Fluids are forms of matter that can flow. Density is a measure of the mass contained

in a given volume. Substances with a lower density will float on substances with a

higher density.

Fluids and Density7.2

Key Termsdensitydisplacementfluid

260 MHR • Unit 3 Fluids and Dynamics

Liquids and gases are fluids, forms of matter that can flow. In this activity, you will share your knowledgeof fluids.

What to DoDivide a piece of notebook paper into quarters. Labelthe sections Box A, Box B, Box C, and Box D.

1. In Box A, list all the fluids you can think of.

2. In Box B, suggest more than one way you can makea fluid flow more quickly.

3. In Box C, suggest several applications (uses) whereheating fluids is important.

4. In Box D, suggest several applications wherecooling fluids is important.

5. Share your information with a partner.

Fluids Can Flow7-5 Think About It

Figure 7.7 Lava flows from an erupting volcano, water flows in a mountain stream, andpancake syrup flows from its container.

What do pancake syrup, water in a mountain stream, and lava flowingfrom a volcano have in common? They are all fluids (Figure 7.7). Afluid is any form of matter that can flow. Liquids and gases are able toflow because they do not have a fixed shape. Solids have a fixed shapeand cannot flow. Therefore solids are not fluids. Your body containsmany fluids, such as blood and the watery cytoplasm inside cells. Airflows into your lungs each time you inhale, and out of your lungswhen you exhale.

ces

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Chapter 7 Kinetic molecular theory explains the characteristics of solids, liquids, and gases. • MHR 261

Using what you already know about particles and the state ofmatter, predict which is likely to be the densest, a solid, a liquid, or agas. Take a look at Figure 7.9. Aluminum (a solid), is denser thanwater (a liquid), and water is denser than air (a mixture of gases)—butwhy? The key to density is the spacing of the particles. The particlesof a piece of solid aluminum are tightly packed, while liquid waterparticles have enough room between them to change position. Theparticles of air are free to move independently and have a largeamount of space between them. Less densely packed particles will“float” on more densely packed particles. As temperature increases, a substance will change from solid, to liquid, to gas. According tokinetic molecular theory, the particles of a substance spread out asthey gain energy when heated. The particles will take up more space,which means that the density of the substance decreases.

Most substances are denser in their solid form than their liquidform, but water is an exception. When water freezes, the particlesmove slightly farther apart as they become fixed in position. Thismeans that ice is actually less dense than liquid water, so it floats (see Figure 7.10).

Section 10.2 has moreinformation about the densityof water and ice.

Connection

Figure 7.8 When traffic gets very dense, it is difficult for vehicles to move.

Solid, Liquid, and Gas Density One property that is useful in understanding both fluids and solids isdensity. Density is the mass of a given volume. In other words, densitydescribes how closely packed together the particles are in a material.

You might think of density in terms of vehicles on a highway. Atraffic jam like the one on the left in Figure 7.8 is a model of highdensity. The photograph of free-flowing loosely packed traffic on theright in Figure 7.8 is a model of low density.

Figure 7.9 A sealed containerholds air, water, and an aluminumblock.

Figure 7.10 The property of icefloating on water makes life infreshwater lakes possible. If icesank as it froze, lakes would freezesolid. Instead, the floating ice buildsslowly from the top down, creatingan insulating barrier against coldtemperatures.

Did You Know?

There is space between grains ofsugar. When you melt sugar tomake candy, the sugar becomesa fluid. After the sugar cools, itcontracts, making the candy even more dense than theoriginal sugar.

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262 MHR • Unit 3 Fluids and Dynamics

Generally speaking, solids are denser than liquids, andliquids are denser than gases. In this activity, you willdiscover whether a fluid could be denser than a solid.

What to DoThe approximate densities of some common substancesat 20°C are listed in the table. The higher the number,the denser the substance. Use the information in thetable to answer the following questions.

1. Which substance in the table is the densest?

2. Which substance is the least dense?

3. Which fluid is denser than lead?

4. Water is denser than which three solids?

5. (a) Which substances would float in water?

(b) Which substances would sink in water?

6. Which common metals are less dense thanmercury?

Dense, Denser, Densest7-6 Think About It

Layers of FluidsImagine two beakers, one filled with water, and one filled with cornsyrup. Does the water or corn syrup have the greater mass? Which hasthe greater density? Recall that density is the mass of a given volume.When you compare the masses of equal volumes of different kinds ofmatter, you are comparing their densities.

