Description: Small metal balls bounce about inside a horizontally oriented clear-walled frame, under the influence of a vibrating frame edge.

Purpose: To illustrate thermal motion in gases.

Procedure: Put the metal balls in the frame and lie the frame horizontally. Arrange the projector so that it projects a clear image of the balls and frame on a screen that everyone can see. Activate the vibrating edge of the frame so that the balls bounce around inside the confines of the frame. Since the frame is horizontal, gravity should have little or no effect on their motions.

Explanation: The metal balls represent air molecules and the vibrating edge ensures that they have "thermal energy."

Description: You inflate an elastic balloon.

Purpose: To show how air pressure can exert forces on surfaces.

Procedure: Stretch the balloon to show that it takes outward forces and work to enlarge the balloon. Now inflate the balloon. Point out that the air that you've pushed into the balloon has provided the outward forces and work needed to stretch the balloon to its new size.

Explanation: By adding more and more air molecules to the volume inside the balloon, you are increasing the pressure inside the balloon. A pressure imbalance appears between the pressure inside the balloon and the pressure outside the balloon, and the balloon's skin experiences outward forces and accelerates outward. While the balloon's own elastic forces tend to oppose this outward acceleration, the balloon gradually grows larger.

Description: Small metal balls bounce about inside a horizontally oriented clear-walled frame, under the influence of a vibrating frame edge. Another edge of the frame is movable and it is pushed outward by the impacts.

Purpose: To illustrate that air pressure is caused by the impacts of countless tiny air molecules.

Procedure: Put the metal balls in the frame and lie the frame horizontally. Arrange the projector so that it projects a clear image of the balls and frame on a screen that everyone can see. Activate the vibrating edge of the frame so that the balls bounce around inside the confines of the frame. Since the frame is horizontal, gravity should have little or no effect on their motions. The balls should gradually push the movable edge of the frame outward and the "gas" of balls should become more dilute.

Explanation: The metal balls represent air molecules and the vibrating edge ensures that they have "thermal energy." Their collisions produce pressure on the frame walls and this pressure pushes the movable frame wall outward.

Description: Small metal balls bounce about inside a horizontally oriented clear-walled frame, under the influence of a vibrating frame edge. As you push the movable edge of the frame inward, the density of the balls increases and the collision rate (and pressure) increases.

Purpose: To illustrate that air pressure is proportional to air density.

Procedure: Put the metal balls in the frame and lie the frame horizontally. Arrange the projector so that it projects a clear image of the balls and frame on a screen that everyone can see. Activate the vibrating edge of the frame so that the balls bounce around inside the confines of the frame. Since the frame is horizontal, gravity should have little or no effect on their motions. Move the movable frame edge in and out gradually to show that packing the balls more tightly increases not only their density, but also their "pressure."

Explanation: The metal balls represent air molecules and the vibrating edge ensures that they have "thermal energy." Their collisions produce pressure on the frame walls and packing the balls more tightly causes them to exert more "pressure" on the frame edges.

Description: Small metal balls bounce about inside a vertically oriented clear-walled frame, under the influence of a vibrating frame edge and gravity. The density and "pressure" of the balls is highest near the bottom of the frame.

Purpose: To illustrate that gravity structures the density and pressure of the atmosphere.

Procedure: Put the metal balls in the frame and prop the frame vertically, with its vibrating edge on the bottom. It needs to have two clear surfaces to confine the balls in a plane. Arrange the projector so that it projects a clear image of the balls and frame on a screen that everyone can see. Activate the vibrating edge of the frame so that the balls bounce around inside the confines of the frame. Since the frame is vertical, gravity should make the density highest near the bottom and lower or even zero near the top of the frame.

Explanation: The metal balls represent air molecules and the vibrating edge ensures that they have "thermal energy." Gravity structures the ball density and "pressure" from highest near the bottom of the frame to lowest near the top of the frame.

Description: Two half-spheres are joined together and the air is removed from between them. With no air inside the sphere, its halves can't be separated by hand.

Purpose: To show that atmospheric pressure exerts enormous forces on large surfaces.

Procedure: Show that the two hemispheres don't normally stick to one another when you simply touch them together. Then touch them together and remove the air from inside the overall sphere with the vacuum pump. Point out that you are removing the air by allowing the air molecules to bounce out through a hole in the spheres and into a machine that prevents them from returning (i.e. you aren't "sucking" the air out of the sphere). Once there is a vacuum inside the sphere, seal off the sphere and allow two students to try to pull the hemispheres apart. Note that the two hemispheres are being pressed together so strongly by the surrounding air pressure that they are inseparable. Now allow air to reenter the sphere and show that the hemispheres separate easily.

