Photo Source: easytwistballoons.com
1. Water-Proof Hanky
A great excuse threatening to pour water over your audience – but with a surprise Twist thanks to physics.
Ingredients
• Large glass
• Ashtray or similar
• Water
• Handkerchief
Instructions
1. Push the centre of the handkerchief into the glass, so that the edges are hanging over the outside of the rim of the glass.
2. Pour water into the glass, through the loose handkerchief. Make sure that your audience can see the water easily passing through the handkerchief into the glass. Keep pouring the water until the glass is roughly half full.
3. Pull the corners of the handkerchief so that the material is taut over the top of the glass. Hold the glass and handkerchief so that the material stays tightly stretched over the opening. For younger audiences you may like to say some magic words that make the hanky water proof.
4. Place the ashtray on the top of the glass and tip it all upside down, being careful to keep the handkerchief pulled tight.
5. Choose a likely suspect from your audience to threaten with a drenching!
Hold the upside-down glass and ashtray above their head, making sure that the glass is vertical and the handkerchief is tight. Remove the ashtray and voila! – Nothing happens! The water stays inside the glass.
How does it work?
This demonstration is based on surface tension and air pressure. When the handkerchief is loose, the water can pour through the gaps in the fabric. However, when the handkerchief is pulled tight, the water molecules can form a single surface or membrane across the handkerchief due to a property called surface tension. The air surrounding us is exerting a force in all directions - air pressure. When the membrane is formed on the surface of the handkerchief, it provides a uniform surface for the air pressure to act upon. The force of the surrounding air acting upon this membrane is sufficient to overcome gravity, and so the water stays in the glass.
Tips for Success
Don't try to substitute a paper tissue for the handkerchief as it won't work! If the glass isn't held vertically, some water may dribble out where the membrane attaches to the edge of the glass.
Serving Suggestions
This trick will work in almost any environment and with any age group.
Did You Know?
Galileo was among the earliest to demonstrate the existence of surface tension on water by showing that an iron needle can be floated lengthways on water, but not on its point.
2. Cloud In A Bottle
Now-you-see-it, now-you-don't! This is a good trick for people to try themselves, watching a cloud appear and disappear before their own eyes.
Ingredients
• Plastic bottle with cap (fairly flexible e.g. from most soft drinks)
• Water
• Match-stick
Instructions
1. Place a splash (~1 teaspoon) of water into the plastic bottle.
2. Light the match and make sure it is burning well, then drop it into the bottle.
3. Quickly screw the cap on, and squeeze the bottle with your hand five or six times (for larger bottles you may have to do it slightly more). You should see a cloud form in the bottle, and then magically disappear when you squeeze it.
4. Pass the bottle around the audience to give everyone a chance to experience it for themselves.
How does it work?
Clouds are formed when water droplets in the air cool and then collect on dust particles. In this demonstration, the dust particles were provided by the smoke from the match. The air inside the bottle was cooled by releasing the pressure after the bottle was squeezed. The temperature is changed by squeezing the bottle: the amount of air within the bottle is constant, but squeezing the plastic bottle changes the volume of the gas. Expanding the bottle causes a lowering of the air temperature – in this case, enough to cause the water gas to form a liquid – the cloud.
Tips for Success
Try adding a small amount of food colouring to the water – it can help to increase the visibility of the effect.
Serving Suggestions
This works well for small groups of people of all ages. It is particularly applicable in outside environments where you can actually see clouds and potentially discuss the science behind them.
Did You Know?
This demonstration involves building a small cloud chamber exactly like those used to first record the tracks of subatomic particles (alpha and beta radiation) by Charles Wilson in 1911. Wilson (who was born on St Valentine’s Day) was awarded the Nobel Prize in 1927 for this discovery.
3. Lifting Lemon
Levitate a slice of lemon using a few simple ingredients found in any pub.
Ingredients
• slice of lemon
• Four matches
• Pint glass
• Ashtray
• Water
Instructions
1. Pour water into the ashtray until the water is ~1cm deep.
2. Push three matchsticks into the slice of lemon, in the shape of a triangular pyramid, with the match heads together at the top point of the pyramid.
3. Place the lemon and matchsticks in the centre of the ashtray, so that they float on the water.
4. Light the fourth match and use it to light the other three together.
5. Invert the pint glass over the lemon and matches, letting it sit inside the ashtray.
6. Watch as the lemon slice magically levitates within the pint glass!
How does it work?
The simplicity of this trick belies the complexity of the physical processes that contribute to the effect…
Firstly, there is a simple air pressure effect caused by the expansion and contraction of the gas within the pint glass as it heats up and cools down. The heat from the three matches causes the air inside the pint glass to get hot. When all the oxygen within the glass is exhausted, the matches go out and the air inside the glass cools down. The cooler air takes up much less space, so water gets sucked up into the glass to take up that extra volume.