Some liquids float on top of others. Liquids will layer in order of density—the less dense liquid floats on the denser liquid if the twoliquids do not mix together. How would corn syrup and water belayered if they were placed in the same beaker? Even though the twoliquids are of the same volume, the corn syrup has more mass, andtherefore it has a greater density than the water. Placing them togetherin the same container would result in the water floating on top of thecorn syrup.

This layering according to density can even occur within the same substance. Air is an excellent example; differences in air densitycontribute a great deal to weather. When air is heated near the groundon a hot summer day, the particles gain energy and move farther apart. The warm air has a lower density than the air around it, and as aresult, it begins to rise (see Figure 7.11 on the next page). As thewarm air rises, cooler air rushes in beneath it, and a breeze is created.

Section 5.1 has informationabout air layers and mirages.

Connection

Fluid Density (g/mL) Solid Density (g/cm3)

hydrogen 0.00009 Styrofoam™ 0.005

helium 0.0002 cork 0.24

air 0.0013 oak 0.70

oxygen 0.0014 sugar 1.59

carbon dioxide 0.002 salt 2.16

ethyl alcohol 0.79 aluminum 2.70

machine oil 0.90 iron 7.87

water 1.00 nickel 8.90

seawater 1.03 copper 8.92

glycerol 1.26 lead 11.34

mercury 13.55 gold 19.32

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10 000 m

8000 m

2500 m

sea level

Chapter 7 Kinetic molecular theory explains the characteristics of solids, liquids, and gases. • MHR 263

Figure 7.11 As low-density warmair rises, it can carry water vapourwith it. When the water vapourreaches the cooler higheratmosphere, it condenses into tinydroplets that we see as clouds.

Air is a mixture of many types of particles, but it is mostly made ofnitrogen and oxygen. Air particles are relatively dense close to Earth’ssurface. If we increase our altitude, we encounter areas of lower airdensity. The higher we go, the farther apart the air particles are spreadout, making it harder for us to get enough oxygen particles into ourlungs with every breath (see Figure 7.12).

Reading Check

1. Explain why gases and liquids are called fluids, but solids are not. 2. What happens to the density of matter when the matter is heated?3. Why does ice float on water?4. Why does water float on corn syrup? 5. How is a breeze created over land on a hot summer day?6. Why is there more oxygen available to breathe at sea level than

there is higher in the atmosphere?

Figure 7.12 At sea level (A), there are more than enough oxygen molecules for us to breathe. Most people can climb to 2500 m with no ill effects (B). But going higher will likelylead to symptoms of lack of oxygen. Oxygen masks are needed at this air density (C). Largeairplanes fly at high altitude because the air density is very low (D). With so much empty space between the particles, the airplane encounters less air friction, making flying moreefficient and requiring less fuel.

Find Out Activity 7-8 on page 268

Suggested Activity

Did You Know?

The cabins in large airplanes are pressurized so that the airdensity in the airplane is similarto the density on the ground.If the airplane loses cabinpressure, oxygen masks drop tothe passengers, providing themwith the oxygen they need.

D

C

B

A

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264 MHR • Unit 3 Fluids and Dynamics

Measuring Density Layering is a useful technique for comparing densities (see Figure7.13). When an object is placed in a less dense fluid, the object willsink down toward the bottom. If the fluid is denser than the object,the object will float. If the object has the same density as the fluid, theobject will “hover” in place. Layering allows you to determine whetherone substance is denser than another substance. However, layeringdoes not provide a specific measurement of density, and layeringcannot be used with solids. Solids do not flow, and their particles areso close together that other substances cannot move through them.How can you measure the density of a substance?

Recall that density is the mass of a given volume. To find thedensity of a substance you need to know its mass and its volume. Masscan be determined using an electronic scale or balance (see Figure 7.14).

The volume of a solid is often measured in cubic centimetres (cm3).A cubic centimetre is the volume of a cube that measures 1 cm oneach side. In other words, the volume of an object equals the numberof 1 cm cubes it takes to fill that object. The volume of an object thathas a simple shape can be determined mathematically (Figure 7.15).

Figure 7.13 The SuperBall® sinksin the oil, but floats on the water!

Figure 7.15 For objects that are block-shaped, volume canbe calculated mathematically by using the equation:volume � length � width � height.

Figure 7.14 This triple beam balance indicates theapple has a mass of 94 g.

