7.2. Fluids and Density. 7-5 Fluids Can Flow. Think About It. Key Terms

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1 7.2 Fluids and Density 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. Key Terms density displacement fluid What do pancake syrup, water in a mountain stream, and lava flowing from a volcano have in common? They are all fluids (Figure 7.7). A fluid is any form of matter that can flow. Liquids and gases are able to flow because they do not have a fixed shape. Solids have a fixed shape and cannot flow. Therefore solids are not fluids. Your body contains many fluids, such as blood and the watery cytoplasm inside cells. Air flows into your lungs each time you inhale, and out of your lungs when you exhale. Figure 7.7 Lava flows from an erupting volcano, water flows in a mountain stream, and pancake syrup flows from its container. 7-5 Fluids Can Flow Think About It Liquids and gases are fluids, forms of matter that can flow. In this activity, you will share your knowledge of fluids. What to Do Divide a piece of notebook paper into quarters. Label the 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 make a fluid flow more quickly. 3. In Box C, suggest several applications (uses) where heating fluids is important. 4. In Box D, suggest several applications where cooling fluids is important. 5. Share your information with a partner. 260 MHR Unit 3 Fluids and Dynamics

2 Solid, Liquid, and Gas Density One property that is useful in understanding both fluids and solids is density. Density is the mass of a given volume. In other words, density describes how closely packed together the particles are in a material. You might think of density in terms of vehicles on a highway. A traffic jam like the one on the left in Figure 7.8 is a model of high density. The photograph of free-flowing loosely packed traffic on the right in Figure 7.8 is a model of low density. Did You Know? There is space between grains of sugar. When you melt sugar to make candy, the sugar becomes a fluid. After the sugar cools, it contracts, making the candy even more dense than the original sugar. Figure 7.8 When traffic gets very dense, it is difficult for vehicles to move. Using what you already know about particles and the state of matter, predict which is likely to be the densest, a solid, a liquid, or a gas. Take a look at Figure 7.9. Aluminum (a solid), is denser than water (a liquid), and water is denser than air (a mixture of gases) but why? The key to density is the spacing of the particles. The particles of a piece of solid aluminum are tightly packed, while liquid water particles have enough room between them to change position. The particles of air are free to move independently and have a large amount 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 to kinetic molecular theory, the particles of a substance spread out as they 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 liquid form, but water is an exception. When water freezes, the particles move slightly farther apart as they become fixed in position. This means that ice is actually less dense than liquid water, so it floats (see Figure 7.10). Figure 7.9 A sealed container holds air, water, and an aluminum block. Figure 7.10 The property of ice floating on water makes life in freshwater lakes possible. If ice sank as it froze, lakes would freeze solid. Instead, the floating ice builds slowly from the top down, creating an insulating barrier against cold temperatures. Connection Section 10.2 has more information about the density of water and ice. Chapter 7 Kinetic molecular theory explains the characteristics of solids, liquids, and gases. MHR 261

3 7-6 Dense, Denser, Densest Think About It Generally speaking, solids are denser than liquids, and liquids are denser than gases. In this activity, you will discover whether a fluid could be denser than a solid. Fluid Density (g/ml) Solid Density (g/cm 3 ) hydrogen Styrofoam helium cork 0.24 air oak 0.70 oxygen sugar 1.59 carbon dioxide 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 mercury gold What to Do The approximate densities of some common substances at 20 C are listed in the table. The higher the number, the denser the substance. Use the information in the table 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 than mercury? Connection Section 5.1 has information about air layers and mirages. Layers of Fluids Imagine two beakers, one filled with water, and one filled with corn syrup. Does the water or corn syrup have the greater mass? Which has the greater density? Recall that density is the mass of a given volume. When you compare the masses of equal volumes of different kinds of matter, 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 two liquids do not mix together. How would corn syrup and water be layered if they were placed in the same beaker? Even though the two liquids are of the same volume, the corn syrup has more mass, and therefore it has a greater density than the water. Placing them together in the same container would result in the water floating on top of the corn syrup. This layering according to density can even occur within the same substance. Air is an excellent example; differences in air density contribute a great deal to weather. When air is heated near the ground on 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 a result, it begins to rise (see Figure 7.11 on the next page). As the warm air rises, cooler air rushes in beneath it, and a breeze is created. 262 MHR Unit 3 Fluids and Dynamics

