Figure P13.40 shows a water-filled syringe with a

Problem 1CQ Which has the greater density, 1 g of mercury or 1000 g of water?

Problem 1P A 100 mL beaker holds 120 g of liquid. What is the liquid’s density in SI units?

Problem 2P Containers A and B have equal volumes. Container A holds helium gas at 1.0 atm pressure and 20°C. Container B is completely filled with a liquid whose mass is 7600 times the mass of helium gas in container A. Identify the liquid in B.

Problem 2CQ A chunk of material has a mass of 3 kg. a. What is the material’s density? b. Would a chunk of the same material have the same mass? Explain. c. Would a chunk of the same material have the same density? Explain.

Problem 3CQ You are given an irregularly shaped chunk of material and asked to find its density. List the specific steps that you would follow to do so.

Problem 3P Air enclosed in a sphere has density . What will the density be if the radius of the sphere is halved, compressing the air within?

Problem 4CQ Object 1 has an irregular shape. Its density is . a. Object 2 has the same shape and dimensions as object 1, but it is twice as massive. What is the density of object 2? b. Object 3 has the same mass and the same shape as object 1, but its size in all three dimensions is twice that of object 1. What is the density of object 3?

Problem 4P Air enclosed in a cylinder has density . a. What will be the density of the air if the length of the cylinder is doubled while the radius is unchanged? b. What will be the density of the air if the radius of the cylinder is halved while the length is unchanged?

Problem 5CQ When you get a blood transfusion the bag of blood is held above your body, but when you donate blood the collection bag is held below. Why is this?

Problem 5P a. 50 g of gasoline are mixed with 50 g of water. What is the average density of the mixture? b. of water. What is the average density of the mixture?

Problem 6CQ To explore the bottom of a 10-m-deep lake, your friend Tom proposes to get a long garden hose, put one end on land and the other in his mouth for breathing underwater, and descend into the depths. Susan, who overhears the conversation, reacts with horror and warns Tom that he will not be able to inhale when he is at the lake bottom. Why is Susan so worried?

Problem 6P Ethyl alcohol has been added to 200 mL of water in a container that has a mass of 150 g when empty. The resulting container and liquid mixture has a mass of 512 g. What volume of alcohol was added to the water?

Problem 7CQ Rank in order, from largest to smallest, the pressures at A, B, and C in Figure Q13.7 . Explain.

Problem 7P Reference: Table 13.1:The average density of the body of a fish is . To keep from sinking, the fish increases its volume by inflating an internal air bladder, known as a swim bladder, with air. By what percent must the fish increase its volume to be neutrally buoyant in fresh water? Use the Table 13.1 value for the density of air at 20°C .

Problem 8CQ Refer to Figure Q13.7 . Rank in order, from largest to smallest, the pressures at D, E, and F. Explain.

Problem 8P The deepest point in the ocean is 11 km below sea level, deeper than Mt. Everest is tall. What is the pressure in atmospheres at this depth?

Problem 9CQ Cylinders A and B contain liquids. The pressure pA at the bottom of A is higher than the pressure pB at the bottom of B. Is the ratio pA /pB of the absolute pressures larger, smaller, or equal to the ratio of the gauge pressures? Explain.

Problem 9P a. What volume of water has the same mass as 8.0 m3 of ethyl alcohol? ________________ b. If this volume of water completely fills a cubic tank, what is the pressure at the bottom?

Problem 10CQ In Figure Q13.10 , A and B are rectangular tanks full of water. They have equal heights and equal side lengths (the dimension into the page), but different widths. a. Compare the forces the water exerts on the bottoms of the tanks. Is ? Explain. b. Compare the forces the water exerts on the sides of the tanks. Is ? Explain.

Problem 10P A 1.0-m-diameter vat of liquid is 2.0 m deep. The pressure at the bottom of the vat is 1.3 atm. What is the mass of the liquid in the vat?

Problem 11CQ Helium-filled weather balloons are spherical when they reach very high altitudes. However, they are only partially inflated with helium before they are released. Explain why this is done.

Problem 11P A 35-cm-tall, 5.0-cm-diameter cylindrical beaker is filled to its brim with water. What is the downward force of the water on the bottom of the beaker?

Problem 12CQ Water expands when heated. Suppose a beaker of water is heated from 10°C to 90°C. Does the pressure at the bottom of the beaker increase, decrease, or stay the same? Explain.

Problem 12P The gauge pressure at the bottom of a cylinder of liquid is . The liquid is poured into another cylinder with twice the radius of the first cylinder. What is the gauge pressure at the bottom of the second cylinder?

