why does the unspillable water experiment work

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Upside Down Glass of Water Science Experiment

Have you ever tried turning a glass of water upside down without spilling it? It seems impossible! Both kids and adults will be amazed by this experiment that appears to defy gravity.

With just a few simple household items, you can try this simple and fun science experiment where kids can get see the effects of air pressure in action. Printable instructions, a demonstration video, and an easy to understand explanation of how it works are included below.

Helpful Tip: Be sure to try this experiment over a sink or large container lest you accidentally make a BIG wet mess!

JUMP TO SECTION:   Instructions  |  Video Tutorial  |  How it Works

Supplies Needed

• Drinking Glass
• Thick Sheet of Paper that is long and wide enough to cover the entire mouth of the glass. (We used a piece of poster board)
• Large Container or Sink

Upside Down Glass of Water Science Lab Kit – Only $5

Use our easy Upside Down Glass of Water Science Lab Kit to grab your students’ attention without the stress of planning!

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Upsidedown Glass of Water Science Experiment Instructions

Step 1 – Begin by filling the empty glass with water. Ensure that the water is completely to the top of the glass. If there is any space between the water and the paper, the experiment won’t work.

Step 2 – Gently place the paper on the top of the glass.

Step 3 – Move the glass over the container or sink. 

Step 4 – Gently place your hand on the paper, then flip the glass over. What do you think will happen if you remove your hand? Write down your hypothesis (prediction) and then follow the steps below.

Step 5 – Remove your hand from the bottom and watch in amazement as the paper stays covering the glass and the water doesn’t spill out. Do you know why this happens? Find out the answer in the how does this experiment work section below.

Upside Down Water Glass Video

How Does the Experiment Work?

The reason this experiment works is because of air pressure! Air pressure is the weight of a column of air pushing down on an area. While we cannot feel it, the air is heavy! The weight of the air pushing down on all objects on Earth is the same as the combined weight of three cars! The reason we don’t feel this extreme weight is that the molecules in air push evenly in all directions – up, down, sideways, diagonally. In this experiment, the air pushing up from underneath the paper is strong enough to overcome the weight of the water pushing down on the paper. Because of the air pressure pushing up on the card, the card will stay on the glass and the water will not spill out.

Do note that while the paper will stay for a while, the paper will become saturated and it will fall eventually.

More Science Fun

If you enjoyed this experiment, then you’ll definitely enough these other cool science experiments that also highlight the power of air.

• Balloon Rocket – Make a balloon that flies across the room like a rocket
• Keep Towel Dry Under Water – Use simple science to keep the paper towel dry after submerging it in water
• Put a Straw through a Raw Potato – Yes, you can easily stick a drinking straw through a hard raw potato

I hope you enjoyed the experiment. Here are some printable instructions:

Upside down Glass of Water Experiment

Instructions.

• Begin by filling the empty glass with water. Helpful Tip: Ensure that the water is completely to the top of the glass. If there is any space between the water and the paper, the experiment won’t work.
• Gently place the paper on the top of the glass.
• Move the glass over the container or sink.
• Gently place your hand on the paper, then flip the glass over.
• Remove your hand from the bottom and watch in amazement as the paper stays covering the glass and the water doesn’t spill out.

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February 18, 2023 at 4:11 pm

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Spillnot: the physics behind the slosh.

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Although the problem of why coffee spills might seem trivial, it actually brings together a variety of fundamental scientific issues. These include fluid mechanics, the stability of fluid surfaces, and interactions between fluids and structures (we'll set aside the biology of walking for now). The SpillNot is a cool tool for getting your students interested in the everyday physics behind why drinks spill while we're carrying them and what has to happen to prevent spillage. Why spilling happens: When the rigid cup is accelerated horizontally the low viscosity fluid remains at rest and is left behind to rise up on the cup's wall. The greater the acceleration is compared to gravity, the more fluid is left behind such that the ratio ahoriz/g is the same as the slope. Later, when the person stops walking forward, the cup is decelerated but the fluid (now in motion) remains in motion toward the other end of the container. In some cases there is an amplifying resonance when the accelerations match the natural frequency of the fluid's back and forth sloshing. Try it!

