the balloons, so they would be able to stretch without bursting.

Staying up there

The balloon stays in the air stream through a balance of forces. Follow these instructions to make a balloon hover in mid air and learn about forces and air pressure.

• What you need • What to do • What's happening

What you need

To do this activity you will need:

• a balloon • a hair dryer.

What to do

1. Blow up the balloon and tie it off. 2. Turn on the hair dryer and point it so the stream of air is blowing towards the ceiling. 3. Hold the balloon in the stream of air and then let go.

If you've never done this activity before, you'd probably expect the balloon to float up and then fall to one side. What actually happens is that the balloon hangs in the middle of the air stream.

Diagram of the 'staying up there' activity.

What's happening

The balloon hangs in the stream through a balance of forces. The downward pull of gravity is balanced by the upward push of the air flow.

The balloon stays in the centre of the air stream because the fast moving air has a lower pressure than the surrounding still air. If the balloon starts to move out of the air stream, the higher pressure of the surrounding air pushes it back.

Try tilting the hair dryer a bit. What happens now?

Bottled balloons

Try blowing up a balloon in a bottle. What happens? Follow these instructions to make your own bottled balloons and learn about how air takes up space.

• What you need • What to do • What's happening

What you need

To do this activity you will need to gather:

• two balloons • two plastic PET bottles (1.25 L soft drink bottles work well) • a metal skewer, large darning needle or a knife.

What to do

1. Ask an adult to make a small hole (less than one centimetre across) in the bottom of one of the bottles. Be very careful when you do this because PET plastic is extremely strong.

2. Stretch the neck of a balloon across the opening of each of the bottles. Gently push the balloons so they sit inside the bottles.

3. Try and blow the balloons up in the bottles. Which one is the easiest to blow up? 4. Take the PET bottle with the hole in it and place your finger tightly over the hole. What

happens when you try and blow it up now? 5. Take the PET bottle with the hole in it and blow the balloon up inside it. At the end of your

last breath, place your finger over the hole and take your lips off the bottle. What happens to the balloon?

6. Gradually release the pressure of your finger. What happens to the balloon now?

What's happening

You probably found that you could blow up the balloon in the bottle with the hole in it. You wouldn't have had much luck with the other one though. Don't worry! There is nothing wrong with your lungs. It's got to do with space.

Air takes up space just like anything else. When you blew up the balloons in the bottles, you were trying to push air into a limited space. There wasn't enough room for the air in the sealed bottle as well as the extra air you were trying to blow into the balloon. And your lungs certainly don't have the power of an air compressor so you can't blow much extra air into the balloon in the hole-less bottle.

You could blow up the balloon when the hole was uncovered because air escaped from the hole and made room for all the extra air you blew into it. So you were blowing some air in the top but some was also escaping through the hole.

You saw the reverse happening when you blew up the balloon in the bottle with the hole and then put your finger over the hole. As you released the pressure of your finger, the elastic sides of the balloon forced the air out of the balloon, returning it to its original size. So air was forced out the top but was replaced by air entering back through the hole at the bottom.

The top diver

Squeeze the bottle and see what happens. Follow these instructions to make your own top diver and learn about buoyancy.

• What you need • What to do • What's happening

What you need

To do this activity you will need:

• a one or two litre plastic bottle • water • a plastic pen top • a piece of Blu Tack or similar • a hook (optional).

What to do

1. Fill the plastic bottle to the brim with water. 2. Stick a small piece of Blu Tack onto one end of the pen top as in the diagram. 3. Put the pen top into the bottle. The Blu Tack weighs down one end so that the top stays

upright with a bubble of air inside. 4. Seal the bottle tightly and squeeze the sides of the bottle.

If you squeeze hard enough, the pen top sinks. Stop squeezing and it rises. Squeeze just the right amount and you can get the top to hang in the middle of the bottle.

What's happening

Diagram of the top diver experiment. Illustration: Alec Ellis

This is a clever demonstration of buoyancy. The top floats because it, and the bubble of air trapped in it, displace a greater weight of water than it weighs itself.

When you squeeze the size of the bottle, you put pressure on the air trapped in the top causing it to shrink in volume. The top and trapped air now displace less water and weigh more than the water.

Try adding a hook to the Blu Tack and use your top diver (also referred to as a cartesian diver) to lift objects off the bottom of the bottle.

Eggs ins and outs

Can you fit the egg in the bottle? Follow these instructions to trap an egg in a bottle and learn about air pressure.

• What you need • What to do • What's happening

Caution: This activity involves fire. You must have an adult present, plus water and other safety equipment that the adult feels is needed. Make sure no flammable items are nearby.

What you need

To do this activity you will need to gather:

• a hard boiled egg (boiled for 15-20 minutes) that has been allowed to cool • a glass bottle or jar with an opening at the top a couple of millimetres smaller than the egg • some newspaper • matches or a lighter • a sink to do the activity on • an adult helper.

What to do

1. Peel the shell off the hard boiled egg. 2. Place the pointy end of the egg in the neck of the glass bottle or jar. 3. Scrunch up a piece of paper and light it then lift the egg up and carefully (but quickly) put

the burning paper into the bottle and replace the egg.

What's happening

Thwump - the egg gets sucked into the bottle.

The burning paper causes the air inside the bottle to expand, pushing some of it out of the bottle. When the paper goes out, the air cools and contracts. The air pressure outside the bottle is greater and the egg is 'pushed' into the bottle.