Problem 13CQ In Figure Q13.13 , is larger, smaller, or equal to ? Explain.

Problem 13P A research submarine has a 20-cm-diameter window 8.0 cm thick. The manufacturer says the window can withstand forces up to . What is the submarine’s maximum safe depth in seawater? The pressure inside the submarine is maintained at 1.0 atm.

Problem 14CQ A beaker of water rests on a scale. A metal ball is then lowered into the beaker using a string tied to the ball. The ball doesn’t touch the sides or bottom of the beaker, and no water spills from the beaker. Does the scale reading decrease, increase, or stay the same? Explain.

Problem 14P The highest that George can suck water up a very long straw is 2.0 m. (This is a typical value.) What is the lowest pressure that he can maintain in his mouth?

Problem 15CQ Rank in order, from largest to smallest, the densities of objects A, B, and C in Figure Q13.15 . Explain.

Problem 15P The two 60-cm-diameter cylinders in Figure P13.15 , closed at one end, open at the other, are joined to form a single cylinder, then the air inside is removed. a. How much force does the atmosphere exert on the flat end of each cylinder? b. Suppose the upper cylinder is bolted to a sturdy ceiling. How many 100 kg football players would need to hang from the lower cylinder to pull the two cylinders apart?

Problem 16CQ Objects A, B, and C in Figure Q13.16 have the same volume. Rank in order, from largest to smallest, the sizes of the buoyant forces on A, B, and C. Explain.

Problem 16P What is the gas pressure inside the box shown in Figure P13.16 ?

Problem 17CQ Refer to Figure Q13.16 . Now A, B, and C have the same density, but still have the masses given in the figure. Rank in order, from largest to smallest, the sizes of the buoyant forces on A, B, and C. Explain.

Problem 17P The container shown in Figure P13.17 is filled with oil. It is open to the atmosphere on the left. a. What is the pressure at point A? b. What is the pressure difference between points A and B? Between points A and C?

Problem 18CQ When you stand on a bathroom scale, it reads 700 N. Suppose a giant vacuum cleaner sucks half the air out of the room, reducing the pressure to 0.5 atm. Would the scale reading increase, decrease, or stay the same? Explain.

Problem 18P Glycerin is poured into an open U-shaped tube until the height in both sides is 20 cm. Ethyl alcohol is then poured into one arm until the height of the alcohol column is 20 cm. The two liquids do not mix. What is the difference in height between the top surface of the glycerin and the top surface of the alcohol?

Problem 19CQ Suppose you stand on a bathroom scale that is on the bottom of a swimming pool. The water comes up to your waist. Does the scale read more than, less than, or the same as your true weight? Explain.

Problem 19P A U-shaped tube, open to the air on both ends, contains mercury. Water is poured into the left arm until the water column is 10.0 cm deep. How far upward from its initial position does the mercury in the right arm rise?

Problem 20CQ When you place an egg in water, it sinks. If you start adding salt to the water, after some time the egg floats. Explain.

Problem 20P What is the height of a water barometer at atmospheric pressure?

Problem 21CQ Submerged submarines contain tanks filled with water. To rise to the surface, compressed air is used to force the water out of the tanks. Explain why this works.

Problem 21P Postural hypotension is the occurrence of low (systolic) blood pressure when standing up too quickly from a reclined position, causing fainting or lightheadedness. For most people, a systolic pressure less than 90 mmHg is considered low. If the blood pressure in your brain is 120 mm when you are lying down, what would it be when you stand up? Assume that your brain is 40 cm from your heart and that for your blood. Note: Normally, your blood vessels constrict and expand to keep your brain blood pressure stable when you change your posture.

Problem 22CQ Fish can adjust their buoyancy with an organ called the swim bladder. The swim bladder is a flexible gas-filled sac; the fish can increase or decrease the amount of gas in the swim bladder so that it stays neutrally buoyant—neither sinking nor floating. Suppose the fish is neutrally buoyant at some depth and then goes deeper. What needs to happen to the volume of air in the swim bladder? Will the fish need to add or remove gas from the swim bladder to maintain its neutral buoyancy?

Problem 22P A 6.00-cm-diameter sphere with a mass of 89.3 g is neutrally buoyant in a liquid. Identify the liquid.

Problem 23CQ Figure Q13.24 shows two identical beakers filled to the same height with water. Beaker B has a plastic sphere floating in it. Which beaker, with all its contents, weighs more? Or are they equal? Explain.