Spilling happens when acceleration is perpendicular to the normal

Why the SpillNot doesn't spill: Instead of accelerating the cup sideways, the handy lever tilts the base of the apparatus so that the cup's walls are always perpendicular to the fluid's surface. The device tips when you accelerate it so that the largest force on the cup comes perpendicularly from the base. Now, even when though the fluid has been sloped compared to the horizontal, the cup has been, too! Simply put, the SpillNot prevents spilling by rotating the bottom of the cup so that the sloshing of the fluid never falls over the edge.

Simply put, the SpillNot rotates the bottom of the cup so that the sloshing of the fluid never falls over the edge. Most teachers are familiar with the demonstration of centripetal force that involves a cup or water in the bottom of a bucket is maintained in the bucket even when the bucket is spun in a vertical circle that goes overhead. This is not a difficult demonstration to do, but the SpillNot makes it more fun and students can safely try the experiment themselves. Of course I recommend practicing with clear water first versus using hot coffee. For the most part spilling is nearly impossible unless one goes out of his way to jounce the string. So long as there is tension in the string, spills generally will not happen. The SpillNot is best for qualitative demonstrations of centripetal force. The idea that it can successfully take an object through a vertical circle so long as its acceleration exceeds the acceleration due to gravity is well demonstrated. But quantitative measurements are technically nuanced and not as convenient. The radius of the circle is often hard to measure and is different for every case of spin. Additionally, the normal force N on the object is not the same as the force acting on the strap. Therefore, one will have to account for the added mass of the apparatus itself if one wishes to measure the force directly; for example by using a spring scale hooked to the loop. Otherwise, one can indeed use the SpillNot to make direct verification of Centripetal Force as being mv2/r.

B A sample procedure for the horizontal circle.

a) Hold the apparatus (loaded with ½ filled cup) out horizontally at an arm's length b) Hook a spring scale into the loop of the SpillNot (this can be used to measure m, the mass of the device and cup, and then later to measure the Tension, T) c) Spin with the device in hand with a sufficient velocity such that the device raises d) Have a partner time five full cycles with a stop watch, determine t for one cycle e) During the spin, note the average value of the force on the scale (T) f) Measure the horizontal radius (if the velocity is sufficient then Rhoriz = R is nearly true, otherwise Rhoriz = R cos θ) g) Compute the velocity using the formula vcircle = 2πR/t or, more accurately, 2πRhoriz / t h) Compare T with mv2/R, determine the percent difference, account for experimental error. (One such error is the assumption that either R or T is horizontal or that the mass of the apparatus is all the way out at R, which it is not!) Diagnosing errors is an important skill in physics. Note, that the centripetal force is only caused by Thoriz = T cos θ.

Alternatively, one could use the tilt of the SpillNot to determine the force. This can be accomplished by perhaps taking a picture or still-frame of a person swinging the apparatus. Then, with a protractor, measure the angle at which the rope falls below the horizontal. One can then compare a and v2/R by using tan(Ɵ)=a/g This lab does not have much to offer pedagogically beyond what a ball on a string can teach, however the device itself is the hook that gets kids interested. It is novel and exciting to be spinning a cup ominously out with the plane of the fluid nearly perpendicular to the floor! Another lab idea that you might try is the small vertical circle demonstration. In this case the radius is much easier to measure because, for all practical purposes, it is simply the height of the SpillNot plus the small rope. Assuming the cup has a fairly low level, one can determine the minimum speed required to spin the device without spilling. It may be wise and more fun – to do this lab outside. The slowest speed possible will be noticed when, at the top, the cup looses contact with the base. The free body diagram at the top of the spin generates Fnet = mv2/r = N+mg (down or centripetal taken to be positive). The statement "losing contact" implies that there is normal force coming from the base. Thus setting N=0 results in g=v2/r. Measure vcircle = 2πR/t similar to step g in the horizontal circle lab. In this case however I would recommend frame by frame video analysis of a video in which the students spin the device progressively slow until the cup falls off. By counting frames, t can be determined (frame rates can vary from camera to camera).

Be careful however, the velocity changes throughout the circle. It will reduce error to use only the top half of the circle. In that case, vsemicircle= πR/t. Post lab analysis might involve comparing g with v2/r and accounting for error; which is usually about 15%.