Reading Check

1. How can you find the volume of a rectangular solid?2. How can you find the volume of an irregularly shaped solid? 3. What two measurements do you need in order to calculate

density?4. What is the volume of a rectangular box that is 10 cm long, 5 cm

wide, and 2 cm high?

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Chapter 7 Kinetic molecular theory explains the characteristics of solids, liquids, and gases. • MHR 265

Practice Problems

1. What is the density of a 2 cm3 sugar cube that has a mass of 3.18 g?

2. A 3 mL sample of oil has a mass of 2.64 g. What is the density of the oil?

3. The mass of 1 cm3 of lead is 11.34 g. The mass of 1 cm3 of iron is 7.87 g.Which solid has the greater density?

Density and Dinosaurs

The densest substance thatoccurs in nature is iridium, ahard, brittle, whitish-yellowmetal, with a density of 22.65 g/cm3. Iridium has aspecial connection to the endof the reign of dinosaurs onEarth. Find out more aboutiridium and its connection todinosaurs. Start your search atwww.bcscience8.ca.

Answers

1. 1.59 g/cm3

2. .88 g/mL 3. lead

DisplacementHow would you measure the volume of an object with an irregularshape? Displacement is the amount of space that an object takes upwhen placed in a fluid. Have you ever noticed how the water level rises in a bathtub when you get into it? The amount of water you are displacing is the volume of your body that is in the water. So bymeasuring the displacement of an object, you can measure the volumeof the object.

Calculating Density Once you know the mass and the volume of a substance, you cancalculate the density. You can calculate the density of both fluids andsolids. The units for density depend on how you measure the mass andvolume of your objects. The density of fluids is usually measured ing/mL, while the density of solids is usually measured in g/cm3

(1 mL has the same volume as 1 cm3).

density (D) �

Read the question:

1 mL of glycerol has a mass of 1.26 g. What it the density of glycerol?

Use the formula:

D �mV

�

State your answer:The density of glycerol is 1.26 g/mL.

mass (m) volume (V)

1.26 g1 mL

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Balloon Lift8-5

In this activity, you will observe whether a balloon can dowork.

Materials• straw • textbooks• balloon • strong elastic band• strong tape

What to Do1. Insert a straw into the mouth of a balloon. Seal the

straw to the balloon with tape. Wind the elasticband around the balloon seal.

2. Place a book on the balloon. Blow steadily into thestraw. Observe what happens.

3. Place one book at a time on top of the first book.Record how many books you can lift by blowinginto the straw.

What Did You Find Out?1. What provided the force to lift the book(s)?

2. Was it more difficult blowing into the balloon whilelifting several books than lifting one book? Why?

3. If you and a partner both blew into balloons so thattwo balloons were lifting the same stack of books,would it be easier than one person blowing into oneballoon? Why?

Find Out ACTIVITY

How would you use the word “pressure” to describe a fast-movingchampionship game of tennis? Would you say the players are under a lotof pressure to do well? Would you say the ball is under pressure as well?Pressure is the amount of force acting over a given area on an object.Take a look at Figure 8.16. As the pressure is applied to the ball, its shapechanges. What do you think has happened to the particles of air inside theball? The increase in pressure forces the particles closer together in asmaller volume. In other words, the space between the particles iscompressed. Compression is a decrease in volume produced by a force.

Pressure is the amount of force applied over a given area on an object. When

pressure is applied to matter, compression can result. Compression is a decrease in

volume produced by a force. Gases can be easily compressed, but it is very difficult

to compress liquids and solids. Pressure can be a good indicator of the kinetic energy

of a gas. The formula for pressure is P = F/A.

Pressure8.2

Key Termscompressionkilopascalpascalpressure

290 MHR • Unit 3 Fluids and Dynamics

Figure 8.16 The fastest tennis serves can travel over240 km/h. The force delivered to the ball is enormous.

Observe what happens when you blow into the balloon.

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Chapter 8 Fluids are affected by forces, pressure, and heat. • MHR 291

Gases Are Compressible A gas can easily be compressed because there is a large amount ofspace between its particles. Less than 1 percent of the volume of a gasis made up of particles. The rest of the volume is the space betweenthe particles. A container full of a gas, such as a tennis ball filled withair or nitrogen, can briefly absorb a great deal of force because of thespace between its particles.

Compressed gas can be useful for doing work. Creating thatcompression can be rather simple. All you need is a container to holdthe gas and the means to apply a force to it. A good example of usingcompressed air to do work is air compressor technology (see Figures8.17 and 8.18). An air compressor uses an electrically powered motorthat drives a piston. The piston repeatedly draws in air and compressesit, driving the air into a holding tank. The compressor effectively takesa large amount of air and compresses it into a much smaller volumeby pushing the particles closer together.