Problem 23P A cargo barge is loaded in a saltwater harbor for a trip up a freshwater river. If the rectangular barge is 3.0 m by 20.0 m and sits 0.80 m deep in the harbor, how deep will it sit in the river?

Problem 24CQ A tub of water, filled to the brim, sits on a scale. Then a floating block of wood is placed in the tub, pushing some water over the rim. The water that overflows immediately runs off the scale. What happens to the reading of the scale?

Problem 24P A 10 cm × 10 cm × 10 cm wood block with a density of floats in water. a. What is the distance from the top of the block to the water if the water is fresh? b. If it’s seawater?

Problem 25CQ Ships A and B have the same height and the same mass. Their cross-section profiles are shown in Figure Q13.26. Does one ship ride higher in the water (more height above the water line) than the other? If so, which one? Explain.

Problem 25P What is the tension in the string in Figure P13.25?

Problem 26CQ Gas flows through a pipe, as shown in Figure Q13.27. The pipe’s constant outer diameter is shown; you can’t see into the pipe to know how the inner diameter changes. Rank in order, from largest to smallest, the gas speeds at points 1, 2, and 3. Explain.

Problem 26P A 10 cm × 10 cm × 10 cm block of steel is suspended from a spring scale. The scale is in newtons. a. What is the scale reading if the block is in air? b. What is the scale reading after the block has been lowered into a beaker of oil and is completely submerged?

Problem 27CQ Liquid flows through a pipe as shown in Figure Q13.28. The pipe’s constant outer diameter is shown; you can’t see into the pipe to know how the inner diameter changes. Rank in order, from largest to smallest, the flow speeds at points 1, 2, and 3. Explain.

Problem 27P Styrofoam has a density of 300 kg/m3. What is the maximum mass that can hang without sinking from a 50-cm-diameter Styrofoam sphere in water? Assume the volume of the mass is negligible compared to that of the sphere.

Problem 28P Calculate the buoyant force due to the surrounding air on a man weighing 800 N. Assume his average density is the same as that of water.

Problem 28CQ A liquid with negligible viscosity flows through the pipe shown in Figure Q13.29. This is an overhead view. a. Rank in order, from largest to smallest, the flow speeds at points 1 to 4. Explain. b. Rank in order, from largest to smallest, the pressures at points 1 to 4. Explain.

Problem 29CQ Wind blows over the house shown in Figure Q13.31. A window on the ground floor is open. Is there an airflow through the house? If so, does the air flow in the window and out the chimney, or in the chimney and out the window? Explain.

Problem 29P River Pascal with a volume flow rate of joins with River Archimedes, which carries , to form the Bernoulli River. The Bernoulli River is 150 m wide and 10 m deep. What is the speed of the water in the Bernoulli River?

Problem 30CQ Two pipes have the same inner cross section area. One has a circular cross section and the other has a rectangular cross section with its height one-tenth its width. Through which pipe, if either, would it be easier to pump a viscous liquid? Explain.

Problem 30P Water flowing through a 2.0-cm-diameter pipe can fill a 300 L bathtub in 5.0 min. What is the speed of the water in the pipe?

Problem 31MCQ Figure Q13.33 shows a 100 g block of copper and a 100 g block of aluminum connected by a massless string that runs over two massless, frictionless pulleys. The two blocks exactly balance, since they have the same mass. Now suppose that the whole system is submerged in water. What will happen? A. The copper block will fall, the aluminum block will rise. B. The aluminum block will fall, the copper block will rise. C. Nothing will change. D. Both blocks will rise..

Problem 31P A pump is used to empty a 6000 L wading pool. The water exits the 2.5-cm-diameter hose at a speed of 2.1 m/s. How long will it take to empty the pool?

Problem 32MCQ Masses A and B rest on very light pistons that enclose a fluid, as shown in Figure Q13.34. There is no friction between the pistons and the cylinders they fit inside. Which of the following is true? A. Mass A is greater. B. Mass B is greater. C. Mass A and mass B are the same.

Problem 32P A 1.0-cm-diameter pipe widens to 2.0 cm, then narrows to 0.50 cm. Liquid flows through the first segment at a speed of 4.0 m/s. a. What are the speeds in the second and third segments? b. What is the volume flow rate through the pipe?

Problem 33MCQ If you dive underwater, you notice an uncomfortable pressure on your eardrums due to the increased pressure. The human eardrum has an area of about , and it can sustain a force of about 7 N without rupturing. If your body had no means of balancing the extra pressure (which, in reality, it does), what would be the maximum depth you could dive without rupturing your eardrum? A. 0.3 m B. 1 m C. 3 m D. 10 m

Problem 33P What does the top pressure gauge in Figure P13.35 read?