Despite that the SpillNot does not offer itself easily to quantitative laboratory work, you will be impressed by how easy it is to use. It is not a quantitative demonstration tool. On the contrary, its best use is to demonstrate that the study of physics can be used to solve practical problems in ordinary life. The bonus is that it makes the classic centripetal force demonstrations much easier to perform.

In conclusion, the SpillNot's ability to demonstrate centripetal force is not unprecedented. Many teachers will already be aware of the demonstration of the "Greek Waiter's Tray" or water in the bottom of a bucket (both vertical and horizontal circles), and of course loop-the-loop rollercoasters. What is unique about the SpillNot is that you don't spill whereas spilling is quite common among these other demonstrations, especially when a novice handles the apparatus. A novice, however, can successfully handle the SpillNot. Of course there is always the possibility that students will try to push the limits of the apparatus; but this is not a bad thing! In fact, having students learn what it takes to spill is a good lesson in the scientific method.

James Lincoln Tarbut V' Torah High School Irvine, CA, USA

James Lincoln teaches Physics in Southern California and has won several science video contests and worked on various projects in the past few years. James has consulted on TV's "The Big Bang Theory" and WebTV's "This vs. That" and the UCLA Physics Video Project.

April 26, 2013 Collin Wassilak

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How does the unspillable water experiment work?

Leigh Metcalfe : Leigh Metcalfe asked the Naked Scientists:     According to the "Unspillable Water Experiment" Air pressure can be stronger than gravity. This unspillable water experiment demonstrates its strength as it keeps the contents of a water glass in place, even upside down. (experiment follows after my question) If that is the case, why does the index card fall when there is only air in the glass but stick when there is water in the glass? Is it really suction or some other property of the water that is holding the card in place? What You'll Need:     * Juice glass     * Water     * 4x6-inch index card Step 1: Fill a juice glass full of water. Let the water run over so that the lip of the glass is wet. Be sure that you fill the glass right up to the top. Step 2: Place a 4x6-inch index card on top of the full glass of water. Be sure to press the card down securely with your hand so that it makes a good seal all around the wet lip of the glass. Step 3: Working over a sink, hold the card in place with one hand as you turn over the glass. Carefully let go of the index card. The card will stay in place, and the water will stay in the glass. What happened? The force of air pressure against the card is stronger than the force of gravity on the water. The air pressure holds the card in place.. What do you think?

daveshorts : exactly the same way as the waterproof hankey kitchen science http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/waterproof-hankey-1/ The air is much easier to stretch than water, so the weight of the water in the glass can stretch the air enough to make enough space between the card and the glass at the bottom for air to leak in, at which point you are going to get wet

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Why doesn't the water spill out?

In this experiment, a number of coins are put into a cup full of water, without spilling it.

http://www.youtube.com/watch?v=N2mKpZHnEzw

Firstly, let me clarify one thing.

If you fill up a cup of water to the brim, in such a way that even another drop of water will cause it to overflow, can this cup take a coin, instead of a water-drop ? - If no, then it doesn't matter and there's no point to this question. - If yes, then my question is this: why another coin and not another water-drop?

---This part exists only if the answer to the question above is yes---

They say it's because of surface tension, but I still don't get how that explains it. If the surface tension can hold together another coin , then why not another drop (which actually has less volume than a coin)?

The only reason I can think of is that the adhesion between the water and the coin is high, which sorta pulls the water molecules towards the coin, increasing the density of water immediately around the coin - thereby making up for the extra space the coin takes. (But thinking about that, that doesn't seem to make much sense either. The coin is at the bottom, and I don't know if adhesive forces can produce such increases in density.)

So: what is the reason for this? Why a coin and not another water-drop?

• experimental-physics
• surface-tension

• $\begingroup$ "If you fill up a cup of water to the brim, in such a way that even another drop of water will cause it to overflow, can this cup take a coin, instead of a water-drop?" I don't understand why you're asking this on the internet, rather than going to your kitchen with a handful of coins... $\endgroup$ –  David Richerby Commented Feb 22, 2014 at 18:47
• $\begingroup$ @DavidRicherby Actually, I did. But it's difficult to get that kinda precision in the kitchen. I don't know when I've achieved that level of 'fullness', if I may. And anyway, that part of the question was thought of later on. :) $\endgroup$ –  mikhailcazi Commented Feb 23, 2014 at 6:04

2 Answers 2

The real issue is that the cup wasn't really full so that adding anything more would make it spill. You can clearly see the the level slowly growing above the top of the cup, as would be expected due to surface tension. Eventually another coin finally exceeded the limit, and a little water spilled. There is really nothing extraordinary going on here.