Explanation: When there is no air inside the sphere, the enormous inward forces exerted by air pressure on the outer surfaces of the hemispheres aren't balanced by outward forces exerted by air inside the hemispheres. Only when air is allowed to reenter the sphere do the outward forces reappear and make it easy to separate the hemispheres.

Description: One or more empty plastic bottles were sealed during a trip at high altitude (on a mountain or in an airplane) and subsequently crushed during the descent to lower altitude.

Purpose: To show that the atmosphere's pressure and density decrease with altitude.

Procedure: Empty and dry one or more plastic soda bottles. Take the bottle(s) to high altitude, open them and seal them tightly. Then descend back to low altitude. The bottles will collapse as the increasing atmospheric pressure squeezes them inward.

Explanation: High altitude air is less dense than low altitude air, so that when you seal a plastic bottle at high altitude, it contains relatively few air particles and its internal pressure is low. When you then take the bottle back to low altitude, the higher air pressure collapses the bottle so that the density of trapped air increases. Only when the internal density and pressure rises high enough to balance the forces on the bottle walls does the bottle stop collapsing inward.

Description: A balloon expands to enormous size in a vacuum chamber and then pops.

Purpose: To show a balloon's size depends on a balance of forces and that when the pressure around the balloon is reduced, the balloon's size increases.

Procedure: Put the balloon in the bell jar and gradually remove the air from around the balloon. As the pressure surrounding the balloon falls, the unopposed pressure inside the balloon will cause it to grow in size. Eventually, the balloon will burst.

Explanation: When you remove the air from around a balloon, only the balloon's elastic character remains to oppose the outward pressure of the gas inside the balloon. The gas inside the balloon pushes its walls outward until it bursts.

Description: Marshmallows, marshmallow cream, and shaving cream expand to enormous sizes in a vacuum chamber.

Purpose: To show a bubble's size depends on a balance of forces and that when the pressure around the bubble is reduced, the bubble's size increases.

Procedure: Put each of the objects (marshmallows, sandwich, and shaving cream) in the bell jar separately, gradually removing the air from around it. As the pressure surrounding the object decreases, the unopposed pressure inside the bubbles in the object will cause that object to grow in size. Eventually, the bubbles will burst and lose their gases. This bursting is most noticeable for a marshmallow; after growing steadily for a while, its size will abruptly shrink. Once this shrinkage has occurred, let the air return to the bell jar. The object will shrink dramatically as its bubbles are crushed by the surrounding air pressure. Marshmallows become withered, ancient-looking things.

Explanation: When you remove the air from around a bubble, only the bubble's elastic character remains to oppose the outward pressure of the gas inside the bubble. The gas inside the bubble pushes its walls outward until they burst.

Description: A cylindrical weight is suspended from a cylindrical container of the same size, which is itself suspended from a spring scale. When the cylindrical weight is submerged in water, the weight reported by the scale decreases. But when the cylindrical container is then filled with water, the weight reported by the scale returns to its original value.

Purpose: To show that an object that is displacing water in a container experiences an upward buoyant force that's equal in magnitude to the weight of the water it's displacing.

Procedure: Suspend the spring scale from the support, suspend the cylindrical container from the spring scale, and suspend the cylindrical weight from the cylindrical container. Observe the weight of the two cylindrical objects on the spring scale. Now raise the container of water so that the cylindrical weight is entirely immersed in the water and place the support under the container of water. The scale will now read less than the weight of the two cylindrical objects. Point out that the water in the container is exerting an upward buoyant force on the cylindrical weight so that the scale doesn't have to pull upward as hard to support the cylindrical weight. Now fill the cylindrical container with water. The scale will report the original value. Evidently, the upward buoyant force on the cylindrical weight is exactly equal in magnitude to the weight of an equal volume of water.

Explanation: The buoyant force experienced by an object immersed in water is equal in magnitude to the weight of the water it displaces.

Description: A plastic soda bottle that's full of water has a small vial floating upside down at its top. When you squeeze the bottle, the bubble of air trapped inside the vial shrinks and the vial sinks.

Purpose: To show that an object floats when its average density is less than that of the surrounding fluid and sinks when its density is greater than the surrounding fluid

Procedure: Fill the bottle with water. Fill the vial with water and then invert it carefully into the mouth of the soda bottle. Let just enough water escape from the vial during this insertion to trap the right amount of air: the vial should just barely float. Seal the soda bottle. When you squeeze the soda bottle, the bubble of trapped air inside the vial will shrink and the vial's average density will increase. The vial will sink.

