Air Pressure Experiments

Sucking Water Through a Straw


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The Cartesian Diver

The buoyant force and gravity compete to determine where the Cartesian Diver goes:

You will need

1) A plastic soft drink bottle full of water and its lid
2) Plasticine
3) A Cup
4) A pen lid. A transparent one works best.

What to do

1) The pen lid will have a hole in it where the pen goes. Stick some plasticine around the hole, so that if you place it in water it will float with the hole pointing down and the tip just barely above the surface. If your pen lid has a hole in the tip, block it off with plasticine. If you are as equiptment challenged as i was when I filmed the video of this, you can experiment with hair ties, and rubber bands to weight the cap. Don't block the hole in the bottom of the pen lid.

2) Use the water in the cup to test if it is floating correctly.

3) Fill the bottle with water, right to the top.

4) Place the Diver in the bottle - make sure he stays right way up so he doesn't fill with water.

5) Screw the lid on the bottle.

If the diver sinks as you screw the lid on, then it just a little too heavy - remove a tiny bit of the plasticine from the diver.

What Happens When You Squeeze the Bottle?

Squeeze the bottle and the diver will sink. Release the bottle and the diver will float.

Dont forget to check out the video of the Cartesian Diver in Action

Be careful: If the bottle is shaken or turned upside-down, the bubble can escape the diver. You will need to take the diver out, shake the water out of it and return it to the bottle. You can pour the water into a bucket and then pour it back into the bottle

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Antigravity Water

How can you turn a full glass upside down and not spill the water?
Turn a glass of water upside down and the water always falls out. Gravity pulls the water towards the earth, right?
Is there a way to turn the class upside down without it spilling out?

What You Will Need

1) Large Bowl of Water

2) Food Coloring

3) Small Clear cup

What to Do

1) Make sure you’re doing this somewhere a spill wont cause a problem – there is a reasonable chance that water will hit the ground during this experiment.

2) Mix food coloring through the water.

3) Submerge the cup in the bowl, right way up so that it fills with water.

4) Keeping it fully submerged, turn the glass upside down.

5) Slowly lift the glass, but don’t lift the top of the glass above the surface of the water.

6) Now, carefully, see what happens.

Watch the video of the Antigravity Water in action

Why does the Water stay inside the glass?

When you lift the base of the cup above the surface of the water, gravity tries to pull it back into the bowl.

However, the pressure of the air pushing down on the surface of the water forces it to remain in the cup.

The atmospheric pressure at the surface of the earth can support a column of water approximately 10 meters high – above that height, even the pressure differential between the air and the vacuum that forms above the column when if drops cannot overcome the weight of the water.

When you lift the glass above the surface, the air can get in to the top of the glass by easily letting the water fall, so the glass empties.

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A Balloon that won’t Burst!

If you push a pin, or a skewer or some other sharp object into a balloon, it bursts. Well, actually, I can think of two cases where it won’t burst:

1) If the balloon isn’t inflated (ok, that’s a bit silly, but it turns out that it’s to the point). If you poke a hole in an empty balloon, then there is no explosion, no “bang” and no little pieces of rubber everywhere. This seems to fit the description of the balloon not bursting.

2) This is the more surprising case. Take you uninflated balloon and blow it up, but don’t blow it up too far. Once you have got a few breaths into it, pinch shut the end and examine the balloon. You’ll notice that around the opening and around another point roughly straight across the balloon from there, the rubber is unstretched.

You can tell it’s unstretched, because unlike the rest of the balloon, it is not translucent (i.e. it doesn’t let any light through), as rubber stretches, it gets thinner (just like if you stretch a piece of gum) and when it gets thin enough, it starts letting light through. If you poke a needle or some other sharp object into these dark areas, the balloon will not burst.
In my experience, I’ve found that barbeque type skewers work very well, with the added advantage that you can usually work them right through the balloon and out the other dark patch. The balloon does have a hole in tit now, and if you listen closely, you’ll be able to hear the air escaping it, especially if you pull the sharp object out again, but in many ways it keeps the properties of a balloon. The most important property for us is “burstability” because the people you show this too are going to want proof that it’s not a trick balloon: so while the balloon is still skewered, poke it with another skewer, on the thin side. It should burst nicely.