To get the egg back out of the bottle, wash the ashes out of the bottle. Turn it upside down and position the egg with the pointy end back in the opening. Take a deep breath and blow in. Continue to hold the bottle upside down, wait and watch.

Film canister rocket

Preparing the fuel for blast off! Follow these instructions to make your own film canister rocket and learn about the chemical reaction that makes it blast off.

• What you need • What to do • What's happening

WARNING: This activity involves a flying projectile. Make sure you have an adult with you and wear eye protection (safety glasses). Do this activity outside or in a high-ceilinged building like a hall. Never launch with anything breakable above the rocket, especially your face. Never point your canister rocket at anything, except the sky.

What you need

To do this activity you will need:

• a film canister • baking soda (sodium bicarbonate) • vinegar (any kind will work, but white vinegar is easiest to clean up) • an ice cream stick or teaspoon • a plate, saucer, tray or similar • eye protection (glasses, sun glasses or safety goggles) • an adult.

What to do

1. Take the lid off the film canister. Before adding the ingredients, practise putting on the lid and placing it upside down as described in step 5.

2. Put on your eye protection. 3. Pour a small amount of vinegar, about 5 millimetres deep, into the body of the canister. 4. Using the teaspoon or icecream stick, place enough baking soda to fill the recess in the lid. 5. Hold the body of the canister in one hand and the lid in the other. Quickly and firmly press

the lid completely on, place the canister lid down on the plate and stand back. Make sure your plate is on a level surface. Your canister rocket will blast off seconds later. The exact timing will depend on the canister, temperature, amount of ingredients and how tightly you packed the baking soda in.

6. Have a close look at the lid and bubbling ingredients left on the plate.

What's happening

When vinegar and baking soda mix together, there is a fast chemical reaction. There are several products of the reaction, although it is the carbon dioxide gas (C02) that pops the lid off.

As more and more carbon dioxide is produced, the bits of carbon dioxide (called molecules) are squashed together and begin to push, or apply a force, on all the inside surfaces of the canister, including the lid.

Pressure is defined as a force over an area. In this case, it's the force of the carbon dioxide pushing over the inside area of the canister. As the carbon dioxide builds up, so does the pressure inside the canister. The pressure quickly pops the lid off.

A good way to understand what is happening is to take a deep breath in, seal your lips and slowly breathe back out into your mouth. Eventually your mouth cannot hold the pressure and your lips will unseal, letting some air out. Caution: don't overdo this as you can hurt your eardrums.

The carbon dioxide gas pushes down on the lid, although as it is sitting on the plate it can't go anywhere when it pops. The carbon dioxide is also pushing on the inside base of the canister (the top of your rocket) and this pushes it into the air.

Blow your own - garden hose - trumpet

Illustration by: Angelo Madrid Follow these instructions to make your own garden hose trumpet and learn about how wind instruments work.

• What you need • What to do • What's happening

Warning: This activity involves using a sharp knife. Ask an adult to help you.

What you need

To do this activity you will need the following items:

• garden hose • stanley knife • plastic funnel • duct tape or electrical tape • scissors.

What to do

1. Cut a 60-centimetre length of hose using the Stanley knife and rinse it clean. 2. Place the small end of the funnel into one end of the hose. If it doesn’t fit tightly, use the

tape to secure it in place. 3. Form the hose into a circle, leaving both the funnel end and the other end of the hose poking

out about 15 centimetres from where the hose crosses over to make the circle. Tape the hose firmly together where the two ends cross over.

4. You’re ready to blow! Hold the looped part of the hose in one hand and the plain end (the mouthpiece) in the other hand. Take a deep breath, tighten your lips, and ‘buzz’ (vibrate) them on the mouthpiece to start playing. Don’t point the funnel into anyone’s ear - the sound can get quite loud!

What’s happening

Wind instruments make sounds when you blow air into them. The air molecules move back and forth inside the instrument, creating sound waves. Brass-type instruments, such as trumpets, trombones and tubas, use lip vibrations on a cupped mouthpiece to create sound. In flute-type instruments, sound is made when air flows over the edge of the mouth-hole and passes over the air inside the flute, causing it to vibrate.

The sound produced by a wind instrument is affected by the shape and length of the instrument body. For example, with the garden hose trumpet, the larger the funnel you use, the louder the sound you can make. Try untaping the hose and cutting it a bit shorter, then remake the trumpet. How does it sound now?

Falling for science

Gravity acts on everything. Photo from www.sxc.hu Try these two activities to explore the effects of gravity on falling objects.

• Activity 1: Balloon in a box • Activity 2: Drop cup

In the following two activities you'll see how gravity affects falling objects. You may need an adult to help you put them together. Have fun and be careful!

Activity 1: Balloon in a Box

Warning: This activity includes the use of nails, a hammer, and a needle. Be careful with sharp objects. You may want an adult to assist you.

What you need

What will happen when you drop it?

To do this activity you will need:

• four pieces of plywood • a balloon • a lead sinker • a needle • rubber bands • tape • nails • a hammer.

What to do

1. Put together a rectangular wooden frame. 2. Using elastic bands, suspend a lead sinker from the top of the inside of the frame so that the

rubber bands are stretched. Attach an upward facing needle to the sinker. 3. Inflate a balloon and stick it to the top of the frame so the needle is suspended below the

balloon. 4. Try dropping the frame from a couple of metres high with someone there to catch it so your

construction isn't damaged when it hits the ground. What happens to the balloon?