Explanation: As the vial's bubble gets smaller and water enters the vial, the floating object (that is, the vial and its contents) experience a rise in average density. When the vial's average density exceeds the density of water, the vial sinks.

Description: A helium balloon is immersed in liquid nitrogen and shrinks to about a quarter of its normal size. When the balloon is removed from the liquid nitrogen, it reinflates and eventually lifts itself up into the air.

Purpose: To show that cooling a gas slows its molecules so that they must become more dense in order to have the same pressure as before. To show that cooling a gas without changing its pressure causes the gas's density to increase.

Procedure: Show that you have a helium balloon by letting it float briefly. Now immerse the balloon carefully into the liquid nitrogen (don't freeze your skin) and observe how the balloon becomes much smaller. The balloon will eventually reach about a quarter of its original size. Now remove the balloon from the liquid nitrogen and allow it to warm up on the table. Once the balloon is almost back to its original size, it will become buoyant enough to rise up into the air.

Explanation: Cooling the helium gas slows its atoms so that its pressure begins to fall and the surrounding atmosphere compresses the balloon. The result is that the balloon's volume decreases while the pressure of the cooling helium inside it remains nearly constant. The helium gas becomes more and more dense. When you then warm up the helium gas, its pressure increases and it pushes the walls of the balloon outward. The balloon's volume increases so that the pressure inside the balloon remains nearly constant.

Description: An air-filled balloon is immersed in liquid nitrogen and shrinks to very small size. When the balloon is removed from the liquid nitrogen, it reinflates.

Purpose: To show that cooling air slows its molecules so that they must become more dense in order to have the same pressure as before. When the molecules have slowed sufficiently, they condense into a liquid, with a dramatic decrease in volume.

Procedure: Inflate the balloon, tie it off, and immerse the it carefully into the liquid nitrogen (don't freeze your skin). The balloon will become much, much smaller. Now remove the balloon from the liquid nitrogen and allow it to warm up on the table. It will eventually return to its original size.

Explanation: Cooling the air slows its molecules so that its pressure begins to fall and the surrounding atmosphere compresses the balloon. The result is that the balloon's volume decreases while the pressure of the cooling air inside it remains nearly constant. Below a certain temperature, the air inside the balloon begins to condense—the molecules begin to stick to one another to form a liquid. When that happens, the volume of material inside the balloon drops dramatically. When you then warm up the air in the balloon, it converts back into a gas, its pressure increases, and it pushes the walls of the balloon outward. The balloon's volume increases so that the pressure inside the balloon remains nearly constant.

Description: You repeatedly add air to a balloon and liquefy that air in liquid nitrogen. After finally tying off the balloon, you allow it to warm up on the table and it eventually explodes.

Purpose: To show that, when its density and pressure are sufficiently high, a gas can exert enormous pressures—and cause things to explode.

Procedure: Blow up the balloon and pinch the nipple to keep the air inside. Now carefully immerse the other end of the balloon in the liquid nitrogen and allow the air inside it to liquefy. Quickly remove the balloon from the liquid nitrogen and blow more air into the balloon. Return the end of the balloon to the liquid nitrogen so that the added air liquefies. Repeat this procedure a total of about 10 times. Finally, tie off the balloon and place it on the table. It will inflate itself to giant size and eventually burst.

Explanation: Cooling the air inside the balloon removes much of its thermal energy. Exposed as it is to atmospheric pressure, the balloon collapses as it cools. The air inside the balloon first becomes denser and then begins to liquefy. Because liquid air occupies so much less volume than gaseous air, even at lower temperatures, you can put a considerable number of air molecules into the balloon as liquid air. When this liquid air turns back into gaseous air and the temperature of this gaseous air returns to room temperature, the molecules will either occupy a very large volume (if their pressure is roughly atmospheric) or have an enormous pressure (if the volume they can occupy is very limited). In this case, the air's volume and pressure both increase until the balloon's skin rips.

Description: Hot air from a heat gun is directed into a large, thin-walled plastic bag. The bag inflates and, when released, floats upward to the ceiling.

Purpose: To show that hot air is less dense than cold air at the same pressure.