Why doesn’t this balloon burst? Why do balloons burst anyway? Balloons are made of rubber, which is an elastic material, meaning, if you stretch it, it pulls back.

In order to make a hole in the balloon, you need to push the skewer into the side of the balloon until the rubber in front of the point is so stretched that it breaks. Then two things can happen – either the balloon bursts, or it doesn’t. If the rubber you’re poking through is generally unstretched (like the end of an inflated balloon or an uninflated one), then the stretching due to the skewer is in a sense “local”, that is, only the rubber very close to the point is stretched and breaks., the rest of the rubber remains outstretched and holds together.

Around the hole the skewer made is a number of little cracks and tears in rubber. If the rubber is slack, these don’t spread and the balloon stays together. On the other hand, if the rubber is stretched, then it pulls on these cracks and tears and makes them larger and larger. Some of these tears very quickly become large enough that the balloon falls to pieces. This is when you burst a balloon by making a small hole in it, you still often end up with the balloon looking like it was torn to pieces.

Another way to prevent these tears catastrophically increasing is to reinforce the balloon in some other way. For example, if you put a strip of sticky tape on the balloon and carefully pierce the balloon through the tape, the tape should hold the tears together and kept he balloon in one piece.

But where does the bang come from?

The balloon is full of air at high pressure, held in by the balloon. Once the balloon is gone, there is nothing holding in the air, so it tries to spread out and equalize the pressure everywhere. This cannot happen instantaneously, so a “wave” of high pressure air spreads out from the balloon. Waves of high pressure air are exactly what sound is, so when this high pressure hits your ears, it “makes the bang”.

High pressure air balloon rockets and momentum.

It is the pressure of the air inside that holds the rubber sides of a balloon out, so it’s fun to play with. Balloons also give us another way to look at our last idea – air will try to move so that it equalizes any variation in pressure – when you blow up a balloon, but don’t tie off the end, the air will rush out so that the pressure inside and outside will be equal.
When the air rushes out the end of the balloon, it makes the balloon shoot forward – another interesting effect called conservation of momentum – one of the most fundamental physics laws in the universe. Momentum is measure of how much “motion” an object possesses. In a sense, it is momentum that determines how hard a thrown ball hits your hand. A ball that is moving fast will hit harder than a ball moving slowly. A heavy medicine ball will hit harder than a baseball (going the same speed). Physicists have conducted hundreds of thousands of these collision experiments and determined that moment (which is equal to mass times velocity) is never created nor destroyed. When you catch a ball, its momentum is transferred to your hand – if you’ve caught a ball moving fast enough, you’ll have noticed that it pushes your hand backwards, which pushes your body backwards.

You don’t go flying back as fast as the ball for two reasons:

1) you’re much heavier than the ball, so the velocity it gives you is much smaller than the velocity it had, and

2) the friction between you and the ground transfers the momentum into the earth, which is so heavy that the momentum going into it is unnoticeable (and pretty much cancel out the momentum taken from it by the person who through the ball in the first place)


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Sucking Water Through a Straw - Air Pressure and Fluids

If you lower the pressure in your mouth, the air pushes the top of the water up the straw!

When you suck water through a straw, what happens?