What's happening

When an object falls the effects of gravity are cancelled out and an object experiences weightlessness. In this case, the frame and the objects inside become weightless. However, if the lead sinker becomes weightless, the elastic bands, which are stretched, will now pull the sinker up causing the needle to puncture the balloon.

Activity 2: Drop Cup

Many strange things happen with falling objects due to the effects of gravity being cancelled out. What's not happening in this experiment that you would expect to happen under normal circumstances?

It's best to do this experiment outside as it will make a mess.

What you need

To do this activity you will need:

• a styrofoam cup • a pen or pencil • water.

What to do

What a difference a drop makes!

1. Fill a styrofoam cup with water and put a hole in the bottom side of the cup with a pencil. As you'd expect, water pours out (as in the cup on the left in the image).

2. Now drop the cup full of water. It's a good idea to have a bucket under the falling cup to prevent a mess when it hits the ground.

What's happening

The water becomes weightless while the cup is falling and, for the duration of the fall, it no longer pours out of the hole.

See if you can come up with your own falling demonstrations that might produce other interesting effects.

For more hands-on activities, sign up for free Science by Email.

The invisible glass

Notice how the submerged glass appears invisible? Follow these instructions to make a glass appear invisible through the science of refractive indices.

• What you need • What to do • What's happening

This activity demonstrates that if light passes through two transparent objects with equal refractive indices, it will not bend or reflect light.

What you need

To do this activity you will need the following items:

• a glass bowl • a small glass • baby oil • a glass eye-dropper.

What to do

1.

Place the small glass inside the glass bowl. Pour baby oil into the bowl until the oil covers the glass. The refractive index of baby oil is very close to that of glass. The small glass should therefore appear practically invisible.

2. Gently place the glass eyedropper into the small glass. It should appear visible because the eye-dropper contains air - the refractive indices of air and glass are different.

3. Retrieve the eye-dropper, fill it with baby oil and place it back inside the glass. The eye-dropper should become almost invisible because the air has been removed. When light passes from the oil into the glass (and visa versa) it is only slightly bent.

Note: If you don't want to waste the baby oil, use the glass to pour it back into the bottle. Make sure you do this over the bowl so it doesn't get everywhere.

What's happening

Could Harry Potter's invisibility cloak really exist? Well, science can begin to explain how Harry's cloak could actually be invisible to the human eye!

For Harry's cloak to be invisible it needs to change the way light interacts with it. An object is only visible if it reflects or bends (refracts) light that lands on its surface. If an object doesn't reflect or bend light it becomes invisible!

When light travels from one material to another it usually changes speed. This change in speed causes the light to bend, and our eyes can detect the difference. For example, when light moves from the air into a raindrop and back out again, the light changes its speed in the raindrop, and bends. When a straw is placed in a glass of water, it may look 'broken' because the light reflecting off the straw in the water is refracted.

Each transparent material bends light by a particular amount depending on the material the light is travelling from. We call the amount of bent light the 'refractive index'. For example, if the refractive index of water were the same as air, light would not bend as it passes through. If light didn't bend as it passed through water we wouldn't see raindrops falling. They would be invisible!

Therefore, the refractive index of Harry's cloak would have to be the same as air to make it appear invisible. But, even if Harry's cloak were invisible, would Harry appear invisible too?

Harry's body would have a different refractive index compared to the air and his cloak. He would therefore appear visible, the same way that you are visible behind a glass window. For the cloak to make Harry invisible, Harry's body would also need to have the same refractive index as the air.

Interestingly, Harry's cloak would not be invisible if he wore it underwater. The refractive index of water is different to air, and therefore different to Harry's cloak - making it visible.

Cristy Byrne

Jelly optics

Which way does the light bend? Follow these instructions to make your own jelly lenses and have fun with bending light.

• What you need • What to do • What's happening

Let's make some simple lenses and see how they bend light.

Caution: This activity uses hot water and a knife. Have an adult assist you with this activity.

What you need

To do this activity you will need:

• a torch • a comb • two packets of jelly crystals, both the same flavour and preferably a light colour or clear. In

some parts of the world, jelly is called jello. • a rectangular plastic container, such as a lunch box, to use as a jelly mould • hot water • a spoon • a fridge • a dark room • a knife with a straight edge • a cutting board • a table • an adult.

What to do

1. Mix up your jelly, but only use half as much water as normal. You need to make very stiff jelly.

2. Pour the jelly into your mould and leave it to set overnight. 3. When the jelly has set, tip it out onto the cutting board. It is easier to tip out if you run a

knife around the edge and then dip the outside of the mould into warm water for about ten seconds.

4. Use the knife to cut the jelly into the shape of some lenses such as: • wide in the middle and thin at the ends • thin in the middle and wide at the ends

• semicircular. 5. Shine light from the torch through the lenses and look at how they bend the light. If you

place a comb between the torch and the lenses, you will get lines of light, which can make it easier to see the light bending.

6. After you have finished, you can eat the jelly.

What's happening

When light hits a boundary between two substances, like the surface of the jelly, it often bends. This is called refraction. When light goes from air into the jelly, it bends away from the surface of the jelly. When it goes from jelly into the air, it bends towards the surface of the jelly. When the jelly surface is curved, rays of light hitting at different spots on the surface will either spread out or move together.

If the jelly makes a concave lens (thick on the ends, thin in the middle) the rays of light will be spread out.

If the jelly makes a convex lens (thin on the ends, thick in the middle) the rays of light will bend towards each other, until they cross over and then spread out again. If you are lucky and your lens is just the right shape, you may even find the rays of light all cross over at one spot.

