Physics-Fluids (Science)

"If there is magic on the planet, it is contained in Water."—Loren Eiseley (1907–1977)

What happens if you push on the surface of water? It flows away from the point at which you are pushing and your finger fills the space formerly filled by water. If the water is frozen, however, this does not happen. Substances that flow under stress are called fluids. Generally, the term applies to liquids and gases. In solids, atoms or molecules are held tightly to the atoms and molecules around them. It is difficult to move one particle away from nearby particles. In gases and liquids, these particles are easily separated from one another. As a result, the physical properties of fluids are often very different from those of solids.

The key difference between materials that are fluid and those that are not fluid is the ability of particles to move independently. Traffic engineers use the physics of fluids to analyze the flow of vehicles on busy highways. The ebb and flow of traffic can often be explained by the mathematics of fluid flow.

A fluid is any substance that continually deforms under stress, no matter how small the stress that is applied. Although all liquids and all gases are fluids, some fluids are neither liquid nor gas.

What is the shape of a raindrop?

Everyone knows the shape of a raindrop, right? It is rounded on the bottom, tapering to a point at the top. Wrong. This classic shape is familiar to everyone—it shows up in books, illustrations, the nightly TV weather forecast, and sometimes even in science textbooks. While it is true that a drop forming on a dripping faucet has approximately that shape as it clings to the spout, a free-falling drop is a different story. It is hard to look at falling rain, though, and analyze the drop as it passes your face. High speed photography has come to the rescue.

In reality, raindrops don’t have a "raindrop" shape or anything resembling it. The shape of a raindrop depends on its size and how fast it is falling, but it tends to be roughly spherical. Look up at the sky on a cloudy day and you see a collection of very small water droplets. They form when water collects on small pieces of dust. The molecules of water are attracted to one another and pull together into a shape that keeps them closest. This shape is a sphere. At the beginning, these spheres are very tiny, ranging from about one one-thousandth to one twentieth of a millimeter in diameter. As these small droplets move around, they bump into one another and form larger drops. Eventually these drops become too heavy to be supported by the air around them and they fall toward the ground.

The speed at which a raindrop falls i s determined by the pull of gravity on the water and friction between the water and the air around it. The smallest drops hang in the air as fog, the largest can reach speeds of about 20 mph before the effects of wind resistance break them apart.

As rain falls, the drops continue to bump into one another, sometimes clinging together to grow larger and sometimes breaking apart. The size of raindrops varies from about 1 to 4 millimeters in diameter. The smallest of them tend to keep a spherical shape as they head for the ground. Larger drops are affected by the force of air pushing against them from below. Because of surface tension from the attraction of water molecules on the outside of the drop, the top of the drop keeps its rounded shape. The bottom flattens, however, due to the air pressure. As the drops grow even bigger, the flat bottom pushes inward and becomes a depression, sort of like a soccer ball that has lost all its pressure and then had one side pushed inward.

As raindrops grow, the depression becomes bigger so they are thinner and thinner in the middle. Eventually, the drop becomes too large (greater than 4 mm in diameter) and it splits into two smaller drops.

Surface tension is a property of a liquid that causes its surface to act as a sheet. Surface tension is a result of forces that cause particles (atoms or molecules) of the liquid to be attracted to one another.

Why does a bicycle pump get hot when you inflate a tire?

Try pumping up a tire with a bicycle pump by rapidly pushing up and down on the handle of the pump several times. If you feel the barrel of the pump, you will find that it has become quite warm. One possible explanation for the heat is friction. However, you can try the experiment in reverse. Close the end of the hose with the handle all the way down. Now pull the handle up very quickly and you will find that the barrel of the pump is cold. Friction won’t explain that temperature change, so what is happening?

Gases, such as air, are different from liquids and solids in that they are compressible— that is, they can be pushed into smaller volumes of space. Gas molecules tend to fly around freely with a lot of space in between. The temperature of a gas is a result of this motion and heat is transferred as particles bump into one another. You detect the temperature of air when its particles strike your skin, transferring energy from the particles to you. On a hot day, faster-moving molecules hit your body more often, so more energy is transferred. On a cold day, the particles are lethargic by comparison, so there are fewer collisions, each transferring less energy.

When you compress air in a bicycle pump, your muscles transfer energy to the handle, which in turn transfers energy to the molecules of air in the pump. This additional energy makes the molecules move faster. As they are compressed into a smaller space, they also collide more often with the wall of the pump, so they transfer more energy to the metal wall and it becomes hot.