First we need to talk about pressure. The gas molecules that make up air wiz around and bump into things. Like a ball bouncing against a wall (or your hand) when the gas bumps into things, it pushes against them. This pushing is exactly what we mean by pressure – and even if you think you’ve never experienced, I’ll bet if we think about it a little, I can convince you of this. Let’s try a little experiment

1) Take a deep breath

2) Breath out slowly and steadily through your mouth

3) Before you’ve finished breathing close your mouth – but don’t let the air out through your nose either, but continue blowing out against the inside of your cheeks

Feel something pushing your cheeks out and trying to part your lips? That is air pressure. When you’ve got your mouth open, the air on the in and out sides of yourcheeks will be at the same pressure – so the air outside will push in with the same force as the air inside pushes out – so you won’t feel anything, but when your ribs squeeze the air out of your lungs, there’s more air jammed into the same space within your mouth – so it’s like having two or three or twenty (depending on how strong your lungs are) times as many balls bouncing against the inside walls – the pushing out is stronger than the pushing in, which stretches out your cheeks.

Another example of this is a balloon – the pressure of the air inside holds the rubber sides of a balloon out, so it’s fun to play with. Balloons also let us into another important idea – air will try to move so that it equalizes any variation in pressure – when you blow up a balloon, but don’t tie off the end, the air will rush out so that the pressure inside and outside will be equal. When the air rushes out the end of the balloon, it makes the balloon shoot forward – another interesting effect called conservation of momentum – one of the most fundamental physics laws in the universe. But it’s the tendency to equalize pressure variations that helps us drink through straws.

Now let’s try the reverse of our last experiment – keeping your mouth closed, suck your cheeks in. Your lungs are expanding, reducing the amount of air in your mouth – now it is the air outside that wins in the contest to push you r cheeks. But as you suck your cheeks in, air will sneak in to your mouth, pushing your lips apart to do so. Air is that determined to bring back the equality of pressure, that it will move parts of your body!

So how is this related to drinking? When you suck through a straw, you’re doing exactly the same thing – but instead of pushing your cheeks in, the air outside has found a better way to get into your mouth – by pushing down on the top of your drink so that it shoots up the straw into your mouth. In order to get into your mouth, the air is willing to push all the drink in your glass all the way up to your mouth!

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When Two Straws are Worse Than One

Here’s another experiment

1) put two straws in your mouth

2) Place the other end of one of the straws in your drink, but leave the second straw in the air

3) Try to drink!

You’ll notice that no matter how hard you try to suck up the drink, all you end up getting is a mouthful of air from the second straw. Although nature is determined to equalize the pressures, it’s lazy – pushing air up through the free straw is much easier than pushing fluid, so only air flows into your mouth.

4) now, if you were lucky, or especially clever, at the last step, you might have succeeded at getting a drink – if you use part of your lip, or your tongue to seal the end of the free straw tightly, air will no longer be able to go up that straw, and you’ll be able to drink

5) Challenge others to “two straw drinking races” – but don’t tell them the secret. Use this to impress your friends, family and coworkers

A subtler version of this trick is to make a very tiny hole (or have an adult make the hole if you’re not allowed to use knives) in the side of a drinking straw about half way up. Now, if you try to drink with this straw (with the new hole outside your mouth and above the top of the drink) , you’ll find that you just get air – now try it with your finger over the hole

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Water The Magic Can - a demonstration of pressure


1) Small coffee can

2) Coffee can lid

The pushing force of air is called air pressure. The closer you are to Earth, the greater the air pressure. The farther away from Earth (in other words the higher your altitude), the less the air pressure. And remember, pressure is coming from all around us.

What to do:

1) Take the coffee can and punch 3 small holes in the bottom. Also punch one hold in the plastic lid.

2) Now fill the coffee about 1/2 full of water and put the lid on.

3) Place your hand over the hole and press down on the lid. Notice how the water streams out of the holes on the bottom due to the pressure you are exerting on the lid.

4) Now slowly stop applying pressure to the lid. Notice how the stream of water stops. You can stop and start the flow of water simply by removing you finger from the hole. (Now would be a good time to hand the can to one of your parents...)

5) When you filled the can only half full, you left some space empty. This space actually was not empty - it was filled with air. Pressure on the lid exerted pressure on this air which in turn exerted pressure on the water forcing it out of the can.
When you stop pressing on the lid, and leave your finger over the hole, the pressure of the air outside the can holds the water up from the bottom.