Figure 8.17 Highly compressed gas can be used to force sand particles at high speed. Thehigh-speed sand particles strike the surface with enough force to remove coatings, paint, or dirt.

Figure 8.18 In this photograph ofsandblasting, a special technique ofphotography allows us to see what isusually invisible to the eye—the high-speed jet of air and radiation of sound.

Did You Know?

Hyperbaric chambers are specialdevices used to place humanbeings under higher than normalair pressures. A person is placedin the chamber and air oroxygen is forced in to create ahigh-pressure environment. Thistreatment delivers more oxygento the tissues, so it speeds thehealing of injuries, helps treatinfections, and even combatscarbon monoxide poisoning.

Find Out Activity 8-7 on page 297

Suggested Activity

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292 MHR • Unit 3 Fluids and Dynamics

Pressure and Kinetic Activity in GasesPressure can be a good indicator of the kinetic energy of a gas. As youlearned in section 7.1, when energy is added to a gas the kineticenergy of the particles is increased, and the particles move faster. Thismeans that a warmer gas will expand in its container faster than acooler gas will. It also means that a gas that is trapped in a containerand heated will increase in pressure as the fast-moving particles bounceagainst the sides of the container. The increased pressure could causean explosion (see Figure 8.19).

Pressure can cause explosions, but did you know that it can causeimplosions as well? An implosion is a collapse inward. A small amountof water was placed in the metal can in Figure 8.20. The can was thenheated with the cap off. When the can was removed from the heat, thecap was quickly screwed on.

As the can cooled, the particles of gas trapped inside also cooled,causing them to lose energy and contract. This contraction caused thepressure inside the can to become lower than air pressure outside thecan. As a result, the air pressure on the outside pushed the walls of thecan inward, crushing the can.

Figure 8.19 Aerosol cans should never be heated because they could explode.

Carbon dioxide car racing isan interesting hobby madepossible by gas compression.Small cylinders of carbondioxide fit into the back ofthe custom-built cars. Whenit is time to race, thecylinders are triggered andcarbon dioxide rushes out athigh speed, propelling thecar forward. Carbondioxide–powered cars canreach speeds of 100 km/h inonly 20 m! Find out moreabout the compressed gaspowered vehicles. Start yoursearch atwww.bcscience8.ca.

internet connect

Figure 8.20 This metalcan has imploded becausethe air pressure outside thecan was greater than theair pressure inside the can.

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Chapter 8 Fluids are affected by forces, pressure, and heat. • MHR 293

Liquids and Solids Are Very Difficult to CompressWhat do you think would happen if you tried to compress a liquid or asolid? The particles of liquids and solids are already so tightly packedtogether that squeezing them together is almost impossible. Instead ofchanging the volume, the applied force is transmitted (passed along), fromone particle to the next, throughout the substance. Solids and liquids aredescribed as incompressible, which means they are not able to becompressed using normal means. See Figure 8.21.

Compression and DeformationIf you have squished a marshmallow or rolled upa foam mattress, have you compressed a solid?This appearance of compression in a solid can beexplained by the presence of air spaces withinthe solid. In Figure 8.22, Styrofoam® cups thatwere exposed to extreme ocean pressure on adeep-sea mission have been noticeably crushed.There are small air pockets in Styrofoam®, and it is these air pockets that are compressed anddestroyed by the pressure.

Solids can also appear to be compressed when they are deformed.Deformation means a change of shape without being forced into asmaller volume. For example, a solid rubber ball may appear tomomentarily compress when it hits the ground, but if it contains no air,it does not compress. So what is happening? The ball’s shape changesslightly as it hits the ground. As the ball deforms, it stores elastic energy.This elastic energy makes the ball bounce back upward.

A B C D

Figure 8.21 A: A bottlefilled with gas

Figure 8.21 B: When force isapplied to the bottle, the gasparticles move closer together.The gas is compressed into asmaller volume.

Figure 8.21 C: A bottlefilled with liquid

Figure 8.21 D: When force isapplied to the bottle, the liquid doesnot compress. There is no room forthe liquid particles to move anycloser together.

Figure 8.22 The Styrofoam®

cups in the foreground used to be the same size as the large cups, until their air pockets weredestroyed by deep-sea pressure.

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Balloon Burst 8-6

In this activity, you can find out the relationshipbetween area and pressure.