Procedure: Insert the hairdryer into the mouth of the plastic bag and begin to inject hot air into the bag. Use a medium power setting until the bag is fairly fully inflated so that you don't melt the plastic. Once the bag is relatively inflated, switch to the highest heat setting. Continue to blow hot air into the bag until all the air inside the bag is hot. If you're using a dry cleaning bag, you can seal the bottom of the bag around the neck of the hairdryer and allow air to flow out of the small hanger hole at the other end of the bag. If your using a solar hot air balloon, leave some space so that cold air can escape as hot air enters the balloon. Once the bag is completely full of hot air, let it go and it should rise up into the air. The solar hot air balloon will float all the way to the ceiling, while a dry cleaning bag will rise several meters upward before it collapses. Point out that the pressures inside and outside the bag are equal—if they weren't, air would accelerate toward the lower pressure.

Explanation: The hot air inside the bag is less dense than the cooler air around it. Because there are fewer air molecules in the bag than there would be if the bag were full of cooler air, the bag's overall weight is less and it's pushed upward by the buoyant force.

Description: An elastic balloon is filled with air, tied off, and released. It slowly sinks downward. A second balloon is filled with helium. This balloon floats upward.

Purpose: To show that helium gas is less dense than air at the same pressure.

Procedure: First fill a balloon with air, tie it off, and release it. It will sink because its average density is slightly greater than that of air. Now fill the second balloon with helium, tie it off, and release it. It will float upward.

Explanation: The number of helium atoms in the second balloon is the same as the number of air molecules in the first balloon. However, helium atoms weigh much less than the average air molecule, so the overall weight of the helium balloon is much less than that of the air-filled balloon. While the air-filled balloon's average density is slightly higher than that of air—the balloon's skin contributes much of this excess density—the helium balloon's average density is well below that of air.

Description: A helium balloon is floating at the top of a clear jug that's only open at the bottom. When helium gas is introduced into the jug and the air is expelled, the helium balloon no longer floats.

Purpose: To show that the buoyant force exerted by helium gas is less than the buoyant force exerted by air.

Procedure: Invert the container, so that the closed end is on top, and put the helium balloon inside it. The balloon will float to the top of the container. Now spray helium gas into the container from below. The air will be displaced and the container will soon be full of helium. The helium balloon will sink to the bottom of the container.

Explanation: The buoyant force on the helium balloon depends on what gas it's displacing. When the balloon is displacing air, the buoyant force is relatively large and is large enough to support it against gravity. But when the balloon is displacing helium, the buoyant force is much smaller and the helium balloon sinks.

Follow-up: Discuss what would happen to a hot air balloon if it were to drift into a region of very hot air? Why do balloonists prefer to fly in cold weather?

Description: Soap bubbles filled with air sink slowly to the ground. But soap bubbles filled with helium rise rapidly.

Purpose: To show that the upward buoyant force on a helium bubble greatly exceeds its downward weight.

Procedure: First dip the plastic ring in the soap solution and blow some normal bubbles (air-filled bubbles). In calm air, they will slowly settle downward. Now use the hose to attach the funnel to the helium tank and dip the wide end of the funnel in the soap solution. Lift the funnel out of the soap and observe that a film has formed across its mouth. Turn on a gentle flow of helium so that the soap film gradually inflates. With a flick of your wrist, break off a helium-filled bubble and watch it float upward to the ceiling.

Explanation: While a helium-filled bubble and an air-filled bubble of the same size contain the same number of particles, the helium atoms in the helium-filled bubble are much lighter than the air molecules in the air-filled bubble. While the air-filled bubble has an average density just slightly greater than that of the surrounding air, the helium-filled bubble has an average density that is substantially less than that of the surrounding air.

Description: Soap bubbles filled with methane (natural gas) rise rapidly until they are ignited. They then burn with a large orange flame.

Purpose: To show that the upward buoyant force on a methane bubble exceeds its downward weight.

Procedure: Be careful—do this experiment only in a room with a high ceiling and no flammable materials around! Use the hose to attach the funnel to the source of methane. Mount the large candle on its base and light the candle. Attach the clothespin to the stick with tape and use it to grab the small candle (or simply tape the small candle to the stick). Light the small candle with the big candle.

Now turn on a gentle flow of natural gas, but keep the two candles well away from the gas flow. Dip the wide end of the funnel briefly into the soap solution and allow a methane-filled bubble to form. When the bubble is reasonably large, flick your wrist to break the bubble free from the funnel. It will float upward rapidly. When it's a safe distance from you, the funnel, and any flammable materials, ignite the bubble with the small candle on the stick. It will burn with a surprisingly large, orange flame. Be sure that the bubbles always fill with essentially pure methane and never a mixture of methane and air—such a mixture can be explosive.

Explanation: A methane molecule (CH₄) is lighter than the average air molecule. Thus a methane-filled bubble has an average density that is well below the density of air.