They could have just as well added some more water as individual drops and gotten the same effect. You say that the cup was filled so that another drop of water would cause it to spill, but that was never stated nor demonstrated in the video.

Once difference between adding a water drop and a coin is that, if done carefully, the coin will cause less of a wave. In fact the coin that caused the spill seemed to be added deliberately to cause a wave, like the person was tired of adding coins and wanted to see the spill already. A coin can be inserted into the water edge-on, and cause a small wave when doing so. A water drop will cause more of a wave because the surface tension of the water in the glass and that of the drop merge when they touch, which causes a sort of snap action that cause a wave. This wave will more likely stress the miniscus at the edge to the breaking point than the tiny rise in water level due to the drop alone.

Edit: regarding your core question: no, if you drop a coin into a glass filled 'more' than the brim-level, such that any more water will cause it to spill, water will spill out.

But, instead of a coin, if you insert something that has other properties that allow it to absorb water whilst not suffering any volumetric changes and has strong cohesive force between it and water, it might be possible.

You may ignore the consideration of adhesive forces between water and coin for the purpose of explaining this effect.

It is surface tension: the strong cohesive forces that allow the water to reach heights above the brim of the glass is responsible for the effect.

In surface tension perspective, the external force in this experiment is gravity which attempts to 'flatten' out that 'bump' of water over the brim by pulling all the molecules downward.

Side experiment to confirm this: add soap to water to affect surface tension and reduce cohesive forces in the medium (water+soap solution). You can't add as many coins.

Related material:

[1] http://physicscentral.com/experiment/physicsathome/h2o-surface-tension.cfm

[2] http://ysp.wustl.edu/KitCurriculum/SurfaceTension/Surface%20Tension-Teacher.pdf

• $\begingroup$ This didn't really answer my question. See, I've edited it to clarify. :) $\endgroup$ –  mikhailcazi Commented Feb 22, 2014 at 13:57

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Science At Play: Upside Down Water

One day I found my daughter Samantha drinking her juice in a very unusual way, the straw was not even in the cup of juice. I thought to myself, how is this possible and of course I found the answer in science. Keep watching to learn exactly how this trick works and how you can try it yourself at home.

Materials to Collect

• Clear straw
• Glass of water
• Thin plastic or oversized playing card that covers the mouth of the glass
• Bowl to catch any spills

Straw Trick:

• Put your straw into the water.
• Cover the top of the straw with your finger.
• Lift the straw out of the water.

No-Spill Cup:

• Cover the top of the glas with the rigid plastic or playing card.
• While holding the plastic in place, tip the glass upside down.
• Release the plastic and be amazed as the liquid does not spill out! 

What is the Science?

The air around us exerts pressure on us. At sea level, it pushes on everything at about 14.7 pounds per square inch. The reason we don’t feel it is because it pushes equally on all sides. The straw has solid sides, so the air can only affect the liquid in it through the openings in the top and bottom. When you pick up a straw from a drink, the air rushes into the top opening of the straw and pushes the liquid down into your cup. But if you put your finger over the top opening, air can longer push on the liquid from the top, leaving only the bottom for air to push on. Water molecules also like to “stick” together, this is called surface tension. (Ever see a liquid in a cup form a dome over the top of a glass?)  Between the air pushing on the liquid and the water molecules sticking together, the water will not fall out of the bottom of the straw. The same thing is happening with the upside down glass. Air pressure cannot push down on the water because the solid glass blocks it. The plastic covering the opening is being pushed up by the air pressure from below, holding it in place. This air pressure is heavier than the water in the glass and keeps the cover on and the water in the cup.