Safety

• Clean up any spills.

Materials • small balloons• water• plastic or ceramic coffee mug with a flat bottom• dull pencil• needle

What to Do 1. Do this test in a deep tub or sink (the balloons may

pop during testing).

2. Fill four balloons equally with water and knot theend of each one. Do not overfill the balloons. Theyshould have a small amount of flexibility.

3. Steady a balloon against the bottom of the tub.Place the bottom of the coffee mug against theballoon and push downward until the balloon popsor the mug touches the bottom of the tub.

4. Steady a balloon against the bottom of the tub.Place your pointed finger against the balloon andpush downward until the balloon pops or yourfinger touches the bottom of the tub.

5. Steady a balloon against the bottom of the tub.Place the dull point of the pencil against theballoon and push downward until the balloon popsor the pencil tip touches the bottom of the tub.

6. Steady a balloon against the bottom of the tub.Place the point of the needle against the balloonand push downward until the balloon pops or theneedle tip touches the bottom of the tub.

What Did You Find Out?1. Were any of the items unable to burst the balloon?

Offer a pressure explanation as to why the item(s)were unable to burst the balloon.

2. Which item poked the balloon with the leastsurface area? The greatest surface area? Explain.

3. Which method required the least amount of forceto burst the balloon?

4. Which method of bursting the balloon producedthe greatest amount of pressure applied to thesurface of the balloon? Explain.

Find Out ACTIVITY

294 MHR • Unit 3 Fluids and Dynamics

Compression and deformation canoccur at the same time, as shown inFigure 8.23. There are air spaces in thehuman skull, and the face is somewhatflexible. Therefore, when the soccer ballapplies a large force, the player’s headboth compresses and deforms. The soccerball also compresses and deforms becauseof hitting the player’s head.

Figure 8.23 Ouch! This soccer player’s face and thesoccer ball are temporarily compressed and deformed.

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Kilopascals are not the onlyacceptable unit of pressure.Meteorologists (scientists whostudy weather) often use theunit millibars (mb) to describepressure in weather systems,such as in the centre of ahurricane. Find out more about how millibars are used.Start your search atwww.bcscience8.ca.

internet connect

Chapter 8 Fluids are affected by forces, pressure, and heat. • MHR 295

Comparing Pressure

The circus can be a good place to learn more about pressure.Consider, for example, the bed of nails trick—it is not really a trick(see Figure 8.24). The performer actually does lie down on a bed ofnails, but the secret is to do it very carefully, making sure large areas ofthe body all touch the nails at the same time. This ensures the weightof the body is spread out over more nails. By spreading force over agreater area, pressure can be reduced.

The effect of area on pressure is reflected in the formula forpressure:

pressure (P) � or P �

Common units used in pressure calculations are newtons (N) forforce and square metres (m2) for area. In honour of the Frenchscientist Blaise Pascal, a single unit of pressure, 1 N/m2 is called apascal (Pa). This is very small unit of pressure, so 1000 Pa, or thekilopascal (kPa), is a much more commonly used unit whencomparing pressures (see Figure 8.25 and turn back to Figure 7.3B).

Vehicle tires commonly haverecommended pressures statedin pounds per square inch(p.s.i.). One p.s.i. is equivalent to6895 Pa. Using the proper p.s.i.can make your bike ride morecomfortable. For riding on arugged trail, a mountain biketire should be inflated to about40 p.s.i. so that it can absorbmore impact. For pavement, thep.s.i. should be 60 to 100 p.s.i.so that the tire rolls easily.

Did You Know?

Figure 8.25A A bull elephant exerts 65 000 N of force on a 1 m2 board. This isequivalent to a pressure of 65 000 Pa or 65 kPa.

Figure 8.25B The circus performerexerts 800 N of force on a 1 m2 board.This is equivalent to a pressure of 800 Pa or 0.8 kPa.

Figure 8.24 Theclown’s weight isspread over manynails.

F A

force (F) area (A)

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Scuba divers who return to thesurface too quickly from greatdepths can experience “thebends,” also known asdecompression sickness (DCS).Strangely enough, astronautsare also at risk of DCS. Inspace, there is no pressure soastronauts must wearpressurized suits. If those suitswere to fail, astronauts wouldsuffer from DCS. Find out moreabout the challenges ofexploring high- and low-pressure environments.Start your research atwww.bcscience8.ca.