4 Air is a mixture of many types of particles, but it is mostly made of nitrogen and oxygen. Air particles are relatively dense close to Earth s surface. If we increase our altitude, we encounter areas of lower air density. The higher we go, the farther apart the air particles are spread out, making it harder for us to get enough oxygen particles into our lungs with every breath (see Figure 7.12). D m C 8000 m Figure 7.11 As low-density warm air rises, it can carry water vapour with it. When the water vapour reaches the cooler higher atmosphere, it condenses into tiny droplets that we see as clouds. A sea level 2500 m 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 likely lead to symptoms of lack of oxygen. Oxygen masks are needed at this air density (C). Large airplanes 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 more efficient and requiring less fuel. B Did You Know? The cabins in large airplanes are pressurized so that the air density in the airplane is similar to the density on the ground. If the airplane loses cabin pressure, oxygen masks drop to the passengers, providing them with the oxygen they need. 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? Suggested Activity Find Out Activity 7-8 on page 268 Chapter 7 Kinetic molecular theory explains the characteristics of solids, liquids, and gases. MHR 263

5 Figure 7.13 The SuperBall sinks in the oil, but floats on the water! Measuring Density Layering is a useful technique for comparing densities (see Figure 7.13). When an object is placed in a less dense fluid, the object will sink 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, the object will hover in place. Layering allows you to determine whether one substance is denser than another substance. However, layering does not provide a specific measurement of density, and layering cannot be used with solids. Solids do not flow, and their particles are so 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 the density of a substance you need to know its mass and its volume. Mass can be determined using an electronic scale or balance (see Figure 7.14). The volume of a solid is often measured in cubic centimetres (cm 3 ). A cubic centimetre is the volume of a cube that measures 1 cm on each side. In other words, the volume of an object equals the number of 1 cm cubes it takes to fill that object. The volume of an object that has a simple shape can be determined mathematically (Figure 7.15). Figure 7.14 This triple beam balance indicates the apple has a mass of 94 g. Figure 7.15 For objects that are block-shaped, volume can be calculated mathematically by using the equation: volume length width height. 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? 264 MHR Unit 3 Fluids and Dynamics

6 Displacement How would you measure the volume of an object with an irregular shape? Displacement is the amount of space that an object takes up when 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 by measuring the displacement of an object, you can measure the volume of the object. Calculating Density Once you know the mass and the volume of a substance, you can calculate the density. You can calculate the density of both fluids and solids. The units for density depend on how you measure the mass and volume of your objects. The density of fluids is usually measured in g/ml, while the density of solids is usually measured in g/cm 3 (1 ml has the same volume as 1 cm 3 ). Density and Dinosaurs The densest substance that occurs in nature is iridium, a hard, brittle, whitish-yellow metal, with a density of g/cm 3. Iridium has a special connection to the end of the reign of dinosaurs on Earth. Find out more about iridium and its connection to dinosaurs. Start your search at density (D) Read the question: mass (m) volume (V) 1 ml of glycerol has a mass of 1.26 g. What it the density of glycerol? Use the formula: D m V 1.26 g 1 ml State your answer: The density of glycerol is 1.26 g/ml. Practice Problems 1. What is the density of a 2 cm 3 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 cm 3 of lead is g. The mass of 1 cm 3 of iron is 7.87 g. Which solid has the greater density? Answers g/cm g/ml 3. lead Chapter 7 Kinetic molecular theory explains the characteristics of solids, liquids, and gases. MHR 265