Secondly, the combustion reaction changes the species present, thereby changing the volume of gas within the glass. When the matches burn they consume the oxygen from the air within the pint glass. The products of this reaction are carbon dioxide and water. The water will be a liquid, thus there will be less gas in the glass, causing the water to be sucked up into the glass to fill the volume.
Tips for Success
The lemon slice needs to be thick enough to support the matches, yet still able to float on the water.
Serving Suggestions
This trick was first developed for use in pubs, and draws on ingredients commonly found in most pubs. However, there's no reason not to use it in other environments also. It is most suitable for demonstrating to small groups of onlookers – it won't attract a crowd by itself, but will maintain the interest of audiences that you are already engaged with.
Did You Know?
This demo is based on the methodology used by Joseph Priestley to demonstrate that
Oxygen is a component of atmospheric air, and to estimate the proportion of Oxygen in the air.
4. The Power Of Words
This demonstration challenges common preconceptions about forces, and demonstrates the strength of atmospheric pressure.
Ingredients
• Table with flat edge
• Ruler
• Newspaper
Instructions
1. Lay the ruler over the edge of the table so that ~1/3 of its length is over the edge.
2. Ask your audience what will happen if you hit the ruler from above.
3. Hit the ruler – as expected it flips off the table.
4. Ask your audience how you might possibly keep the ruler on the table while you hit it, using only newspaper. Hopefully someone will guess that you need to exert an opposing force on the far end of the ruler – you may need to prompt them.
5. Tell your audience that you can only use a sheet of newspaper. Try first by folding up a sheet of newspaper as small as possible and placing it at the back end of the ruler so that it acts as a counterweight. Get an audience member to hit the ruler again – still it flips off the table, this time along with the folded up newspaper!
6. Ask your audience how else you might be able to use a sheet of newspaper to hold the ruler down. If your audience guesses the trick, ask them to explain the physics behind the idea. Lay a single sheet of newspaper flat on the table so that the ruler is roughly in the center. When you hit the ruler it will stay on the table!
How does it work?
It all comes down to air pressure. Atmospheric pressure is exerting a downward force on the single sheet of newspaper. The area of a single sheet of newspaper is fairly large; therefore the downward force of the atmospheric pressure exerted on the newspaper is strong enough to counter the upward force of hitting the ruler. It didn't work with the folded-up newspaper because the surface area over which the atmospheric pressure could act was far too small.
Tips for Success
For optimal effect, make sure as little air as possible is under the newspaper by smoothing it out flat prior to hitting the ruler.
Serving Suggestions
This works well with audiences of all ages, but does require a sturdy table or bench, so is best suited for performing when an audience has already been gathered.
Did You Know?
During the scientific revolution it was common to think of air pressure in terms of the total weight of a column of air pressing down on a unit area. In 1643 Evangelista
Torricelli, a pupil of Galileo, inverted a mercury filled glass tube, sealed at one end, into a basin also containing mercury. He found that the weight of air over the basin was sufficient to support a column of mercury to a height of 76 cm. This invention is the basis of using ‘millimeters’ of mercury’ as a unit of air pressure.
5. Magical Match
A very visual physics-related magic trick to wow your audience.
Ingredients
• Matchboxes (one empty)
• matches
Instructions
1. Lie the empty matchbox flat on the table, slightly open, with the empty section facing upwards.
2. Insert two matches into the empty matchbox so that the heads point out from the matchbox. Close the matchbox as much as possible so that the two matches are securely held.
3. Angle the two matches so that the heads are touching.
4. Rest a third match so that the head meets the other two, and the tail is resting on the table, pointing away from the empty matchbox. The three matches should now form a triangular pyramid shape with the three heads at the top point of the pyramid.
5. Use a fourth match to light the others from underneath.
6. Watch as the third match 'magically' rises off the table!
How does it work?
When the three match heads ignite they fuse together. In the case of the third match, the top and bottom surfaces of the match burn at different rates, causing a gradient in the surface temperature and tension across the match. The differing tensions across the third match cause an upward force to be exerted upon it. The two original matches are fixed in place, therefore the three match heads form a pivot point, from which the third match 'magically' rises.
Tips for Success
For the greatest amount of lift make sure that the matches are placed as symmetrically and as securely as possible.
Serving Suggestions
This is a good demonstration to perform for small groups, i.e. once you have attracted a small crowd.
Did You Know?