  Ask Your Young Scientists

While you are doing this experiment, ask your scientist:

• What happens when you release your finger from the straw while there is liquid in it?
• With water in the straw and your finger over one end, tip the straw so the opening is pointing up. What happens to the water? Why do you think this happens?
• How big of a glass can you get to be upside down with water in it? (provided you have a big enough cover.) 

More to Explore

Some other things to try:

• Try putting your finger over the straw BEFORE you put it in the water. What do you notice?
• Try different width straws. Does the size make a difference?
• How big of a glass can you get to be upside down?
• Try different types of containers. Find one with a narrow opening and compare to one with a larger opening. Do you see any difference? 
• Try placing an upside down cup in a bowl of water. What do you notice happening inside the cup? Does the water get inside?

Check out some of these activities from the Connecticut Science Center!

https://ctsciencecenter.org/blog/science-at-play-air-pressure-pranks/

https://ctsciencecenter.org/blog/science-sunday-experimenting-with-heat-and-air-pressure/

Find more air pressure experiments here!

https://www.asme.org/topics-resources/content/5-ways-to-demonstrate-air-pressure-to-children

We want to see what you try at home. Share your experiments with us on social media by using the #ScienceAtPlay and tagging @CTScienceCenter.

Andrew Fotta is a STEM educator at the Connecticut Science Center. He has currently holds a CT teaching certification for grades K-6, and has spent time in the classroom in nearly all grades, and taught middle school science. In addition to teaching classes for the Science Center, Andrew is also part of a team of educators currently creating new programs aligned with the new Next Generation Science Standards for grades PreK-9. Andrew is an avid photographer, who enjoys blending science and art in his work.

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The Physics of a Spill-Proof Cup

The tumbler's primary weapon is a suction cup, which uses atmospheric pressure to form a rubber seal with the surface of the table.

• Play/Pause Button Pause 01 The atmosphere is like a bunch of tiny balls that are constantly moving around—and when they collide with a surface, they exert (also tiny) force in the direction they were traveling.
• 02 There's less air pressure underneath the suction cup than above it, which creates a net downward force.
• 03 That pressure differential creates a healthy grip on the table, but the big question is: what does it take to knock the cup over?
• 04 When we push the cup near the top like this, we see all the forces at play.
• 05 If we assume the suction force and contact area, we can calculate the suction force.
• 06 It takes 11 pounds of force to knock the mug over—or roughly five times the force of a normal tap. In other words, you *can* do it, but you're not gonna do it accidentally. Feel free to gesticulate wildly\!

Science in School

Fantastic feats: experimenting with water teach article.

Author(s): David Featonby

How can air hold the water in an upturned glass? Why does water stay in a bottle with a hole in its base? Find out with these entertaining experiments.

From their earliest years, children enjoy playing with water, and so do many older students. In this set of experiments, we look at the forces that are significant when dealing with water, demonstrating some basic science principles – and some surprising results. All the experiments are safe to do at home as well as at school, and require just simple household objects as the equipment, plus plenty of water.

Experiment 1: the upside-down glass

Many people have tried this experiment in some version, but can you work out what’s really going on?

• Straight-sided water glass
• Piece of thin card (large enough to cover the open end of the glass)
• Pour water into the glass until it is nearly full.
• Place the piece of card on top of the beaker.
• Turn the beaker upside down with one hand, holding the card in place with the other hand.
• Remove the hand holding the card (figure 1).
• Note what happens. Does the card fall off and the water fall out? Can you explain why not?

Surprisingly, when the glass is inverted, the card and the water remain in place. Why is this?

Let’s consider the forces on the card. These are:

• gravity, from the weight of the card itself (acting downwards)
• gravity, from the weight of the water pushing on the card (acting downwards)
• air pressure, which pushes on the outer surface of the glass and card, acting at 90 0 to the surface of the card (so producing an upward force on the card where this has just water above it).

So in this experiment, the force of air pressure pushes upwards on the card at the open end of the glass, opposing the force of gravity and keeping the water in the glass.

Extension: estimating the upward and downward forces

How do we know that the upward force of air pressure is enough to oppose the downward force gravity, to hold the water in the glass? We can estimate these forces quite easily.