Description: When you hold a water-filled straw horizontally, the water remains stationary.

Purpose: To show that when a fluid in a horizontal pipe experiences equal pressures at both ends, it doesn't accelerate.

Procedure: Fill the straw with water by dipping it in the colored water, sealing one end with your finger, lifting it out of the colored water, turning it horizontally, and finally releasing the seal. The water will remain motionless in the horizontal straw.

Explanation: With the water's weight being supported by the wall of the horizontal straw, the water's acceleration is determined only by the pressures at its two ends. Since those pressures are equal, the forces on the two ends of this little column of water cancel one another perfectly and the net force on the water is zero. It doesn't accelerate.

Description: You hold a water-filled straw horizontally and blow on one end. The water accelerates away from your mouth and sprays into the room.

Purpose: To show that when a fluid in a horizontal pipe experiences a pressure imbalance, it accelerates toward lower pressure.

Procedure: Fill the straw with water by dipping it in the colored water, sealing one end with your finger, lifting it out of the colored water, turning it horizontally, and finally releasing the seal. The water will remain motionless in the horizontal straw. Now blow hard on one end of the straw. The water will accelerate away from your mouth and spray out of the straw.

Explanation: By exposing the water in the straw to a pressure imbalance, you cause it to accelerate. It accelerates toward the lower pressure, which is on the side of the straw opposite your mouth. It quickly acquires a velocity away from your mouth and leaves the straw.

Description: You squeeze a water-filled plastic bottle with its opening plugged. Its pressure rises. To confirm that pressure rise, you release the plug and the water accelerates, perhaps spraying around the room.

Purpose: To show that squeezing water causes its pressure to rise.

Procedure: Fill the bottle with water (air gaps just complicated the story because air is compressible). Plug the bottle's opening and squeeze the bottle. Note that the water's pressure rises because you feel the resulting pressure imbalance (high pressure inside the bottle, atmospheric pressure outside the bottle) pushing outward on the bottle's wall and keeping that wall from accelerating. Now remove the plug and let the water accelerate from high pressure inside the bottle to atmospheric pressure outside the bottle.

Explanation: By squeezing the bottle's walls inward, you cause the water's pressure to rise. That water pressure is keeping the walls of the bottle from accelerating in response to your inward forces. When you open the bottle, the water can accelerate from the high pressure inside the bottle to the lower, atmospheric pressure outside the bottle. The water does accelerate, quickly acquiring a velocity that sends it spraying around the room.

Description: You operate a transparent piston water pump to lift water from one container to another higher container.

Purpose: To show that a piston pump is basically a squeeze bottle with one-way valves.

Procedure: Fill one container with water. Immerse the input end of the piston pump in the full container and put its output end in the empty container. Now operate the pump. Show that each time you withdraw the piston from the cylinder, the pressure inside the cylinder drops below atmospheric, so that the water accelerates toward the cylinder and begins to flow into it through a one-way valve. Show that each time you push the piston into the cylinder, the pressure inside the cylinder rises above atmospheric, so the water accelerates away from the cylinder and begins to flow out of it through a one-way valve.

Explanation: You are operating a self-refilling squeeze bottle -- you are filling and emptying that squeeze bottle and doing work in the process.

Description: You squeeze a water-filled eye-dropper, squirt bottle, or squirt gun and a jet of high-speed water sprays out of its nozzle.

Purpose: To show that pressurized water converts its pressure potential energy into kinetic energy as it speeds up in flowing through a nozzle.

Procedure: Fill the eyedropper completely full with water. Hold the eyedropper horizontally and squeeze its bulb hard. The pressurized water will flow slowly through the eyedropper until it enters the narrow nozzle. There its speed will increase and its pressure will decrease—it will exchange pressure potential energy for kinetic energy—and it will spray across the room.

Explanation: As it flows slowly through the eyedropper, the water is under substantial pressure and has considerable pressure potential energy. But as it flows through the nozzle, its speed must increase in order for enough water to make it through the narrowing each second. As the water's speed increases, its pressure decreases—as required by Bernoulli's equation, it's exchanging pressure potential energy for kinetic energy. The water spraying through the air is at atmospheric pressure, so all of its pressure potential energy has become kinetic energy.

Follow-up: Spray the water straight up. Now its kinetic energy will gradually become gravitational potential energy!

Description: When you hold a water-filled straw vertically, the water falls downward. Only when you seal the bottom or top of the straw, thereby allowing a pressure imbalance to develop, does the water stop falling.