7 8.2 Pressure 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. Key Terms compression kilopascal pascal pressure How would you use the word pressure to describe a fast-moving championship game of tennis? Would you say the players are under a lot of 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 As the pressure is applied to the ball, its shape changes. What do you think has happened to the particles of air inside the ball? The increase in pressure forces the particles closer together in a smaller volume. In other words, the space between the particles is compressed. Compression is a decrease in volume produced by a force. Figure 8.16 The fastest tennis serves can travel over 240 km/h. The force delivered to the ball is enormous. 8-5 Balloon Lift Find Out ACTIVITY In this activity, you will observe whether a balloon can do work. Materials straw textbooks balloon strong elastic band strong tape Observe what happens when you blow into the balloon. What to Do 1. Insert a straw into the mouth of a balloon. Seal the straw to the balloon with tape. Wind the elastic band around the balloon seal. 2. Place a book on the balloon. Blow steadily into the straw. Observe what happens. 3. Place one book at a time on top of the first book. Record how many books you can lift by blowing into 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 while lifting several books than lifting one book? Why? 3. If you and a partner both blew into balloons so that two balloons were lifting the same stack of books, would it be easier than one person blowing into one balloon? Why? 290 MHR Unit 3 Fluids and Dynamics

8 Gases Are Compressible A gas can easily be compressed because there is a large amount of space between its particles. Less than 1 percent of the volume of a gas is made up of particles. The rest of the volume is the space between the particles. A container full of a gas, such as a tennis ball filled with air or nitrogen, can briefly absorb a great deal of force because of the space between its particles. Compressed gas can be useful for doing work. Creating that compression can be rather simple. All you need is a container to hold the gas and the means to apply a force to it. A good example of using compressed air to do work is air compressor technology (see Figures 8.17 and 8.18). An air compressor uses an electrically powered motor that drives a piston. The piston repeatedly draws in air and compresses it, driving the air into a holding tank. The compressor effectively takes a large amount of air and compresses it into a much smaller volume by pushing the particles closer together. Did You Know? Hyperbaric chambers are special devices used to place human beings under higher than normal air pressures. A person is placed in the chamber and air or oxygen is forced in to create a high-pressure environment. This treatment delivers more oxygen to the tissues, so it speeds the healing of injuries, helps treat infections, and even combats carbon monoxide poisoning. Figure 8.18 In this photograph of sandblasting, a special technique of photography allows us to see what is usually invisible to the eye the highspeed jet of air and radiation of sound. Figure 8.17 Highly compressed gas can be used to force sand particles at high speed. The high-speed sand particles strike the surface with enough force to remove coatings, paint, or dirt. Suggested Activity Find Out Activity 8-7 on page 297 Chapter 8 Fluids are affected by forces, pressure, and heat. MHR 291

9 Pressure and Kinetic Activity in Gases Pressure can be a good indicator of the kinetic energy of a gas. As you learned in section 7.1, when energy is added to a gas the kinetic energy of the particles is increased, and the particles move faster. This means that a warmer gas will expand in its container faster than a cooler gas will. It also means that a gas that is trapped in a container and heated will increase in pressure as the fast-moving particles bounce against the sides of the container. The increased pressure could cause an explosion (see Figure 8.19). internet connect Carbon dioxide car racing is an interesting hobby made possible by gas compression. Small cylinders of carbon dioxide fit into the back of the custom-built cars. When it is time to race, the cylinders are triggered and carbon dioxide rushes out at high speed, propelling the car forward. Carbon dioxide powered cars can reach speeds of 100 km/h in only 20 m! Find out more about the compressed gas powered vehicles. Start your search at Figure 8.19 Aerosol cans should never be heated because they could explode. Pressure can cause explosions, but did you know that it can cause implosions as well? An implosion is a collapse inward. A small amount of water was placed in the metal can in Figure The can was then heated with the cap off. When the can was removed from the heat, the cap 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 the pressure inside the can to become lower than air pressure outside the can. As a result, the air pressure on the outside pushed the walls of the can inward, crushing the can. Figure 8.20 This metal can has imploded because the air pressure outside the can was greater than the air pressure inside the can. 292 MHR Unit 3 Fluids and Dynamics