The weight of the card is much less than that of the water, so to simplify we can ignore the weight of the card itself. This means that the downward gravitational force on the card is the weight of the water column

= h x A x ρ x g

where h is the height of the water column, A is the cross-sectional area of the glass, ρ  is the density of water (1 000 kg/m 3 ) and g is the acceleration due to gravity (approximately 10 m/s 2 ).

So if h is 10 cm (0.1 m) and A is approximately 25 cm 2 (0.0025 m 2 ), the downward force is approximately

0.1 x 0.0025  x 1000 x 10  = 2.5 N

For the upward force, this is the atmospheric pressure, P , multiplied by the area over which it acts, A.

Atmospheric pressure is approximately 100 000 Pa (pascals, or N/m 2 ).

So the upward force on card = P  x A

= 100 000 x 0.0025  = 250 N

So for a 10 cm water column, the upward force due to the atmosphere on the card far exceeds the downward force of gravity on the card due to the water.

We also need to recognize that the air above the water plays a significant role.  If this remained at atmospheric pressure, the weight of the water would be sufficient to remove the card, however as soon as the water exerts a downward pressure on the card, it reduces the pressure of this trapped air, which is sufficient to enable the upward atmospheric pressure on the card to support the water. A 1/100 change in volume of this air is sufficient to balance the water, i.e., the pressure reduces by 1/100 th  which is equivalent to the pressure of the water.

Further investigation

You can also think about the questions below, and perhaps carry out further experiments to answer some of them:

• Does this experiment work if the glass is completely full of water?
• How does the ratio of air to water change the experiment outcome?
• Would this experiment still work, no matter how tall the glass is?
• What other shaped containers (e.g., bottles) can be used?

Experiment 2: water’s invisible ‘skin’

In this experiment, we discover how cohesive forces within water act like an invisible ‘skin’ that can keep the liquid in an upturned cup – sometimes.

• Piece of thin woven nylon cloth (large enough to cover the open end of the cup)
• Elastic band
• Thin card (large enough to cover the open end of the cup)
• Cover the open end of the cup with the nylon cloth (figure 2, left).
• Pull the cloth tight, and secure it with the elastic band (or glue it to the cup around the rim).
• Pour water into the cup through the cloth, nearly filling it.
• Place the card over the nylon and the open end of the cup.
• Turn the cup upside down.
• Note what happens: the water should stay in the cup, as in experiment 1.
• Now carefully remove the card. Does the water flow out through the nylon cloth? If not, why? Water was poured in through the cloth, so why doesn’t it pour out again?
• To pour the water out turn the cup upright again quickly, then tip up the cup slowly while pressing a finger on the nylon (figure 2, right).

The reason why the water does not flow out through the very small holes in the nylon is because there are forces of cohesion between the molecules in the water. These forces make the surface of the water act like a ‘skin’ between the tiny holes in the nylon cloth. This effect is known as surface tension, and it is the same principle that keeps you dry under a woven nylon umbrella: there are tiny holes in the cloth, but the rain won’t get through due to the cohesive forces of surface tension between water molecules.

Further investigations

There are plenty more experiments you can do with surface tension and molecular cohesion. Perhaps look up ‘surface tension experiments’ on the internet and see what other activities you can find?

Here are two further simple experiments you can try.

Paperclip boat

Take a dish of clean water and a paper clip. Hold the paperclip in a strip of paper towel and lower it into the water. Then allow the paper to sink or carefully sink it with a toothpick. The paperclip will appear to float but is in fact being held by the surface tension of the water.

What else can you ‘float’ – for example, a ring pull from a drinks can? What happens to the paper clip if a drop of washing-up liquid is added to the water? How can you explain what you see?

This effect can also be used to make a ‘ soap boat ’.

Joined-up water jets

Take a clean empty drinks can, plastic cup, or bottle and make three small holes close together, near the base. Fill the can with water, and when three jets come out, use your fingers to try and join the jets together. You will be able to do this, because of the cohesive forces between water molecules.

Another fun experiment to illustrate cohesion is pouring water down a string .

Experiment 3: bottled water

These activities use a simple bottle of water to reveal some surprising effects due to surface tension and gravity. It’s a good idea to do them outdoors because water spillage is likely (see figure 3).