The strength of a lens depends on its shape and the material it is made from. Different materials will bend light by different amounts.

Jet balloon

How far will your jet balloon fly? Follow these instructions to make your own jet balloon and learn how aeroplane jets work.

• What you need • What to do • What's happening

What you need

To do this activity you will need

• a balloon (not blown up) • a short piece of drinking straw • a long piece of string • sticky tape • two friends.

What to do

Cartoon of the jet balloon experiment

1. Blow up the balloon and hold the neck to stop the air escaping. 2. Ask your friends to hold either end of your string so the string is taut. 3. Thread the straw onto the string and tape it to the balloon so that one end of the straw points

to the neck of the balloon. 4. Hold the balloon at one end of the string and let go of it.

What's happening

The balloon should fly to the other end of the string as it deflates.

Many years ago, a scientist named Isaac Newton realised that when an object applies a force onto another object, the other object pushes back with an equal force in the opposite direction.

When you let go of the balloon, the air is pushed out of the neck of the balloon. As it does, the air pushes on the balloon with equal force in the opposite direction, so the balloon is pushed along the string. The air comes out of the balloon faster than the balloon is pushed along, because the air is much lighter than the balloon.

All rockets work by using the same principle. In a rocket, fuel is burned in a combustion chamber, which is open at one end. As the fuel burns, it produces hot gases, which rush out the open end of the chamber. Since the gases are being pushed in one direction by the rocket, the rocket is pushed in the opposite direction with equal force.

Kaleidoscope

The art of reflections. Follow these instructions to make your own kaleidoscope and have fun with reflections.

• What you need • What to do • What's happening

The word kaleidoscope means 'seeing beautiful shapes'. Images of the outside world pass through a kaleidoscope to your eye but they change on the way. Through lenses and mirrors, the kaleidoscope bends and distorts light waves.

What you need

To make your own kaleidoscope you will need to gather:

• three rectangular pieces of mirror or mirrored cardboard cut to the same size (about 15 cm long and 4 cm wide)

• some tape • greaseproof, tracing or baking paper • plastic kitchen film or an overhead transparency sheet • scissors • sequins, confetti or coloured paper.

What to do

1. Tape the mirrors together along the long sides to form a triangular tube with the mirrored surfaces on the inside. It's easiest if you leave a two millimetre gap between each mirror.

2. Cover one end of the tube with greaseproof, tracing or baking paper. Secure this with tape.

3. Place the confetti, sequins or very small pieces of coloured paper onto the outside of the paper.

4. Stick a piece of clear plastic film or transparency on the very outside over the small coloured pieces to ensure they will not escape.

5. Point the kaleidoscope down at a bright light source. While looking through the viewing hole, rotate the kaleidoscope so the confetti or sequins move about. You will see beautiful shapes that constantly change.

6. You can experiment with different coloured cellophane filters, or increase the number of mirrors used to build the tube.

What's happening

A kaleidoscope is made from mirrors placed at angles to each other. The viewer looks in one end and light enters the other end. This light is reflected again and again by the mirrors.

Light bounces off a surface at exactly the same angle at which it hits the surface. As the tube is rotated, the tumbling coloured objects present the viewer with varying colours and patterns. These show up as symmetrical patterns due to the reflections in the mirrors. When the mirrors are set at 60 degree angles, it creates six sets of reflected images.

Light water

The light bends inside the water. Follow these instructions to try two activities that will introduce you to the principles behind fibre optics.

• What you need • What to do • What's happening • Applications

Working with real fibre optics is quite difficult as it requires fairly high tech equipment (including LASER light sources). However, here is an activity you can do at home that demonstrates the properties of optical fibres.

What you need

For this activity, you will need:

• a small soft-drink bottle with its label removed (one with straight sides works best) • a torch • milk • correction fluid (such as liquid paper) or paint • a very dark room (try to do the experiment when it is dark outside) • sink, bucket or jar • a thumbtack or safety pin • sticky tape • scissors • paper.

For this activity, we will need to make a thin, straight beam of light. Most torches produce a fairly wide beam, so you may need to use the paper to cover most of the end so only a thin beam of light comes out. Try for a beam with a width of one centimetre or less.

What to do

There are two parts to this activity. The first part shows how light bounces underwater.

The light beam is reflected back into the water.

1. Fill your bottle partway with water. 2. Add a drop of milk, to make it easier to see the path of the light through the water. 3. Place the bottle near the edge of a table in your dark room. 4. Hold the torch down fairly low and shine it up through the side of the bottle onto the bottom

of the water's surface. You should find the light travels up through the water, through the surface and into the air.

5. Keep the beam of light aimed at the same point on the surface, but slowly lift the torch up so the angle between the light and the water becomes smaller and smaller.

6. When the angle between the water and the beam of light becomes small enough, the light will not go through the surface any more but will bounce off it, like it was a mirror.

The next part shows how to trap light in a stream of water.

1. With the thumbtack, make a small hole in the bottle, a couple of centimetres from the bottom. You might find this easier if the bottle is filled with water so the sides don't bend when you push on them.

2. Empty the bottle, dry off the outside and paint around the hole with correction fluid or paint. Paint at least one centimetre in each direction, and a couple of centimetres downwards. This ensures that the beam of light only travels down the stream of water.

3. Fill the bottle with water until the water squirts out the hole in a steady stream. Make sure you have a sink or bucket set up to catch the water.

4. Shine your torch at the hole from the other side of the bottle. 5. As the water comes out of the bottle, it will look clear until it starts to break up into drops.