Answers

1. 500 Pa 2. 200 Pa 3. 700 Pa

296 MHR • Unit 3 Fluids and Dynamics

Calculating PressureThe pressure an object exerts on a surface can be calculated if you aregiven the weight of the object and the dimensions of the surface it ispressing on.

For example, a BMX rider and bike weigh 1200 N. They are on arigid sheet of steel that is 1.0 m by 2.0 m. How much pressure doesthe sheet of steel exert on the ground?

Recall that the area of a rectangle is equal to length times width.Therefore,

P �

�

�

�

�

� 600 Pa

Practice Problems

Try the following pressure calculations yourself.

1. A vehicle loaded with garbage exerts 18 000 N of force on a scale thatmeasures 6.0 m by 6.0 m. What pressure does the scale put on thespring below?

2. A student sings on a stage while standing on a 2.0 m by 2.0 mplatform. If the student weighs 800 N, what pressure does the platformput on the stage?

3. A large cement block is carried in the back of a pickup truck. Thebottom of the cement block has dimensions of 8.5 m by 4.0 m and itweighs 23 800 N. What pressure does it put on the truck?

Reading Check

1. What term describes the amount of force acting over a given areaon an object?

2. What term describes a decrease in volume produced by a force?3. Why are gases easily compressed?4. Why will a container of gas explode if it is heated?5. Why will a container implode?6. Why is it almost impossible to compress a liquid or a solid?

F A

F (length � width)

1200 N(1.0 m � 2.0 m)1200 N2.0 m2

600 Nm2

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Checking Concepts1. Explain what is happening to the particles of

air inside a volleyball when compressedduring a serve.

2. Why are gases easily compressible but liquidsare not?

3. Helium balloons float upward when they arereleased. As they gain altitude they continueto expand until they burst. Why do theballoons burst?

4. Diving into the deep end of a swimming poolcan be accompanied by pain inside the ears. (a) Why might you feel pain inside the ears if

you dive deep? (b) Why does the pain go away when you

come back to the surface?

Understanding Key Ideas5. In the demonstration shown below left, a

soft drink can containing a small amount ofwater is heated over a Bunsen burner. Thecan is then overturned with the opening ofthe can placed in cold water (below on theright). After being placed in the cold water,the can quickly crushes inward. Explain theresult of this experiment. Why did the softdrink can implode?

6. (a) Calculate the pressure in each of thethree situations shown below. Show allyour work.

(b) Rank the three situations from highest tolowest pressure.

(c) The heaviest object does not have thegreatest pressure. Why not?

Humans cannot survive without protection inareas of very low or very high pressure. In thefuture, we may want to inhabit areas such asthe ocean bottom or the surface of Mars forextended periods of time. Write a paragraphdescribing how humans might cope withspending extended periods of time in areasof pressure extremes.

Pause and Reflect

Chapter 8 Fluids are affected by forces, pressure, and heat. • MHR 299

1 m

2.5 m

0.5 m

2 m

950 N

6000 N

2 m

2 m

8500 N

BCS_G8_U3C08_J19 5/4/06 4:32 PM Page 299

Fluids under pressure have many uses in industry and daily life. Fluids naturally

move from an area of high pressure to an area of low pressure. The movement

of fluids from an area of higher pressure to an area of lower pressure occurs in

natural systems as well as in constructed systems.

Most babies are born healthy, but sometimes a baby is born with a condition known as hydrocephalus. A normal brain has fluidsurrounding it. This fluid cushions the brain and helps transportnutrients. In hydrocephalic babies, there is too much fluidsurrounding the brain. The extra fluid creates an increase of pressure inside the skull, causing a bulging skull, seizures, and even brain damage (see Figure 9.1).

If hydrocephalus is treated, the baby can grow up to be normaland healthy. The standard treatment for hydrocephalus is to use atube to drain the fluid to the abdomen or another part of the body. A check valve in the tube helps ensure that the pressure in the brain is correct and the fluid does not flow in the wrong direction.Hydrocephalus is an example of a problem in a natural fluid systemthat is solved by a common piece of technology in a constructed fluidsystem.

Fluids under pressure are used in devices that assist our everydaylives. Compressed gases can produce forces that can be used in manyways. Liquids may not be compressible, but by placing them underpressure in confined places such as pipes, we can force them to movewhere we want them to go.

Fluids Under Pressure9.1

Key Termsbuoyant force buoyancyconvectionone atmosphere

314 MHR • Unit 3 Fluids and Dynamics

Figure 9.1 (A) A normal skull and brain. (B) The abnormally large skull of ahydrocephalic baby

Did You Know?