10 Liquids and Solids Are Very Difficult to Compress What do you think would happen if you tried to compress a liquid or a solid? The particles of liquids and solids are already so tightly packed together that squeezing them together is almost impossible. Instead of changing the volume, the applied force is transmitted (passed along), from one particle to the next, throughout the substance. Solids and liquids are described as incompressible, which means they are not able to be compressed using normal means. See Figure A B C D Figure 8.21 A: filled with gas A bottle Figure 8.21 B: When force is applied to the bottle, the gas particles move closer together. The gas is compressed into a smaller volume. Figure 8.21 C: A bottle filled with liquid Figure 8.21 D: When force is applied to the bottle, the liquid does not compress. There is no room for the liquid particles to move any closer together. Compression and Deformation If you have squished a marshmallow or rolled up a foam mattress, have you compressed a solid? This appearance of compression in a solid can be explained by the presence of air spaces within the solid. In Figure 8.22, Styrofoam cups that were exposed to extreme ocean pressure on a deep-sea mission have been noticeably crushed. There are small air pockets in Styrofoam, and it is these air pockets that are compressed and destroyed by the pressure. Solids can also appear to be compressed when they are deformed. Deformation means a change of shape without being forced into a smaller volume. For example, a solid rubber ball may appear to momentarily compress when it hits the ground, but if it contains no air, it does not compress. So what is happening? The ball s shape changes slightly as it hits the ground. As the ball deforms, it stores elastic energy. This elastic energy makes the ball bounce back upward. Figure 8.22 The Styrofoam cups in the foreground used to be the same size as the large cups, until their air pockets were destroyed by deep-sea pressure. Chapter 8 Fluids are affected by forces, pressure, and heat. MHR 293

11 Compression and deformation can occur at the same time, as shown in Figure There are air spaces in the human skull, and the face is somewhat flexible. Therefore, when the soccer ball applies a large force, the player s head both compresses and deforms. The soccer ball also compresses and deforms because of hitting the player s head. Figure 8.23 Ouch! This soccer player s face and the soccer ball are temporarily compressed and deformed. 8-6 Balloon Burst Find Out ACTIVITY In this activity, you can find out the relationship between 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 the end of each one. Do not overfill the balloons. They should have a small amount of flexibility. 3. Steady a balloon against the bottom of the tub. Place the bottom of the coffee mug against the balloon and push downward until the balloon pops or 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 and push downward until the balloon pops or your finger touches the bottom of the tub. 5. Steady a balloon against the bottom of the tub. Place the dull point of the pencil against the balloon and push downward until the balloon pops or 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 balloon and push downward until the balloon pops or the needle 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 least surface area? The greatest surface area? Explain. 3. Which method required the least amount of force to burst the balloon? 4. Which method of bursting the balloon produced the greatest amount of pressure applied to the surface of the balloon? Explain. 294 MHR Unit 3 Fluids and Dynamics