Purpose: To show that when a fluid in a vertical pipe experiences equal pressures at both ends, it's weight causes it to accelerate downward, and to show that it can only be prevented from falling by allowing the pressure at the bottom of the water to become greater than the pressure at the top of the water.

Procedure: Fill the straw with water by dipping it in the colored water, sealing one end with your finger, lifting it out of the colored water, and turning it so that your finger is at the bottom of the vertical straw. After holding it there for a second, release the seal that your finger is making and allow the water to fall out of the straw. Point out that the pressures at the top and bottom of the column of water are the same, so that the water experiences no overall force due to pressure, but that the water's weight causes it to fall. Now show that as long as you seal either the top or the bottom of the straw, the water won't fall. Note that when you seal the bottom of the straw, the weight of the column squeezes the water at the bottom of the straw so that the pressure there rises above atmospheric pressure. A pressure imbalance develops between the top and bottom of the water column so that there is just enough upward force due to pressure to support the column's weight. Then note that when you seal the top of the straw, the water initially begins to fall but as it does, the pressure at the top of the column of water drops below atmospheric pressure. Again, a pressure imbalance develops between the top and bottom of the water column so that there is just enough upward force due to pressure to support the column's weight. But whenever you release the seal, whether at the bottom or the top of the straw, the pressure imbalance vanishes and the water falls.

Explanation: To support a column of water in a vertical pipe, the pressure in the column must increase by 10,000 Pa for each meter of depth. Without such a pressure increase, the water will fall.

Description: A full bottle of water is inverted and its mouth is placed in a shallow container of water. The water remains inside the bottle of water. Only when an air bubble is allowed to enter the bottle of water will the level of water in that bottle descend.

Purpose: To show that water falling out of an inverted bottle causes a natural pressure imbalance to develop and to support that water.

Procedure: Invert the bottle of water and immediately put its mouth below the surface of the water in the shallow pan. The water won't descend out of the bottle because the pressure inside the top of the bottle quickly drops below atmospheric pressure. The resulting pressure imbalance supports the water against the force of gravity. But when you lift the bottle high enough to allow an air bubble to enter its mouth, the water will descend and some of it will flow out of the bottle.

Explanation: The number of air molecules trapped between the water and the top of the bottle is limited and as the water falls downward in the bottle, those air molecules spread out into a greater volume. As the gas's density drops, its pressure also drops and soon the pressure inside the top of the bottle is significantly below atmospheric pressure. With atmospheric pressure pushing upward on the water at the mouth of the bottle and less than atmospheric pressure pushing downward on the water at the top of the bottle, the water is experiencing enough upward force due to pressure to prevent it from descending further. But whenever an air bubble is allowed to enter the bottle, these additional air molecules increase the pressure at the top of the bottle and allow more water to descend out of it.

Description: A collection of oddly shaped water containers (Pascal's vases), that are connected at their bottoms, is gradually filled with colored water. The water always flows through the connections so that the water levels in each container are exactly equal.

Purpose: To show that water will naturally flow until its surface is uniformly at the same height.

Procedure: Slowly pour the colored water into one of the vases. As the height of water in that vase rises, the pressure at the bottom of that vase will also rise and the water will begin to flow through the connections to the other vases. If you proceed slowly enough, the water levels in all the vases will remain essentially equal. When you stop pouring, the water levels will soon become exactly equal.

Explanation: If the water level in one of the vases is higher than in the others, the pressure at the bottom of that vase will exceed the pressures in the other vases. Water will accelerate toward the less deeply filled vases and their water levels will soon rise. Only when the water levels are exactly equal will there be no pressure imbalances and no flow between vases. Another way to look at this problem is to realize that when the levels aren't equal, the water has more than its minimum total potential energy -- it can reduce its gravitational potential energy by letting the highest water descend into the lower empty regions. Since any system accelerates in the direction that reduces its total potential energy as quickly as possible, the water in these vases accelerates so as to make its surface level uniform. After oscillating about that stable equilibrium briefly, the water settles down at a uniform level.

Description: A large cup of water has two holes in its lower sides through which water squirts. But when the cup is dropped, the water stops squirting out of the holes.

Purpose: To show that the elevated pressure at the bottom of a cup of water disappears when the cup of water is falling.

Procedure: Cover the two holes with your fingers and fill the cup with water. Climb onto the stool or ladder. Now uncover the holes and water will begin to squirt out of the holes. Point out that this water is propelled outward by the elevated pressure near the bottom of the water. Now drop the cup. As it falls, the pressure inside the cup will be uniform—all the water will be at atmospheric pressure. With no elevated pressure inside the cup, water will no longer squirt out of the holes.