At that point, you may see some light glittering on the drops. Hold your finger in the water stream above the point where it breaks up. You should see a spot of light on your finger.

6. If you look closely, you should find that the point of light is actually below the level of the hole. The light has stayed inside the stream of water as it bent down.

What's happening

Both of these activities rely on an effect called total internal reflection.

The beam of light stays inside the stream of water.

When light hits a boundary between two substances, like the surface of water, it often bends. This is called refraction. In the case of water and air, light bends towards the surface of the water when it goes from water into air.

As the angle on the water side becomes smaller, the angle on the air side gets smaller even faster. When the angle on the water side is just right, the angle on the air side would have to be zero, so the light would be trying to go along the surface.

If the angle on the water side is any smaller, then instead of going through into the air, the light bounces off the surface of the water. This is called total internal reflection. For the surface between water and air, the critical angle is 48.5 degrees.

In this activity, the light stayed inside the stream of water because of total internal reflection. To start with, the light and the water both come out of the hole horizontally. As the water curves down, the light eventually hits the surface. Since the angle between the surface and the light is very slight, the light bounces off.

As the water keeps bending, the light inside it keeps bouncing off the surface, until the water starts to break up.

The same effect happens inside other clear materials such as long thin strands of glass or plastic, even if the strand curves or goes around in circles. This is called an optic fibre.

Applications

Doctors can look inside a person's body using a bundle of optic fibres connected to a television camera outside the body. This has lead to 'keyhole surgery', where doctors carry out complicated operations through small holes in the skin, instead of having to make large incisions.

Optic fibres are also used to carry information, like telephone calls. By sending the information as carefully controlled pulses of different coloured light, it is possible for a single fibre less than a millimetre wide to carry thousands of phone calls.

Optic fibres are becoming more important as scientists and engineers are developing technology called photonics, which is the study of ways to generate and harness light and other forms of radiant energy.

Can you match this star?

Moving matches Follow these instructions for a great party trick that uses capillary action and turgor pressure.

• What you need • What to do • What's happening • Applications

What you need

For this activity you will need:

• matchsticks • an eye-dropper • water • a plate.

What to do

1. Bend five matchsticks in the middle. Be careful not to break them.

2. Arrange the matchsticks on the plate so they are all touching, with the bends in the centre. It should look like a five-pointed asterisk.

3. Use the dropper to place three or four drops of water in the centre of the matches.

4. Watch the matches for a couple of minutes. What happens? 5. The matchsticks should straighten up a little bit, turning the asterisk into a star.

What's happening

There are actually two processes happening here. The first is capillary action.

The matchsticks are made of dry wood. Most of the water has been dried out from the cells of the wood, leaving empty space behind. There are also gaps between the wood cells. The surface tension of the water pulls the water into these gaps, so it is sucked into the wood of the matchsticks. This capillary action leads to the second process.

When you bent the matchstick, the cells and the gaps between them were squashed at the point where the wood bent. As the water filled the gaps inside and between the cells, the pressure of the water pushed out on the inside of the gaps, so they tried to expand back to their original shape.

When the pressure of a fluid inside an object pushes it into a certain shape, it is called turgor pressure. The turgor pressure was enough to slightly straighten the matchsticks, so they pushed against each other to form the star shape.

Applications

Living things use turgor pressure inside their cells to hold the cells in shape - like a balloon blown up hard inside a sock or washing-up glove. If plants do not receive enough water, they will go limp because there is not enough pressure in the cells to maintain their shape.

Eye Eye, Captain

Armed with science, pirates sailed the high seas. Illustration by: Ed Radclyffe Follow these instructions and learn how to harness the Sun's power.

The well-known image of a pirate wearing the sinister eye patch and a black hat emblazoned with skull and crossbones probably comes from literature rather than from reality. These days, the popular image of the pirate comes from the movies, and they tend to look more like Captain Jack Sparrow than 'old-school' pirates such as Peter Pan's Captain Hook.

There is no real evidence that pirates wore eye patches more frequently than other people. However, one theory to explain how pirates could have damaged one eye is concerned with constant navigation at sea.

Pirates roaming the high seas navigated using the coastline and the movement of the Sun and stars. The navigator would spend a great deal of time using a telescope, which is basically a tube with two magnifying lenses in it. While staring through the telescope during the day, navigators would constantly see sunlight reflecting off the sea. Prolonged exposure to this focused light could cause serious eye damage, perhaps even blindness.

In this activity, you can see the effect of focusing light from the Sun through a magnifying glass.

WARNING: This activity involves fire. Adult supervision is needed, and it's a good idea to have a bucket of water nearby.

What you need

To do this activity you will need the following items:

• magnifying glass • metal cooking tray • piece of paper • pencil • styrofoam cup • crayon • sunny day.

What to do

1. Use the pencil to make a tiny dot in the middle of the paper.

Illustration by: Ed Radclyffe

2. Place the piece of paper on the cooking tray. 3. Place the cooking tray on the ground or on an outside table. Make sure the area you are

using is clear of flammable materials. 4. Hold the magnifying glass between the Sun and the paper and adjust the height and angle

until you see a bright spot of light on the paper. 5. Move the magnifying glass back and forth, closer and farther away from the paper, until the

bright spot is a single small spot about the size of a pinhead. This is called the focal point. 6. Direct the bright spot onto the dot you drew on the paper. 7. Hold the magnifying glass steady and wait. 8. Repeat the activity twice, using a Styrofoam cup and a crayon instead of a piece of paper.