12 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 of nails, but the secret is to do it very carefully, making sure large areas of the body all touch the nails at the same time. This ensures the weight of the body is spread out over more nails. By spreading force over a greater area, pressure can be reduced. The effect of area on pressure is reflected in the formula for pressure: pressure (P) force (F) area (A) or P Common units used in pressure calculations are newtons (N) for force and square metres (m 2 ) for area. In honour of the French scientist Blaise Pascal, a single unit of pressure, 1 N/m 2 is called a pascal (Pa). This is very small unit of pressure, so 1000 Pa, or the kilopascal (kpa), is a much more commonly used unit when comparing pressures (see Figure 8.25 and turn back to Figure 7.3B). F A Figure 8.24 The clown s weight is spread over many nails. Did You Know? Vehicle tires commonly have recommended pressures stated in pounds per square inch (p.s.i.). One p.s.i. is equivalent to 6895 Pa. Using the proper p.s.i. can make your bike ride more comfortable. For riding on a rugged trail, a mountain bike tire should be inflated to about 40 p.s.i. so that it can absorb more impact. For pavement, the p.s.i. should be 60 to 100 p.s.i. so that the tire rolls easily. internet connect Kilopascals are not the only acceptable unit of pressure. Meteorologists (scientists who study weather) often use the unit millibars (mb) to describe pressure in weather systems, such as in the centre of a hurricane. Find out more about how millibars are used. Start your search at Figure 8.25A A bull elephant exerts N of force on a 1 m 2 board. This is equivalent to a pressure of Pa or 65 kpa. Figure 8.25B The circus performer exerts 800 N of force on a 1 m 2 board. This is equivalent to a pressure of 800 Pa or 0.8 kpa. Chapter 8 Fluids are affected by forces, pressure, and heat. MHR 295

13 Scuba divers who return to the surface too quickly from great depths can experience the bends, also known as decompression sickness (DCS). Strangely enough, astronauts are also at risk of DCS. In space, there is no pressure so astronauts must wear pressurized suits. If those suits were to fail, astronauts would suffer from DCS. Find out more about the challenges of exploring high- and lowpressure environments. Start your research at Calculating Pressure The pressure an object exerts on a surface can be calculated if you are given the weight of the object and the dimensions of the surface it is pressing on. For example, a BMX rider and bike weigh 1200 N. They are on a rigid sheet of steel that is 1.0 m by 2.0 m. How much pressure does the sheet of steel exert on the ground? Recall that the area of a rectangle is equal to length times width. Therefore, F P A F (length width) 1200 N (1.0 m 2.0 m) 1200 N 2.0 m N m Pa Answers Pa Pa Pa Practice Problems Try the following pressure calculations yourself. 1. A vehicle loaded with garbage exerts N of force on a scale that measures 6.0 m by 6.0 m. What pressure does the scale put on the spring below? 2. A student sings on a stage while standing on a 2.0 m by 2.0 m platform. If the student weighs 800 N, what pressure does the platform put on the stage? 3. A large cement block is carried in the back of a pickup truck. The bottom of the cement block has dimensions of 8.5 m by 4.0 m and it weighs N. What pressure does it put on the truck? Reading Check 1. What term describes the amount of force acting over a given area on 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? 296 MHR Unit 3 Fluids and Dynamics

14 Checking Concepts 1. Explain what is happening to the particles of air inside a volleyball when compressed during a serve. 2. Why are gases easily compressible but liquids are not? 3. Helium balloons float upward when they are released. As they gain altitude they continue to expand until they burst. Why do the balloons burst? 4. Diving into the deep end of a swimming pool can 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? 6. (a) Calculate the pressure in each of the three situations shown below. Show all your work. (b) Rank the three situations from highest to lowest pressure. (c) The heaviest object does not have the greatest pressure. Why not? 1 m 950 N 0.5 m 6000 N 2 m Understanding Key Ideas 2.5 m 5. In the demonstration shown below left, a soft drink can containing a small amount of water is heated over a Bunsen burner. The can is then overturned with the opening of the can placed in cold water (below on the right). After being placed in the cold water, the can quickly crushes inward. Explain the result of this experiment. Why did the soft drink can implode? 2 m 8500 N 2 m Pause and Reflect Humans cannot survive without protection in areas of very low or very high pressure. In the future, we may want to inhabit areas such as the ocean bottom or the surface of Mars for extended periods of time. Write a paragraph describing how humans might cope with spending extended periods of time in areas of pressure extremes. Chapter 8 Fluids are affected by forces, pressure, and heat. MHR 299