The temperature, pressure, and volume of a gas are related to one another. Temperature increases as the volume occupied by the gas decreases and/ or its pressure increases. When you compress it, air gets hotter. When you expand air, it gets colder as you remove energy. The particles move slower and don’t collide as often.

It is this relationship between pressure, volume, and temperature that explains the temperature difference between the top and the bottom of a mountain. Air at the bottom of the mountain is compressed by the weight of the air above it, so it tends to be warmer.

If a mass of air rises in the atmosphere, the pressure drops and the volume of the air expands so it becomes cooler. If a mass of cool air drops from the upper atmosphere to a lower level, it becomes compressed. The molecules are pushed closer together and the air becomes warmer. These changes in pressure, temperature, and volume of air masses can release large amounts of energy that fuel thunderstorms and even hurricanes.

Air is compressed by the mass of air above it, causing the air at lower elevations to have a greater density (density is the mass of material divided by its volume) and higher temperature, if all other factors are equal. The temperature at the bottom of the Grand Canyon is normally about 25°F higher than the temperature at the canyon’s rim due to the difference in air pressure.

Why does an oil tanker float?

Everyone knows that some things float in water and some sink. A small stone, weighing a few ounces, sinks. Meanwhile, an oil supertanker, which weighs hundreds of thousands of tons, has no trouble staying afloat. What keeps the steel ship on top of the water but not the stone?

Buoyancy is an upward force exerted on an object in a fluid as a result of the difference in pressure exerted by the fluid on the top and the bottom of the object.

The answer to that question is based on the physics principle called buoyancy, a force that pushes upward on an object in a fluid. Because water is a fluid, an object placed on top of it pushes some of the water out of the way. The amount of force used to push the water is equal to the force of gravity on the object. The object—a stone or a tanker— pushes water out of the way until the mass of water moved is equal to the mass of the object itself. The buoyant force is equal to the mass of the water that an object displaces.

No matter what the weight of the object, it will float if the buoyant force is greater than the pull of gravity. That means that a stone, which has more density than water, sinks to the bottom. It continues falling through the water, pulled by gravity, until it reaches the bottom. If the object is less dense than water, it sinks until it has displaced an amount of water equal to its weight. After that, the buoyant force causes it to float.

Ships are full of air pockets and voids inside the hull that cause them to reach this displacement before the water reaches the top of the boat. Although steel is denser than water, the shape of the ship and the empty spaces inside make the ship less dense than water. However, if you replace the air inside the ship with water, it quickly sinks. Watch The Poseidon Adventure to see what happens as density increases.

"Any solid lighter than a fluid will, if placed in the fluid, be so far immersed that the weight of the solid will be equal to the weight of the fluid displaced."

Materials that are less dense than water don’t need to be hollowed out in order to float. Place a wooden block and a foam block in a container of water. Both materials will float. However, much more of the wooden block will be beneath the surface because it has a greater mass and displaces more water.

How does a submarine rise and submerge?

Ships float, rocks sink—and then you have submarines. They can float on top of the water or dive beneath it. How do submarines go up and down?

The buoyant force occurs because the pressure in a fluid increases with its depth. You can feel this pressure difference in your ears when you dive into a pool. The deeper you go, the more the water pushes on your eardrums. When an object is placed in water, the water pushes on it. The push is stronger as depth increases, so the overall force is upward, opposite the force of gravity. When you hold an object underwater, it feels lighter than in air because of this upward force.

Submarines use this principle when they move up or down in the water. A submarine has a double hull. The space between the two hulls forms tanks that can be filled with water or air. When these ballast tanks are filled with air, the buoyant force is greater than the force of gravity so the submarine floats on the surface. As the air in the tanks is replaced with water, the weight of the ship increases although its volume remains the same. When the downward pull of gravity exceeds the buoyant force, the submarine sinks toward the bottom of the ocean.

By adjusting the amounts of air and water, the crew can hit the point where the two forces are just equal. Then the sub floats freely, neither sinking nor rising in the water. To return to the surface, compressed air is blown into the tanks, forcing the water out and again reducing the weight of the sub until its density is less than that of water.

Submarines must be designed to withstand great pressure due to the weight of water above them. If you dive into a deep swimming pool, you feel discomfort at a depth of 10 feet because the pressure is 1.3 times as great as the pressure of the atmosphere. At 100 feet, a submarine experiences a pressure that is about 4 times atmospheric pressure, or 4 atmospheres. At 300 feet the pressure increases to almost 10 atmospheres.

How do hot air balloons and helium balloons rise in the air?

Ballooning is a popular pastime, so popular that some races have as many as a thousand contestants. How does the propane burner in the basket beneath a large balloon cause it to rise in the air?