What’s happening

A magnifying glass is a single lens with two curved (convex) surfaces. The lens is shaped like two saucers placed rim to rim – it bulges on both sides in the centre and becomes thinner towards the edges.

Sunlight is very strong, but spread out. Although looking directly into the Sun is very dangerous for your eyes, you can still be outside in the Sun without being blinded. When light from the Sun travels through the lens of a magnifying glass, it is bent towards the centre of the lens and is concentrated, or focused, into a single point some distance from the other side of the lens. This point is called the focal point.

In this activity, intense light energy from the Sun is concentrated through the magnifying glass, generating heat at the focal point. The paper should have started to smoke a bit and then turn brown. If you hold the magnifying glass in place long enough, a hole will form. Some types of paper may even catch fire – so be ready!

By Philippa Rowlands

Poles apart

Does the water rise when ice cubes are added? Image by: Rebecca Kilburn Try this activity to see how melting ice and snow affect sea levels.

• What you need • What to do • What's happening

What you need

To do this activity you will need the following:

• a film canister filled with soil, with the lid on • two clear plastic glasses • water • two ice cubes • a marker pen.

What to do

1. Place the film canister upside down into one cup. This represents an island. 2. Half-fill each glass with water. 3. Place one ice cube on top of the ‘island’ and the other ice cube in the water in the second

glass. Mark the level of the water on each glass. 4. Once both ice cubes have melted, see whether the water level has risen.

What’s happening

The ice cube floating in the water has already shifted, or displaced, the water in the glass; so when it melts, the level will barely rise. But the ice cube on the land (film canister) will not displace the water until it melts and drips into it, making the water level rise.

Only the melting of land-based ice and snow (like Antarctica) will increase the sea level. The melting of floating ice (like the North Pole) will not affect the sea level much.

For more hands-on activities, join CSIRO's Double Helix Science Club.   

Splitting light

White light is made up of various colors. Discover how to split white light into a rainbow of colours, just as Sir Isaac Newton did hundreds of years ago.

• You will need • What to do • What is happening?

Astronomers are able to determine the composition of the the Sun by studying its light, with a technique known as spectroscopy. This technique can also be used to determine the chemicals elements present in a star or planet's atmosphere.

What you need

To do this activity you will need:

• sunlight • a piece of card with a one millimetre wide slit cut into the middle • a straight-sided glass filled with water • a sheet of white A4 paper.

What to do

1. Fill a straight-sided glass with water and tape the card onto the side of the glass. 2. Place the white sheet of paper close to a window where sunlight is entering. 3. Stand the glass on the paper with the slit facing towards the Sun. 4. The sunlight should pass through the slit and split into its colour components as it enters the

glass. The colours should appear on the paper.

What's happening?

White light is a mixture of many different colours. Sir Isaac Newton proved this more than 300 years ago when he directed a beam of sunlight through a slit and prism in a darkened room in 1666.

The prism bent, or refracted, the white light so that it fanned out into a rainbow (spectrum) of colours.

Splitting light using prisms is known as spectroscopy. Each chemical element has a unique signature when its light is split up.

Astronomers use spectroscopy to determine what planets and stars are made of by examining their light.

Backyard bergs

How buoyant are icebergs? Photo by: Tanya Patrick/CSIRO/AAD Follow these instructions to make your own icebergs and learn about the buoyancy of ice.

• What you need • What to do • Colourful bergs • Keeping track of icebergs

An iceberg is a floating piece of freshwater ice that has broken off the seaward end of a glacier or polar ice sheet. The separation or calving of icebergs from glaciers mostly occurs during spring and summer in Greenland and Antarctica.

Only about 11 per cent of an iceberg is visible above the water’s surface because ice is only slightly less dense than water. The ice only just floats; the larger part of the ice sinks beneath the surface. You can make your own backyard bergs to test how much ice floats above the water and how much hides underneath.

Note: This simplified activity only uses height, and not volume, to demonstrate that usually more of an iceberg is underwater than above water.

What you need

To do this activity you will need the following items:

• a balloon • a plastic bag (zip-lock bags work well) • a rubber band • a large bowl, bucket or fish tank • a tray of ice-cubes • a ruler • a calculator.

What to do

This activity involves making two icebergs: one in a balloon, and one in a plastic bag. You could make lots of other different shaped icebergs using a variety of balloons, bags and containers.

1. Fill the balloon with water until it is about the size of a grapefruit. 2. Tie off the end of the balloon and place it in the freezer.

3. Repeat steps 1 and 2 using the plastic bag, except this time, seal the top using the rubber band. Be careful not to overfill the bag.

4. Wait for 12 to 24 hours to fully freeze your icebergs. 5. Fill the bowl, bucket or fish tank with cool water. 6. Add the tray of ice-cubes and stir until they have melted. 7. Take your icebergs from the freezer and remove the balloon or bag. 8. Place the icebergs on the sink and measure the height of each one. 9. Place the icebergs in the bowl, bucket or fish tank and measure how much of your iceberg is

floating above the water. 10. Put your results into this easy calculation: (height above water) divided by (total height)

multiplied by 100 = percentage of ice above the water. The answer should fall somewhere between 11 per cent (1/9 of the height) and about 12.5 per cent (1/8 of the height) of your iceberg floating above the water. (Note: to make this calculation easy, only height is taken into account and not the volume of ice or water, which is more complicated).