15 9.1 Fluids Under Pressure 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. Key Terms buoyant force buoyancy convection one atmosphere A B Figure 9.1 (A) A normal skull and brain. (B) The abnormally large skull of a hydrocephalic baby Did You Know? Some dentists use air abrasion to drill a tooth. Compressed air blows particles of aluminum oxide powder at high speed to remove the areas of decayed tooth. Most babies are born healthy, but sometimes a baby is born with a condition known as hydrocephalus. A normal brain has fluid surrounding it. This fluid cushions the brain and helps transport nutrients. In hydrocephalic babies, there is too much fluid surrounding 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 normal and healthy. The standard treatment for hydrocephalus is to use a tube 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 system that is solved by a common piece of technology in a constructed fluid system. Fluids under pressure are used in devices that assist our everyday lives. Compressed gases can produce forces that can be used in many ways. Liquids may not be compressible, but by placing them under pressure in confined places such as pipes, we can force them to move where we want them to go. 314 MHR Unit 3 Fluids and Dynamics

16 9-1 Fluid Circus Find Out ACTIVITY What devices do you know that operate on fluids under pressure? In this activity, you can bring them to school for a fluid circus! Materials Bring objects from home that use fluid motion. Some examples are whistles, balloons, and pump-up toys. You might think of many others. What to Do 1. Copy the following table into your notebook. Give your table a title. Object Observations Fluid Used Reason for Movement 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 these objects work. Atmospheric Pressure Earth s atmosphere extends more than 160 km above Earth. Every layer of air exerts pressure on the layers below because all of the air particles are pulled toward Earth by the gravitational force. You can observe the effects of air pressure with a simple experiment such as the one shown in Figure 9.2. Air pressure changes with altitude. You may recall from Chapter 7 that air is less dense at higher altitudes because the air there is less compressed. As you climb higher in the atmosphere, fewer air particles press against you on the outside of your body. The number of particles pressing from the inside out is still the same at the top of the mountain as it was when you were at the base of the mountain. How do you feel this difference in pressure between the inside and outside? Your eardrum is a very thin membrane that can move in response to a difference in air pressure. If the difference in pressure on either side of the eardrum becomes great, you experience a pop inside your ear as the pressure equalizes. Figure 9.2 Blow across, under, and over the paper to try to make it move off the books. This is harder to do than you might think. Air pressure keeps the paper in place. Suggested Activity Find Out Activity 9-2 on page 319 Chapter 9 There are both natural and constructed fluid systems. MHR 315

17 Figure 9.3 Air pressure pushes the drink up the straw and into your mouth. 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 straw removes air, creating an area of low pressure in your mouth. As a result, the relatively higher pressure of the surrounding atmosphere pushes 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 unbalanced force. You may have noticed this imbalance when drinking juice from a juice box. The straw makes such a tight seal that as you draw the juice up the straw and reduce the air pressure inside the juice box, the box buckles inward. The air pressure outside the juice box pushes the 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 create a pressure difference. Consider the example of how compressed air is used in the water rocket (Figure 9.5). Pumping the toy compresses air and forces the air into the rocket. The more you pump, the greater the air pressure in the rocket becomes. When an opening at the bottom is triggered, the compressed air forces the water out of the rocket, propelling the rocket in the opposite direction (Figure 9.5). Figure 9.4 Air pressure can provide enough force to chip concrete. Suggested Activity Find Out Activity 9-3 on page 320 Figure 9.5 A: Pump, pump, pump B: Fire! 316 MHR Unit 3 Fluids and Dynamics