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Many years ago WHAM-O sold a plastic air-puff gun. The puffs of air could fly across a room and knock over cardboard targets.

It turns out that this gun used ring-vortices, or "invisible smoke rings" as its ammunition. Also turns out that smoke-ring guns are extremely easy to make.

What you need:

1) A soup can

2) A piece of cardboard

3) A balloon

What to do:

1) Take a soup can, cut out the top and bottom, tape a piece of cardboard over one end, and cut a 1" hole in the center of the cardboard.

2) snip a balloon in half and stretch it across the other end.

3) When you gently whack the covered end of your vortex launcher, a transparent ring of spinning air will shoot out of the hole. Aim the device at your face or arm, and you'll feel the puff of air when it hits your skin.

4) The vortex rings can be made visible with a bit of smoke. I use stick incense, and just shove the end of the stick into the hole for awhile (don't set the cardboard on fire!!)

5) Tap the bottom gently, and slowly spinning smoke rings will be launched. Tap it hard, and the smoke rings will zoom so fast that you'll only see a grey blur. Tap it too hard and you generate air turbulence but no smoke rings.

To see the details of the smoke rings it helps to have bright lights and a dark background. Work in a darkened room while placing your device between you and a bright table lamp. The light should shine towards you, through the smoke, but position things so you observe the smoke against a darkened wall. Smoke rings are similar to tornadoes, but the ends of the tornado is curved around so its ends are joined into a circle.
Try shooting slow rings then immediately shoot faster ones. The faster ones will catch up to the slower ones and move through them (the slower ones open wider to allow the fast ones to pass.)
Rather than using smoke, you could instead use scent. Any fumes in the can will end up inside the air in the smoke ring. Try putting perfume in the can. When you launch your ring vortices, they will be invisible. But if you target a distant nose, your victim will know when they've been hit.

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More Air Pressure Experiments

Two Separate Experiments:

First experiment:

First we'll show that there is air pressure pushing on us, from every direction while we're on this Earth.

You will need:

1) Newspaper,

2) ACE' hardware yardstick (1/8" thick)

3) a flat table

What to do:

1) Place a thin yardstick on a flat table with a little less than half of it hanging off of the edge of the table.

2) Place a sheet of newspaper over the yardstick flat against the table (have as little air as possible under the paper) so that the fold line of the newspaper is at the yardstick.

3) Quickly strike the end of the yardstick hanging off the edge of the table. If you strike it quick enough, the yardstick will break near the table edge.

The Earth is covered in a layer of air that is nearly 80 miles thick and at sea level (the bottom) exerts or 'pushes' almost 15 pounds of pressure per square inch. That means that a full sheet of newspaper laid out flat has nearly 9,300 pounds of air above it. When you break the yardstick above, you are able to break it because of that 'heavy' air pushing down on the paper while you quickly strike the yardstick. Initially, the table is pushing back on the paper, and if you move the yardstick quick enough, other air around the edges of the paper can't get under the paper fast enough, so you are trying to lift that 9,300 pounds with the yardstick! Some air gets under the paper, but not enough, so the yardstick breaks.

Second Experiment:

Now, we're going to make a balloon 'rocket' that shoots along a kite string.

You will need:

1) Kite string

2) plastic straws

3) balloons

4) cellophane or masking tape

What to do:

1) Cut a plastic straw in half and tie a length of string (at least 20 feet long is more fun) between two chairs or something.

2) Before you tie the second knot in the string, slip the straw on to the string. Try to get the string fairly tight (the two chairs work well because you can pull the chairs apart to get the string tight).

3) Blow up a balloon, but don't tie off the end and tape it to the straw so that it resembles the drawing below.

4) Let go of the balloon and the 'rocket' should shoot along the string (very quickly) towards the other chair. Try a different kind of balloon!

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