Colourful bergs

Most icebergs look white, because they are full of tiny bubbles. All wavelengths of visible light are reflected off the bubbles in the ice in equal amounts, which makes the iceberg look white.

Icebergs can also be blue, green, brown or black. In blue icebergs, the ice is compressed so much that the air bubbles are pushed out. Without air bubbles, blue light is reflected and the other wavelengths of light are absorbed. Sometimes icebergs can appear striped blue and white; this happens when crevasses in the parent glacier fill up with meltwater that refreezes so fast, that no bubbles form.

The unusual and vivid green of some icebergs is a result of algae growing in the ice. When you see a green iceberg, you are actually looking at what was once the underwater side of the iceberg. It has rolled over, exposing the previously underwater sections to view.

Brown or black icebergs are just dirty. Dust, rocks and dirt can accumulate in the glacier as it travels over the land. When an iceberg breaks off the glacier, it can have dirt layers deep within the ice giving it a brown or black appearance.

Keeping track of icebergs

The US National Ice Centre monitors all icebergs and ice conditions in the Antarctic, Arctic, Great Lakes and Chesapeake Bay regions. The data is collected by polar orbiting satellites, and information about iceberg size and position is relayed to ships. Thanks to this early warning system, collisions with icebergs have become a very unusual occurrence.

By Philippa Rowlands

Sugar and eggs

Which is the sugary water? Use an egg to find out which glass of water contains sugar using the science of density.

• What you need • What to do • What's happening

Someone puts two glasses of water in front of you and tells you one is fresh water, the other is very sugary water. You're given an egg and asked to determine which glass contains the sugar water without tasting the water. How would you do it?

What you need

To do this activity you will need to gather:

• an egg (uncooked) • sugar • hot water • two containers, such as glass jars.

What to do

Pour about seven centimetres of water into each jar. Add a tablespoon of sugar to the water in one jar then stir until it has dissolved. Repeat this until no more sugar will dissolve into the water. This is called a saturated solution.

Carefully lower the egg into each jar. Does it float or sink?

What's happening

By adding sugar to water you increase the density of the water. Very sugary water is denser than an egg, while fresh water is less dense than an egg. This means that the egg will float in the sugary water and sink in the fresh water.

Try it yourself with a range of sugary solutions. Can you figure out a way of creating a solution which is sugary at the bottom and fresh at the top? The egg would then 'float' midway 

Spooky screaming balloon

Can you make the balloon scream? Illustration by: Ed Radclyffe Follow these instructions to make your own screaming balloon and learn about inertia.

Make a seriously spooky sound. Feel the hairs stand up on your neck.

You will need

To do this activity you will need the following items:

• Balloon • Hex nut - this is a metal nut with six sides.

What to do

1. Before you blow the balloon up, squeeze the nut inside the balloon. Then blow up the balloon and tie off the end.

2. With your whole hand firmly holding the top of the balloon, swirl it around and around. The nut should start to spin around inside of the balloon.

3. Stop spinning and listen.

What's happening

The spooky sound is made when the sides of the hex nut scrape quickly around the inside of the balloon. This makes the balloon and hex nut vibrate, and this vibration is sound that travels through the air to your ears.

The hex nut keeps spinning inside the balloon even when you have stopped spinning it, as it wants to keep doing what it is doing - spinning around and around. This is called inertia. The hex nut will keep moving at the same speed and in the same direction until the force of friction slows it down and it falls back to the bottom of the balloon.

The soap-propelled boat

Watch it go, with bubble power! Photo by: David McClenaghan Make a boat that uses detergent to move through the water and learn about surface tension.

• What you need • What to do • What's happening

What you need

To do this activity you will need the following:

• thick cardboard or thin plastic (like the lid of a margarine container) • scissors • a large shallow bowl or a baking dish full of water • dishwashing liquid • a toothpick, pencil or eye dropper.

What to do

1. Carefully cut out a boat shape from the cardboard (around 8 centimetres long by 6 centimetres wide).

2. Cut out a triangular notch at the back of the boat with the triangle pointing towards the back of the boat.

3. Gently place the boat on the water in the dish. 4. Using the pencil, toothpick or dropper, place a drop of detergent into the notch at the back of

the boat. 5. Watch your boat zip across the water!

What's happening?

It looks like the boat is being pushed along by the detergent, but in fact it is being pulled by the water in front of it.

In many liquids, the molecules of the liquid are attracted to each other. This attraction makes the surface of the liquid act like a stretched-out balloon skin. Any point on the surface of a liquid is under tension. In water, the tension is only very slight and it is fairly easy to break through the

surface, but if you have ever done a 'belly-flop' into a swimming pool, you have felt the effect of surface tension.

Different liquids have different amounts of surface tension. A mixture of water and detergent (or soap) has much less surface tension than water.

Normally, surface tension pulls the boat in every direction.

When you add the detergent, the surface tension at the back is reduced. The tension at the front pulls the boat forward. The triangular notch helps the boat drag a little bit of the detergent with it, so it travels further.

You may find the boat only works once or twice. This is just because the water mixes with the detergent you have dropped in, so the surface tension of the water in the container is reduced and it can't pull the boat as well.

Static electricity

It's an attractive activity. Follow these instructions for a static electricity activity that demonstrates positive and negative charges.

• What you need • What to do • What's happening • Applications

Make small objects fly up and stick to your balloon like magic, and find out about static electricity with this activity.

What you need

To do this activity you will need:

• a balloon • a woollen jumper or clean hair • torn up paper • pieces of Styrofoam • salt.