Hot air balloons use the same relationships among pressure, volume, and temperature that we discussed concerning air pumps. As the burner rages beneath the air in the balloon’s envelope, the temperature rises so the air molecules move faster. When the air gets hotter, its volume or its pressure must increase at the same time. Because the balloon is open on the bottom, air can escape freely, so the pressure remains the same as the pressure of the air around it. That means the volume of air increases. As the volume increases, the balloon inflates, until it reaches its maximum size. After that, any increase in volume forces air out of the balloon through the opening.

As air is forced out of the balloon, the mass of air that it holds decreases while the volume stays the same. That means the density of the balloon decreases. Because the principles of buoyancy apply to all fluids, whether they are liquid or gas, the change in the balloon’s density has the same effect as the change in density of a submarine in water. If enough air is forced out, the average density of the balloon, its basket, and the people in the basket becomes less than the density of the air surrounding it. Then the balloon rises due to the upward force of buoyancy. To lower the balloon, the pilot opens a flap at the top. Warm air escapes through the top and is replaced by denser, cooler air from the surrounding atmosphere.

It is sometimes stated that a hot air balloon rises because heat rises. Heat, being a form of energy rather than a substance, cannot rise. The heated balloon rises because its density is less than that of the cooler air surrounding it. If you heat air in a closed container so that its volume cannot change, the container will not rise in the air. However, the pressure inside the container will increase.

A helium balloon also depends on buoyancy to rise into the atmosphere. Helium is much less dense than air because its particles have smaller mass than the particles of nitrogen and oxygen. One liter of helium has a mass of 0.179 grams while 1 liter of air has a mass of 1.25 grams. So if you have a 1-gram balloon filled with helium, its mass is less than the mass of air that it displaces. In a vacuum, it would read 1.179 grams when placed on a scale, but in air, it rises above the scale.

The balloon rises because the buoyant force is greater than the force of gravity that pulls it down. If you have enough helium displacing air, the upward force can lift a large mass. Think about the blimps that carry camera crews above sporting events.

The blimp itself is very heavy and it is capable of lifting a large load. The difference in the mass of helium that fills the blimp and the mass of the air that would fill the same space provides all the lift.

What happens if the helium leaks from the balloon? You have no doubt seen the result. As the helium leaks through the wall of the balloon or around the closure, the balloon shrinks. The volume changes but the mass of the balloon does not change much at all because most of the mass is contributed by the balloon material, not the helium. As a result, the density of the balloon gradually increases. When the pull of gravity is stronger than the upward push of the air, the balloon sinks to the floor.

Like any other substance, helium has mass and is attracted to Earth by the force of gravity. Helium will only rise if it is surrounded by something (such as nitrogen and oxygen) with a greater density. If all the air were removed from a room so that there is a complete vacuum, a helium balloon and a bowling ball would fall from a shelf to the floor at exactly the same rate.

How can an airplane that weighs many tons stay in the air?

The difference in density of a helium balloon and the air around it can explain the rise of a balloon. An airplane, however, is a different story. You know from experience that the metal components of the airplane, the passengers, and the cargo have a density that is much greater than air. Did you ever look down at the ground, 5 miles below, and wonder just what it is that is keeping you in the air?

The basic reason that an airplane can fly is the same as the reason a balloon stays aloft—the force pushing it upward is equal to the force pulling it down. The biggest difference is that an airplane is much denser than air, so it is not buoyed up by a static difference in air pressure. The lift of an airplane comes from moving air. According to Bernoulli’s principle, the pressure of a fluid decreases as it moves faster.

Bernoulli’s principle states that the pressure exerted by a fluid decreases as its velocity increases.

If you look closely at an airplane wing, you will see that it is curved on top and flat on the bottom. As the air flows past the moving wing, the air above the wing has to move farther because of the curvature. In order to do that, the air on top must move faster and therefore have a lower pressure. The pressure below the wing is higher, so it pushes the plane upward.

Most people have investigated the Bernoulli principle without realizing it. Have you ever held your hand outside the open window of a moving car? If you hold it perfectly flat, you feel pressure along the front edge but no force up or down. Angle your hand slightly upward, though, and you feel a strong lift that tries to push your hand up and away.

Another common illustration of the Bernoulli principle requires a sheet of paper and a puff of air. Hold the paper in front of your mouth and blow above it. You might think the moving air would force the paper downward, but you observe just the opposite. The moving air above the paper exerts less force than the stationary air below it and the paper moves upward.