Explanation: When the cup is motionless, an elevated pressure develops at the bottom of the cup as the water there acts to support the water above it. It's this elevated pressure that causes the water to accelerate toward the holes and squirt out into the air. But when the cup is in free fall, the water at the bottom of the cup no longer has to support the water above it. No pressure gradient develops and the water is uniformly at atmospheric pressure inside the cup. Without any pressure imbalance between the atmospheric pressure water near the bottom of the cup and the atmospheric pressure air outside the holes, the water doesn't accelerate toward the holes as the cup falls. No water squirts out into the air.

Description: You pour water into a tank at the top of a long hose with a nozzle at its lower end. The water sprays vigorously from that nozzle.

Purpose: To show that as water descends in a uniform pipe, its pressure rises. That high pressure is responsible for spraying water hard from the nozzle.

Procedure: Connect the hose between the elevated tank and lower nozzle. Pour water into the tank and watch it spray out of the nozzle. After a second or two, steady state flow will be established and the water will be transforming its gravitational potential energy in the tank to pressure potential energy in the hose (and then to kinetic energy in the nozzle).

Explanation: The descending water is giving up gravitational potential energy. Since the hose has a uniform cross section, the water can't speed up or slow down, so its kinetic energy can't change. Instead, its pressure potential energy rises. Evidence of that high pressure comes from letting the water spray out of the nozzle. The higher the tank, the more vigorous the spray.

Description: You suck water up a straw.

Purpose: To show that when you remove the air from the top of a straw, the atmospheric pressure at the bottom of the straw pushes the water up the straw.

Procedure: Insert the straw into the container of colored water and suck the water into your mouth. Point out that you aren't "attracting" the water toward your mouth—you are reducing the pressure inside the top of the straw so that the pressure at the bottom of the straw can push the water upward toward your mouth.

Explanation: By expanding the volume inside your mouth and the top of the straw, you reduce the density of the air trapped inside that volume and reduce its pressure. Since the atmospheric pressure at the bottom of the straw doesn't change, there is a pressure imbalance. This pressure imbalance exerts enough upward force on the column of water in the straw to lift it upward to your mouth. The water flowing up the straw toward your mouth is converting its pressure potential enegy into gravitational potential energy.

Description: You suck water up a very tall hose and have great difficulty raising it more than about 8 m. (This experiment requires a tall lecture hall, with access to the upper space at the front of the hall.

Purpose: To show that because atmospheric pressure is limited, it can't support a column of water that's taller than about 10 m, even when there is no pressure above that column.

Procedure: Hang the hose vertically from the upper space of the room and insert the bottom of the hose into the container of colored water. Go up to the top of hose and suck the water toward your mouth. Again, point out that you aren't "attracting" the water toward your mouth—you are reducing the pressure inside the top of the hose so that the pressure at the bottom of the straw can push the water upward toward your mouth. As the water column gets taller, you will have more and more trouble making it rise. While you may be able to draw a column 8 m high, you won't be able to reach or exceed 10 m.

Explanation: By expanding the volume inside your mouth and the top of the straw, you reduce the density of the air trapped inside that volume and reduce its pressure. Since the atmospheric pressure at the bottom of the hose doesn't change, there is a pressure imbalance. This pressure imbalance exerts enough upward force on the column of water in the straw to lift it upward to your mouth. But the highest that atmospheric pressure can lift the water is 10 m, even if you remove all of the air molecules from above the water column. Since your mouth isn't capable of reaching a complete vacuum, you can't suck the water upward even 10 m. Another way to look at this problem is in terms of energy. The water is converting its pressure potential energy into gravitational energy as it rises up the tube. Since its pressure potential energy is limited because atmospheric pressure is limited, the water can only rise so high. Even with no pressure above it, the water can only convert its limited pressure potential energy into limited gravitational potential energy. Its height can only rise so far.

Description: Two containers of water are connected by a water-filled tube. When one container is raised so that the levels of water are different in the two containers, water flows from the higher container to the lower one until their water levels are again equal.

Purpose: To show that the tendency of water to level its surface even applies when the water must flow upward a short distance to flow between two containers.

Procedure: Fill both containers with colored water. Now fill the hose with water and insert its ends in the two containers. Water will flow through the hose until the water levels in the two containers are equal. If you now raise one of the containers by placing the block under it, water will flow out of that container through the hose and into the lower container until their water levels are again equal.