Some dentists use air abrasionto drill a tooth. Compressed airblows particles of aluminumoxide powder at high speed toremove the areas of decayedtooth.

A B

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Chapter 9 There are both natural and constructed fluid systems. • MHR 315

Fluid Circus 9-1 Find Out ACTIVITY

Atmospheric PressureEarth’s atmosphere extends more than 160 km above Earth. Every layer of airexerts pressure on the layers belowbecause all of the air particles are pulledtoward Earth by the gravitational force.You can observe the effects of airpressure with a simple experiment such asthe one shown in Figure 9.2.

Air pressure changes with altitude.You may recall from Chapter 7 that air isless dense at higher altitudes because theair there is less compressed. As you climbhigher in the atmosphere, fewer airparticles press against you on the outside of your body. The numberof particles pressing from the inside out is still the same at the top ofthe mountain as it was when you were at the base of the mountain.How do you feel this difference in pressure between the inside andoutside? Your eardrum is a very thin membrane that can move inresponse to a difference in air pressure. If the difference in pressure oneither side of the eardrum becomes great, you experience a “pop”inside your ear as the pressure equalizes.

What devices do you know that operate on fluidsunder pressure? In this activity, you can bring them to school for a fluid circus!

MaterialsBring objects from home that use fluid motion. Someexamples are whistles, balloons, and pump-up toys.You might think of many others.

What to Do1. Copy the following table into your notebook.

Give your table a title.

2. Add your objects to the class display.

3. Examine the objects and consider how and why they work. Complete the table as you look at the display.

4. Be prepared to explain to the class how theseobjects work.

Object Observations Fluid Used Reason for Movement

Figure 9.2 Blow across, under, andover the paper to try to make itmove off the books. This is harder todo than you might think. Air pressurekeeps the paper in place.

Find Out Activity 9-2 onpage 319

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316 MHR • Unit 3 Fluids and Dynamics

Pressure Differences Fluids move naturally from an area of high pressure to low pressure.Consider the simple example of drinking juice from a juice box (Figure 9.3). The motion you make when you drink with a strawremoves air, creating an area of low pressure in your mouth. As aresult, the relatively higher pressure of the surrounding atmospherepushes the juice into your mouth.

If the air inside of a closed container is at a lower pressure than the air pushing on the outside of the container, there is an unbalancedforce. You may have noticed this imbalance when drinking juice from a juice box. The straw makes such a tight seal that as you draw thejuice up the straw and reduce the air pressure inside the juice box, the box buckles inward. The air pressure outside the juice box pushesthe walls of the box together. When high-pressure air moves to an area of lower pressure, we can take advantage of its force to drive all sorts of mechanical devices and to carry, push, or move objects (Figure 9.4).

The key to performing tasks with compressed gases is to first createa pressure difference. Consider the example of how compressed air isused in the water rocket (Figure 9.5). Pumping the toy compresses airand forces the air into the rocket. The more you pump, the greater theair pressure in the rocket becomes. When an opening at the bottom istriggered, the compressed air forces the water out of the rocket,propelling the rocket in the opposite direction (Figure 9.5).

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Figure 9.3 Air pressure pushesthe drink up the straw and into yourmouth.

Figure 9.4 Air pressure canprovide enough force to chipconcrete.

isel

Figure 9.5 A: Pump, pump,pump… B: Fire!

Find Out Activity 9-3 onpage 320

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Chapter 9 There are both natural and constructed fluid systems. • MHR 317

Liquid PressureIf you are underwater, you can feel the pressure of thewater all around you. The deeper you go into the water,the more water will be pressing down upon you. Thepressure of all fluids increases with depth. For example, if abarrel of oil is punctured near the top and near the bottom,oil will be forced much farther from the bottom holebecause the pressure is greater at the bottom of thecontainer than at the top (see Figure 9.6).

The atmospheric pressure at sea level is 101.3 kPa, also called one atmosphere (1 atm). For every 10 m you descend into water, the pressure increases by oneatmosphere. That means if a submarine were to descend to a depth of500 m, the water surrounding it would be squeezing it with a force of50 atm. A single atm is equivalent to almost 100 000 N pressing on asquare metre. At a depth of 500 m, a square metre of the submarinesurface has to endure a force equivalent to the weight of a 500 000 kgobject (see Figure 9.7). That is a lot of pressure!