What to do

1. Spread out the torn-up pieces of paper, small pieces of Styrofoam and salt on a table. 2. Blow up the balloon and rub it on a woollen jumper or on your hair (your hair needs to be

clean and dry). 3. Hold the balloon near the things on the table. What happens?

What’s happening

Everything around us is made of tiny particles called atoms. Circling around atoms are even smaller particles called electrons. When you rub things together, some electrons rub off onto the other object – this creates an electric charge.

When you rub the balloon, electrons from the jumper move onto balloon. The balloon becomes ‘negatively charged’ because it has extra electrons. On the other hand, the jumper becomes ‘positively charged’ because it has lost some electrons.

The more you rub the balloon, the more static electricity you build up. The objects on the table react to the charge by becoming attracted to the balloon. After a while the balloon loses its charge, and the objects drop off.

Have you ever heard the phrase ‘opposites attract’? Things that have opposite charges attract each other better than a charged object attracting a neutral object (one with no charge). So when you rub the balloon with the jumper, there is a stronger attraction between the (positive) jumper and the (negative) balloon than between the (neutral) paper and the (negative) balloon.

Static electricity is different to the electricity used to power your lights at home – static electricity has no currents running through it and doesn’t use wires.

Applications

Photocopiers work by using static electricity. Inside a photocopier is a special drum that has a positive charge that can be affected by light.

When you copy an image, a light beam moves across the page. The light reflects off the white areas of the paper and hits the drum below. Where light hits the drum, it neutralises the positive charge. Dark areas on the paper don’t reflect light back onto the drum, so these areas of the drum remain positively charged.

Negatively charged black particles called toner are spread over the drum and stick to the positively charged areas of the drum. Positively charged paper is then passed across the drum, picking up the toner. Heating and pressing the paper fuses the toner to the paper, creating a copy of the original image.

Make a thaumatrope

Merge two pictures into one. Follow these instructions to make your own thaumatrope. Use the thaumatrope to explore persistence of vision and learn how movies work.

• What to do • What's happening

What to do

1. Cut a circle from a piece of cardboard. 2. On one side draw a head with an apple on it. 3. On the other side of the cardboard, draw an arrow (or pick two other related pictures like a

fish and a fishbowl). 4. Use sticky tape to attach the circle onto a straw. 5. Roll the straw back and forth between the palms of your

hands, while you look at the cardboard. What do you see?

What's happening

Modern movies use technology to produce an optical illusion and fool our brains.

When you look at a picture, your eye and brain retain the image for a fraction of a second after it has gone. This is called persistence of vision.

If you are shown more than ten pictures a second, your brain will merge the separate images into a series of moving images.

Motion pictures show 24 still frames per second that give the illusion of smooth motion. The brightness of an image also affects the length of time the image will remain in your brain.

Home made thermometers

What happens when it warms up? Photo from www.sxc.hu Follow these instructions to make your own thermometer and learn about expansion and contraction.

• What you need • What to do • What's happening

What you need

To do this activity you will need to gather:

• a tomato sauce or mayonnaise squeeze bottle* • plasticine or adhesive putty • a clear, narrow drinking straw • food colouring • a waterproof marking pen • water • a wooden skewer, wire, or pipe cleaner (optional) • an eye-dropper (optional).

*You can also use a plastic container or bottle with a water-tight lid but you will need to make a hole in the lid.

What to do

1. Half fill the bottle with water and add a few drops of food colouring. You may need to add more water depending on the size of your bottle and length of your straw.

2. Place the bottom of a straw in the bottle so it touches the water. The top of the straw should be sitting well above the mouth of the bottle.

3. Holding the straw in place, tightly seal around the straw and the top of the bottle with plasticine. Be careful not to crush the straw.

4. Blow a little air through the straw into the bottle so that the coloured water to rises up into the straw above the stopper. Be careful when you blow into the straw. If you blow too much air into the bottle a jet of water will squirt back at you.

5. If the water level in the straw drops, it means air is escaping through the seal. You need to make sure you have no leaks in your seal.

6. When there is water in the straw you may need to remove any air bubbles inside the straw by moving a skewer up and down in the straw.

What happens to the water level in the straw?

7. You may need to use an eye-dropper to add water to the straw so the water level is about five centimetres above the top of the bottle.

8. Mark the level of the water in the straw with a pen. 9. You have now calibrated your thermometer to room temperature. 10. Cup your hands around the bottle or place it near something warm in the room. Be careful

not to place plastic too close to a heat source or it will melt. What do you notice about the water level in the straw?

11. Place the bottle in the fridge and after about ten minutes take it out and look at the water level in the straw.

What's happening

The thermometer uses the fact that most things expand as they warm up and contract when they cool down.

If you raise the temperature of a gas, the particles that make up the gas absorb heat energy and begin to move faster. This causes the gas to expand.

When the air inside the bottle expands, the pressure inside the bottle increases, pushing down on the liquid inside the bottle and pushing more liquid up the straw. When you cool the air again, it loses energy and decreases the pressure. The coloured water will then be pushed back down the straw by the pressure of the air outside.

A simple bulb thermometer works on a similar principle involving the expansion of liquids.

Standard thermometers use alcohol. Liquid alcohol contracts upon cooling and expands upon heating. Alcohol has a lower freezing point than water, so it will measure temperature below freezing. Adults might have noticed that when some alcohols are stored in the freezer at home they remain a liquid.

A bulb thermometer uses a very small amount of liquid so that it changes temperature quite easily and the tube is extremely small, so slight changes are easily noticed.