• Plastic bottle (250 ml) with screw cap
• Large needle or nail

Use the needle or nail to make one (or more) very small holes near the base of the plastic bottle by heating it over a flame (safely held) until it is hot enough to melt the plastic. In schools, this should be done by the teacher in advance of the experiment, for safely.

• For fun, you can add a label to the bottle saying ‘Do Not Open’ – and see if people ignore this.
• With the cap off, quickly fill the bottle with water, holding your finger over the hole, and then replace the cap.
• Hold the bottle still (or hand it to someone else) with the cap closed. What happens to the water?
• If the warning is ignored and the bottle cap is opened, what happens?

Once a container is sealed, water will only flow out of a small hole if that water can be replaced by air or more water. A bottle with one small hole can therefore hold water if the cap is sealed. Once the cap is unscrewed the water will flow out, due to the weight of the water. The hole needs to be small enough for the surface forces to hold the water.

Another interesting experiment to try with a full bottle of water with a hole near the base is: what happens when you throw the bottle up and catch it?

If you fill the bottle with water and hold it with the cap unscrewed for a few seconds (figure 4), the water will flow out of the hole.

Now throw the bottle up in the air (figure 5), and watch it carefully as it falls. Observe each part of its journey – on the way up, at the top of its flight, and on the way down.

When the water is in free fall (i.e. on the way down), water will cease to flow out of the bottle. This is because the water within the bottle becomes weightless relative to the bottle itself, as both the bottle and its contents are in free fall. Thus, in this situation the weight of the water does not force it out of the bottle.

This effect can also be demonstrated with a water-filled hollow tube (around 50 cm, with a diameter that can easily be covered by a finger). The bottom of the tube is covered with one hand while it is launched into the air, with this hand exerting the launching force and the other just supporting the tube. However, this can be a little tricky to demonstrate because it is essential that the hand covering the tube end is the last to let go when throwing the tube up and the first to contact the tube again on catching it, to avoid accelerating/decelerating the tube without the water column.

You can also try to answer these final questions:

• What happens to the water when the bottle is travelling upwards during the throw? Can you explain this?
• If you try to catch the bottle, what happens to the water? Can you explain this?
• What else can you throw in the air so that there is a change in what happens when it is in free fall, compared with when it is stationary? Hint: Think of toys or devices that work with gravity, e.g. where particles or moving parts or liquid fall through a gap.
• A more detailed version of the thrown water bottle experiment: Tsakmaki P, Koumaras P (2017)  When things don’t fall: the counter-intuitive physics of balanced forces . Science in School 39 :36–39
• Try a similar experiment to the paper cup and nylon cloth activity:  https://www.stevespanglerscience.com/lab/experiments/water-screen/
• Watch this video with more activities to try with your students using water:  https://www.youtube.com/watch?v=CCxbI1qRsWY&ab_channel=DrewtheScienceDude
• Learn how to make a soap boat:  https://www.youtube.com/watch?v=OU76wwmg9Hs
• Watch a video on the running water experiment:  https://www.youtube.com/watch?v=8nOU7jbRPPo&ab_channel=DrBoydTheChemist
• Read other Teach articles from the Fantastic Feats series:
• Featonby D (2017)  Fantastic feats.   Science in School 39 :45–47
• Featonby D (2018)  Further fantastic feats: falling and bouncing .  Science in School   43 :37–54
• Featonby D (2019)  Fantastic feats: magic with money .  Science in School   47 :46–50

David Featonby taught physics throughout his career in a large UK comprehensive school, and now shares his ideas across Europe through the organisation Science on Stage, of which he is a board member, helping to organize its activities. He has presented workshops in various European countries and written articles for both Science in School and Physics Education , including a regular series called What Happens Next? in the latter. David has a particular interest in making physics relevant to all ages through experiments that use everyday equipment.

The simple experiences shown in the article, easy to reproduce and using materials that are easy to find, allow students to approach the topics of surface tension and pressure in liquids. The author, also, by highlighting the “magic side” of some experiments, makes them more interesting and also suitable for the general public. The article offers the possibility to make interdisciplinary links to biology topics, such as pulmonary respiration (and how nature provides the alveoli with surfactants to decrease surface tension), capillarity in plants, or how the surface tension allows some insects to walk on the surface of the water

Maria Teresa Gallo, Math and science teacher, Italy

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