BuoyancyBuoyancy is the tendencyof objects in fluids to riseor sink because of densitydifferences with theirsurroundings (see Figure9.8). The upward forceexerted by a fluid is calledthe buoyant force. Thetransportation of nutrientsthrough our bloodstream,pollen floating in the air,and boats and planesmoving around the worldwould not be possiblewithout the buoyant force.

If the object exerts agreater average forcedownward (due to gravity) than the fluidexerts upward, then theobjects sinks. If the objectexerts less force downwardthan the fluid exertsupward, the objects rises.

Figure 9.6 The greater the depthof the fluid, the greater the pressure.

Figure 9.7 The pressure on eachsquare metre of a submarine at adepth of 500 m is equivalent to theweight of 40 elephants.

Word Connect

A buoy is a floating objectthat is anchored in the waterto warn or guide swimmersand boaters. The word “buoy”can also mean to support oruplift.Figure 9.8 Heating the air in this hot-air

balloon causes the particles in the air to moveapart, creating a lower density inside theballoon. Therefore, the balloon rises.

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318 MHR • Unit 3 Fluids and Dynamics

Figure 9.9 The submarine floatswhen its weight is equal to thebuoyant force. The submarine sinkswhen its weight is greater than thebuoyant force.

Reading Check

1. Why might your eardrums “pop” when you quickly changealtitude?

2. Why does juice rise up a straw when you drink?3. Why does a juice box collapse inward when you drink from it?4. What are two different ways to express the atmospheric pressure

at sea level?5. Why is water pumped into a submarine?6. What happens when the weight of an object is greater than the

buoyant force?

A Cartesian diver is a greatdemonstration of pressure and buoyancy. Find out what a Cartesian diver is and thenbuild one. Start your search atwww.bcscience8.ca.

Rising and SinkingDesigners of submarines need to be aware of the forces that are theresult of pressure deep in the ocean. The designers must also considerhow to control the buoyancy of the submarine (see Figure 9.9). Whenthe submarine takes on water, its average density (steel, water, air, etc.combined) becomes greater than the surrounding water, so it sinks.When compressed air is used to blow out the water from the inside ofthe submarine, the average density of the submarine becomes less thanthe surrounding water, so the submarine rises.

This vertical movement in fluids due to density differences can beobserved in natural systems such as the atmosphere. When air warms, it expands, becomes less dense, and rises. When air cools, it becomesdenser and sinks. This is called convection. Convection is a verticalmovement of fluids caused by density differences. These movementscause heat to be distributed evenly throughout the fluid.

Section 11.3 has informationabout convection and oceancurrents.

Connection

pumps air under pressure

Buoyant Force

ballasttank

water inwater in

waterout

wateroutair outair out

air inair in

RISINGSINKING

FLOATING

Gravitational Force

Find Out Activity 9-4 on page 320 Design an Investigation 9-5 on page 321

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Checking Concepts1. A spray bottle like the one shown here must

first be pumped several times before it willbe able to shoot water.(a) What is the purpose of the pumping?(b) What is the role of air pressure in making

water shoot from the bottle?

2. Explain why the leak in the bottom of awater barrel will spray water farther than aleak at the top of the barrel.

3. Tara blows into a thick balloon, but as it fillsit gets more difficult to continue to blow.Soon the point is reached where Tara can nolonger add air to the balloon. Explain thissituation in terms of air pressure.

4. If an airplane door were to open at a highaltitude, in which direction would the airmove—into or out of the airplane? Why?

Understanding Key Ideas5. Would it be possible for a submarine to stay

still deep in the water, not sinking or rising? (a) Draw an illustration of this situation

using force arrows. (b) Explain whether it is possible or not.

6. After sleeping all night on an air mattress,Sascha removes the plug to empty the air outof the mattress. The flow of air out of themattress becomes slower and slower andseems to stop, so Sascha lies down on themattress hoping to make it empty faster. Use diagrams that include particle detail and labels for areas of high and low pressureto explain the difference in air pressurebetween the room and the inside of themattress: (a) as Sascha slept on the mattress(b) when air stopped moving out of the

mattress as Sascha was emptying it(c) when Sascha lay down on the mattress to

speed up the emptying

Think back to the beginning of this sectionand the problem of excess pressure in thebrain of hydrocephalic babies. Make a chartthat describes three other situations whereexcess pressure is dangerous. Propose asolution to each problem. Make sure youinclude a diagram of the problem and solution.

Pause and Reflect

Chapter 9 There are both natural and constructed fluid systems. • MHR 323

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