The same effect can be seen in many domestic thermostats. It can be amplified by attaching two strips of material together with different rates of expansion (a bimetallic strip). As the temperature rises, the greater expansion on one side of the strips causes it to curve further and further. This can be incorporated into a switch that will connect and cut off a heating apparatus when a particular temperature has been reached top view side view.
6. Cartesian Diver
Density and pressure effects are explored using a simple visual demonstration.
Ingredients
• Large plastic bottle (~2L) with lid, filled with water
• Glass filled with water (for testing the ‘diver’)
• Straw
• BluTack
• Scissors
Instructions
1. Cut the straw to ~4cm in length.
2. Use small blobs of BluTack to seal each end of the straw.
3. Check that the sealed straw just floats in the glass of water. It should slowly rise to the top of the water after you drop it in. If it sinks, remove some of the BluTack. If it floats too easily add some more BluTack.
4. Drop the straw 'diver' into the bottle.
5. Fill the bottle with water from the glass if necessary.
6. Screw the cap on the bottle.
7. Squeeze the sides of the bottle (fairly firmly). The straw diver will sink through the water. Let go of the bottle and the diver will float back up to the top.
How does it work?
The straw diver contains an air bubble sealed inside. The combined density of the straw, air and BluTack is slightly lower than that of the water, so it floats. When you squeeze the sides of the bottle you increase the pressure pushing on the air bubble, making it compress into a smaller space. This decrease in volume causes the air bubble to increase in density. Therefore the overall density of the straw diver will now be greater than that of the surrounding water, making it sink. Releasing the pressure (by stopping squeezing the bottle) allows the air bubble to expand back to its normal size, and so the straw diver will float again.
Tips for Success
Don't be scared about pushing hard on the sides of the bottle – they can take a lot of force. It may be easier to rest the bottle on a flat surface and then use both hands to push the sides.
If your diver keeps floating then take it out and add more BluTack, making sure to test it in the glass of water to be sure that it doesn't immediately sink – you'll find it easier to get it out of the glass than out of the bottle if it does sink!
Serving Suggestions
This is a good visual demonstration when you have a small crowd. However it works best if everyone can have a try for themselves, so it's not recommended for large crowds.
Did You Know?
Sperm whales regularly dive to depths of at least 1000m where the water pressure is on the order of 100 atmospheres. In the process, the rib cage will fold and collapse, and the lungs will compress down to one per cent of their size at the surface. The associated change in buoyancy is essential to keep the animal at such a great depth.
7. Lager Lamp
Traditional retro lava lamps are familiar to most people, but try this updated version in your local pub.
Ingredients
• Pint of lager (or other fizzy drink)
• Nuts (a small handful)
• Pint glass
• Spoon or other stirrer
Instructions
1. Drink some of the beer so that the glass is approximately 3/4 full.
2. Leave the beer for approximately half an hour to reduce the amount of bubbles – it may also be useful to stir the lager a few times.
3. Tip in the nuts.
4. Watch as the nuts gradually float up to the surface of the liquid, then fall back down again, just like a traditional lava lamp.
How does it work?
This effect relies on the gas contained within the fizzy drink. The nuts at the bottom of the drink have imperfect surfaces, where small pockets of air will form. The longer they stay still, the larger the bubbles of air that will accumulate. Eventually the bubbles will be large enough to provide enough buoyancy to counter the weight of the nuts, thereby lifting them off the bottom of the glass. When they get to the top, the bubbles burst, removing the buoyancy from the nuts, causing them to fall back to the bottom of the glass.
Tips for Success
This trick works best if the lager (or other fizzy drink) doesn't have TOO much gas in it.
If the nuts all float on the top then there is too much gas. Conversely, if the drink is too flat then they will mostly stay on the bottom – a great excuse to have another beer…
Serving Suggestions
Another pub-oriented trick, but one that will transfer to other venues very easily. If your audiences are too young to use beer then substitute any other fizzy drink – light colored ones are best, so that the audience can see the nuts moving within the liquid. This is a good demonstration to leave on the side for a while, and let your audience notice it while you're performing some of the other demonstrations.
Did You Know?
This demo has been in use for several centuries and was very popular at Versaille where a single raisin would be dropped into a flute of champagne and would then bob up and down all afternoon.
8. Shrinking Coin
This trick requires the audience to think very much "outside the box". Most people simply won't believe it can be done – but with some 3D thinking it's easy!
Ingredients
• 1x 1p coin
• 1x 2p coin
• Piece of paper (approx 10cm x 10cm)
• Pencil
• Scissors
Instructions
1. Lay the 1p coin in the centre of the piece of paper. Trace around it using the pencil.
2. Cut out the centre of the circle so that you are left with a piece of paper with a hole in the centre.
3. Demonstrate that the 1p coin slips easily through the hole.
4. Challenge your audience to get the 2p coin through the 1p-sized hole – without ripping the paper or altering it in any way. Give them some time to try (multiple coins & paper to distribute amongst your audience can help at this stage).