A baseball pitcher uses Bernoulli’s principle to throw a curve ball. The ball is launched with a strong spin so that one side of the ball has a greater velocity toward the batter than the other side. That means that on one side the air is moving faster relative to the ball and therefore exerts a lower pressure. On the other side the air is moving slower, exerting a higher pressure. The ball is pushed toward the lower-pressure side, causing its path to curve.

Watch what happens to the shower curtain the next time you turn on the water. The curtain immediately moves inward. The falling water from the shower head carries air along with it. The lower pressure on the inside of the shower curtain causes it to move inward—yet another example of the Bernoulli principle.

Why do your ears pop during takeoff and landing?

If you have flown in a large jetliner, you likely experienced a "popping" in your ears during the takeoff and landing parts of the flight. Sometimes the same thing happens as you drive in the mountains. What causes your ears to become uncomfortable and then pop as you travel?

The popping comes as a result of changes as you travel through a fluid—air. The air in the atmosphere exerts a downward pressure on everything below it, including the gases that make up the lower atmosphere. That means the pressure decreases as you go upward through the air and increases as you go downward. In general, you don’t feel these changes inside your body. However, your inner ear is filled with air, a gas.

You can usually help your Eustachian tubes in the job of equalizing pressure by yawning, chewing, or other motions that open and close the tubes. Doctors do not recommend closing your nose and mouth and blowing, however, because you risk forcing fluids into your ears or damaging the eardrum.

The moving parts of your ear, especially the eardrum, need for the pressure inside and outside the ear to be fairly close to the same in order to work correctly. Small tubes, called the Eustachian tubes, provide an opening to pump air into or out of the inner workings. Under normal conditions, air pressure changes very gradually and the Eustachian tubes do their job without your knowledge.

An airplane moves up and down through the atmosphere very rapidly, however, so the tiny tubes can’t always keep up with the change. As you ascend, air becomes trapped in the inner ear, pushing your eardrum outward. You feel discomfort and have trouble hearing because the eardrum can’t vibrate normally. When the Eustachian tubes open, the air escapes rapidly. You hear a popping sound and feel the pressure inside and outside your ear become equal. The opposite effect occurs on the way down. Air pressure outside your ear increases, pushing the eardrum inward. The pop comes as air rushes through the tubes to your inner ear.

The cabins of airplanes that fly at high altitudes are normally pressurized for the comfort of passengers, but the difference in pressure between the cabin and the outside is not enough to cause the effects often seen in movies. A small crack in the window or hole in the structure will allow air to escape but it will not suck passengers to their deaths. Opening a doorway is still not recommended, however, due to the risk of falling through the large opening.

Does quicksand really pull people under?

Many older movies set in the American West or in the jungle include a quicksand scene. Someone falls into quicksand and is slowly pulled under. The evil victim disappears beneath the surface while the good victim is rescued in the nick of time when our hero drops a vine from above and pulls the victim straight up and out. But does quicksand really pull people beneath its surface?

You may not think of quicksand as a fluid because sand itself is a solid. Quicksand is ordinary sand in which a large amount of water reduces the friction between particles and allows them to move easily past one another. It is this ability of the particles to move that makes quicksand a fluid with properties similar to a liquid.

The "quick" in quicksand does not refer to motion. It comes from an old usage of quick as "living," because quicksand seems more alive than regular sand. Quicksand can occur anyplace at which there is a source of moving water beneath a layer of sand or gravel. Normally it is no more than 3 or 4 feet deep.

Quicksand occurs when sand or sandy soil becomes saturated with water, making a batter like mixture. Generally, there must also be some kind of agitation, such as water flowing from a spring. Earthquakes can also provide the agitation needed to turn very wet sand into quicksand, sometimes causing buildings to sink into the ground.

If you happen to fall into quicksand, all is not lost. In the movies, quicksand pulls people under. In real life, the density of quicksand is greater than the density of the human body. Based on the principle of buoyancy, that means you will float in quicksand. Normally you will sink only as deep as your waist. But getting out is a bit more complicated than grabbing the vine that your hero dangles. If you place pressure against the wet sand, it tends to lose water and become more solid. Pull straight upward and it will feel like your foot is in concrete.

The experts advise that the first step in getting free comes straight from The Hitchhiker’s Guide to the Galaxy: don’t panic. Then slowly and carefully get rid of anything that adds weight and density to your body. Wiggle your legs slowly to get more water around them. Finally move toward the edge with a slow, gentle swimming motion.

"Let us hope that the advent of a successful flying machine, now only dimly foreseen and nevertheless thought to be possible, will bring nothing but good into the world; that it shall abridge distance, make all parts of the globe accessible, bring men into closer relation with each other, advance civilization, and hasten the promised era in which there shall be nothing but peace and goodwill among all men."

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