Explanation: The top surface of the water in each container is at atmospheric pressure. When those surfaces are at the same height, the water at the highest point in the hose isn't experiencing any pressure imbalance. That's because its heights above the atmospheric pressure levels in the two containers are equal. But when one surface is lower than the other, the pressures on opposite sides of the highest point of the hose are no longer balanced and water accelerates toward the side with the lower water level. Even though the hose may meander up and down on its way between the two containers, what matters most is the height difference between the atmospheric pressure water at the top of one container and the atmospheric pressure water at the top of the other container.

Description: You pour water into a tank at the top of a long hose with a nozzle at its lower end. The water sprays vigorously from that nozzle.

Purpose: To show that high pressure water can acquire high kinetic energy when its sprays out of a nozzle.

Procedure: Connect the hose between the elevated tank and lower nozzle. Pour water into the tank and watch it spray out of the nozzle. After a second or two, steady state flow will be established and the water will be transforming its gravitational potential energy in the tank to pressure potential energy in the hose and then to kinetic energy in the nozzle. Aim the nozzle upward and show that the water spray rises to almost the height of the water in the tank. Show that as you raise the tank's level, the spray's height rises correspondingly. Point out that overall, the water's energy is going from gravitational (in the tank) to pressure potential energy (in the hose) to kinetic (after the nozzle) to gravitation (as it reaches its highest point in the spray) to kinetic (as the spray falls back to the ground)...

Explanation: The high pressure water in the hose accelerates toward lower pressure beyond the nozzle and sprays rapidly out of the nozzle. Since the total energy of the water is limited by the height of the tank, the water can't spray higher than that tank. In fact, the water doesn't reach the tank because it has wasted some of its total energy en route (a topic for later in the course). The spray slows as it rises (it is converting kinetic energy into gravitational potential energy) and it speeds up as it descends (it is converting gravitational potential energy into kinetic energy).

Description: A baseball is fired out of a plastic pipe at incredible speed when a stopper is knocked away from the other end of that pipe.

Purpose: To how large the forces of air pressure can be on an object.

Procedure: Attach the valve assembly to the side of the PVC pipe so that you can pump the air out of the pipe using the vacuum pump and then seal the pipe with the valve. Use rubber cement to glue the gasket ring to the exit end of the PVC pipe. Drill the rubber stopper so that the steel rod can fit through it. The rod should seal the hole in the stopper tightly. The rod is necessary so that you can knock the rubber stopper away with the mallet. The wide end of the rubber stopper will seal the air-entry end of the pipe, with the narrow end of the stopper facing away from the pipe and the steel rod projecting outward another 6 or 8 inches. In this arrangement, you should be able to hit the steel rod with mallet and abruptly open the end of the pipe to air. Don’t stick the stopper in narrow-end first because you won’t be able to remove it quickly enough.

Tie one end of the elastic cord to a hole drilled on the outer edge of the fiberglass disk and the other end to an object beside the exit end of the cannon. The cord should have to pull taut in order for the disk to cover the exit end of the cannon. This disk will seal the exit end of the cannon until the last possible moment, when rising pressure inside the cannon will allow the disk to slip away from the exit end of the pipe and be pulled clear by the elastic cord.

To operate the cannon, put the baseball in the air-entry end of the pipe and seal both ends. Two people are needed: one to hold the fiberglass disk in place and another to hold the rubber stopper in place. Now turn on the vacuum pump and pump all the air out of the cannon. As soon as the fiberglass disk clings tightly enough to the exit end of the pipe, let go of that disk and get away from the exit end of the pipe. Never, ever stand in the line of fire from the cannon! When all the air is out of the cannon, seal the valve and turn off the vacuum pump. To fire the cannon, use the mallet to knock the rubber stopper away from the air-entry end of the pipe. Air will then rush into the pipe, accelerate the baseball forward, allow the fiberglass disk to snap free, and fire the baseball out the exit end of the pipe at roughly 150 mph. The ball will travel about 500 feet in an open space if you angle the cannon properly. You can also fire just about any spherical object that fits in the pipe, including apples, oranges, ping-pong balls, foam-core balls, etc. They come out at up to 250 mph and may explode when they hit things. BE CAREFUL!

Explanation: When you remove all the air from the pipe, you do work on that air. Allowing the air to return suddenly through one end of that pipe releases the stored energy. With a huge pressure imbalance on its two halves, the baseball accelerates toward lower pressure in front of it and blasts out the exit end of the pipe at astonishing speed. Viewed in terms of energy, the atmospheric pressure air accelerates as it flows through the pipe's opening (effectively through a nozzle!) and the air converts its pressure potential energy into kinetic energy. That high-speed air sweeps the ball along with it and sends it out the end of the pipe at tremendous speed.