5. Show them how it can "really" be done: Take the piece of paper and bend it in half. Hold the paper so that the bend is at the bottom. Drop the 2p coin between the sides of the paper into the centre of the hole.
ii. Grasp the paper between finger and thumb near the bend, on either side of the coin. Slide your fingers upwards around the coin. Allow the paper to buckle outwards in the dimension perpendicular to the coin.
iii. The coin should slip through the hole!
How does it work?
This is all to do with non-Euclidean geometry. The small 2D hole may be stretched in the third dimension to produce a slit that is large enough to allow the larger coin through.
Tips for Success
Don't use the same piece of paper too often or it will develop permanent folds in it, which can cause the coin to get stuck and help your audience guess the solution to the trick!
Serving Suggestions
This is a good hands-on activity for younger audiences, where they can all have a go at cutting out the hole and trying the trick. However, older audiences also won't believe that it's possible. A good trick to use once you have already formed an audience.
Did You Know?
One of the more intellectually difficult aspects of string theory for the lay person is the concept of folded dimensions. If you are feeling brave, you might want to use this trick as a way to demonstrate how multiple dimensions can be folded into tiny spaces.
9. Extra Bounce
Most people are familiar with physics experiments performed with balls of different sizes. But this demonstration questions our intuitive understanding of the forces at work.
Ingredients
• Large ball that bounces (e.g. football or basketball)
• Small ball that bounces (approx 10cm in diameter)
Instructions
1. Hold out your hand at shoulder height. Ask your audience to imagine a scale from 0 to 10, with 0 being the ground and 10 being the height of your hand.
2. Pick up the large ball and hold it out at shoulder height. Tell your audience that you are going to drop the ball, and that they should judge (on the imaginary scale) how high it bounces.
3. Drop the ball and ask the audience how high it went (get a rough value).
4. Pick up the small ball and repeat the demonstration.
5. Hold the small ball on top of the large ball at shoulder height and ask the audience what they think will happen to each of the balls if they are dropped.
6. Drop the balls. The small one will shoot off much higher than the sum of the original bounces put together. Repeat the demonstration, asking the audience to closely watch the larger ball. You will see that it hardly bounces at all.
How does it work?
This experiment is all about conservation of energy and momentum. When the balls are dropped together most of the momentum from BOTH balls is transferred to the small ball. Both the kinetic energy and the momentum of any moving object depend on its mass. If the smaller ball receives all the kinetic energy and momentum from the larger ball it will bounce much higher than the original larger ball because it is so much lighter. Add to that the original energy and momentum in the smaller ball and you get a bounce that is much greater than the sum of the two original bounces.
There are also complications due to the materials used to make the balls ('bouncy' balls go wild!). This experiment can also be used as a good demonstration of Chaos effects – small changes in the initial conditions (e.g. exactly how the two balls are held above one another) can cause large differences in the end result.
Tips for Success
For indoor spaces use a small ball that isn't too bouncy or it will go crazy and could potentially do damage. You could try using a long ruler to help the audience judge how high the balls bounce (some audiences have difficulty with that part and don't get so involved).
Serving Suggestions
This is a good visual trick that can draw in audiences – especially in outside venues. In outdoor venues it can be a lot of fun to use a bouncy ball as the smaller ball – although you may have to chase after it quite a bit!
10. Egg-Citing Physics
Ever mixed up your eggs? Some simple physics will enable you to avoid that problem in the future…
Ingredients
• Raw egg
• Hardboiled egg
Instructions
1. Place the two eggs on a flat surface and set them both spinning.
2. Gently and briefly place your finger on the top centre of each egg.
3. Notice that the hardboiled 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.
How does it work?
Momentum is the key to this demonstration. A raw egg is filled with a liquid, whereas a hardboiled 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 hardboiled 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 hardboiled 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!
Serving Suggestions
This trick is of most interest to adult audiences as younger people generally haven't experienced the difficulty of distinguishing between raw and hardboiled eggs.
You may want to demonstrate that the egg really is hard or soft boiled by breaking it in front of the audience.
Did You Know?
There is a classic physics trick that involves placing a lit match or piece of paper into an old milk bottle, and setting a peeled hardboiled egg on top of the bottle. The egg is pushed into the bottle in a very similar way to the water being sucked into the pint glass in the ‘Lifting Lemon’ trick in this Pack. Unfortunately it is very difficult these days to locate old-style glass milk bottles so this trick seems to be going out of fashion, although it is possible to use a water balloon and a wide-necked bottle instead.