18 Liquid Pressure If you are underwater, you can feel the pressure of the water all around you. The deeper you go into the water, the more water will be pressing down upon you. The pressure of all fluids increases with depth. For example, if a barrel of oil is punctured near the top and near the bottom, oil will be forced much farther from the bottom hole because the pressure is greater at the bottom of the container than at the top (see Figure 9.6). The atmospheric pressure at sea level is kpa, also called one atmosphere (1 atm). For every 10 m you descend into water, the pressure increases by one atmosphere. That means if a submarine were to descend to a depth of 500 m, the water surrounding it would be squeezing it with a force of 50 atm. A single atm is equivalent to almost N pressing on a square metre. At a depth of 500 m, a square metre of the submarine surface has to endure a force equivalent to the weight of a kg object (see Figure 9.7). That is a lot of pressure! Figure 9.6 The greater the depth of the fluid, the greater the pressure. Buoyancy Buoyancy is the tendency of objects in fluids to rise or sink because of density differences with their surroundings (see Figure 9.8). The upward force exerted by a fluid is called the buoyant force. The transportation of nutrients through our bloodstream, pollen floating in the air, and boats and planes moving around the world would not be possible without the buoyant force. If the object exerts a greater average force downward (due to gravity) than the fluid exerts upward, then the objects sinks. If the object exerts less force downward than the fluid exerts upward, the objects rises. Figure 9.8 Heating the air in this hot-air balloon causes the particles in the air to move apart, creating a lower density inside the balloon. Therefore, the balloon rises. Figure 9.7 The pressure on each square metre of a submarine at a depth of 500 m is equivalent to the weight of 40 elephants. Word Connect A buoy is a floating object that is anchored in the water to warn or guide swimmers and boaters. The word buoy can also mean to support or uplift. Chapter 9 There are both natural and constructed fluid systems. MHR 317

19 Suggested Activity Find Out Activity 9-4 on page 320 Design an Investigation 9-5 on page 321 Connection Section 11.3 has information about convection and ocean currents. Rising and Sinking Designers of submarines need to be aware of the forces that are the result of pressure deep in the ocean. The designers must also consider how to control the buoyancy of the submarine (see Figure 9.9). When the 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 of the submarine, the average density of the submarine becomes less than the surrounding water, so the submarine rises. This vertical movement in fluids due to density differences can be observed in natural systems such as the atmosphere. When air warms, it expands, becomes less dense, and rises. When air cools, it becomes denser and sinks. This is called convection. Convection is a vertical movement of fluids caused by density differences. These movements cause heat to be distributed evenly throughout the fluid. Buoyant Force Gravitational Force pumps air under pressure Figure 9.9 The submarine floats when its weight is equal to the buoyant force. The submarine sinks when its weight is greater than the buoyant force. air in air in ballast tank FLOATING air out SINKING air out water out RISING water out water in water in Reading Check A Cartesian diver is a great demonstration of pressure and buoyancy. Find out what a Cartesian diver is and then build one. Start your search at 1. Why might your eardrums pop when you quickly change altitude? 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? 318 MHR Unit 3 Fluids and Dynamics

20 Checking Concepts 1. A spray bottle like the one shown here must first be pumped several times before it will be 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 a water barrel will spray water farther than a leak at the top of the barrel. 3. Tara blows into a thick balloon, but as it fills it gets more difficult to continue to blow. Soon the point is reached where Tara can no longer add air to the balloon. Explain this situation in terms of air pressure. 4. If an airplane door were to open at a high altitude, in which direction would the air move into or out of the airplane? Why? Understanding Key Ideas 5. 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 out of the mattress. The flow of air out of the mattress becomes slower and slower and seems to stop, so Sascha lies down on the mattress hoping to make it empty faster. Use diagrams that include particle detail and labels for areas of high and low pressure to explain the difference in air pressure between the room and the inside of the mattress: (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 Pause and Reflect Think back to the beginning of this section and the problem of excess pressure in the brain of hydrocephalic babies. Make a chart that describes three other situations where excess pressure is dangerous. Propose a solution to each problem. Make sure you include a diagram of the problem and solution. Chapter 9 There are both natural and constructed fluid systems. MHR 323