Problem 34MCQ An 8.0 lb bowling ball has a diameter of 8.5 inches. When lowered into water, this ball will A. Float. B. Sink. C. Have neutral buoyancy.

Problem 34P The 3.0-cm-diameter water line in Figure P13.36 splits into two 1.0-cm-diameter pipes. All pipes are circular and at the same elevation. At point A, the water speed is 2.0 m/s and the gauge pressure is 50 kPa. What is the gauge pressure at point B?

Problem 35MCQ A basketball has a mass of 0.50 kg and a volume of 8.0 × 10?3 m3. What is the magnitude of the net force on a basketball when it is fully submerged in water? A. 4.9 N B. 74 N C. 78 N D. 83 N

Problem 35P A rectangular trough, 2.0 m long, 0.60 m wide, and 0.45 m deep, is completely full of water. One end of the trough has a small drain plug right at the bottom edge. When you pull the plug, at what speed does water emerge from the hole?

Problem 36MCQ An object floats in water, with 75% of its volume submerged. What is its approximate density?

Problem 36P What pressure difference is required between the ends of a 2.0-m-long, 1.0-mm-diameter horizontal tube for 40°C water to flow through it at an average speed of 4.0 m/s?

Problem 37MCQ A syringe is being used to squirt water as shown in Figure Q13.39 . The water is ejected from the nozzle at 10 m/s. At what speed is the plunger of the syringe being depressed? A. 0.01 m/s B. 0.1 m/s C. 1 m/s D. 10 m/s

Problem 37P Water flows at 0.25 L/s through a 10-m-long garden hose 2.5 cm in diameter that is lying flat on the ground. The temperature of the water is 20°C. What is the gauge pressure of the water where it enters the hose?

Problem 38MCQ Water flows through a 4.0-cm-diameter horizontal pipe at a speed of 1.3 m/s. The pipe then narrows down to a diameter of 2.0 cm. Ignoring viscosity, what is the pressure difference between the wide and narrow sections of the pipe? A. 850 Pa B. 3400 Pa C. 9300 Pa D. 12,700 Pa E. 13,500 Pa

Problem 38P Figure P13.40 shows a water-filled syringe with a 4.0-cm-long needle. What is the gauge pressure of the water at the point P, where the needle meets the wider chamber of the syringe?

Problem 39GP The density of gold is . 197 g of gold is shaped into a cube. What is the length of each edge?

Problem 39MCQ A 15-m-long garden hose has an inner diameter of 2.5 cm. One end is connected to a spigot; 20°C water flows from the other end at a rate of 1.2 L/s. What is the gauge pressure at the spigot end of the hose? A. 1900 Pa B. 2700 Pa C. 4200 Pa D. 5800 Pa E. 7300 Pa

Problem 40GP The density of copper is 8920 kg/m3. How many moles are in a 2.0 cm × 2.0 cm × 2.0 cm cube of copper?

Problem 41GP The density of aluminum is . How many atoms are in a 2.0 cm × 2.0 cm × 2.0 cm cube of aluminum?

Problem 42GP A 50-cm-thick layer of oil floats on a 120-cm-thick layer of water. What is the pressure at the bottom of the water layer?

Problem 43GP An oil layer floats on 85 cm of water in a tank. The absolute pressure at the bottom of the tank is 112.0 kPa. How thick is the oil?

Problem 44GP The little Dutch boy saved Holland by sticking his finger in the leaking dike. If the water level was 2.5 m above his finger, estimate the force of the water on his finger.

Problem 45GP a. In Figure P13.47, how much force does the fluid exert on the end of the cylinder at A? b. How much force does the fluid exert on the end of the cylinder at B?

Problem 46GP A friend asks you how much pressure is in your car tires. You know that the tire manufacturer recommends 30 psi, but it’s been awhile since you’ve checked. You can’t find a tire gauge in the car, but you do find the owner’s manual and a ruler. Fortunately, you’ve just finished taking physics, so you tell your friend, “I don’t know, but I can figure it out.” From the owner’s manual you find that the car’s mass is 1500 kg. It seems reasonable to assume that each tire supports one-fourth of the weight. With the ruler you find that the tires are 15 cm wide and the flattened segment of the tire in contact with the road is 13cm long. What answer will you give your friend?

Problem 47GP A diver 50 m deep in 10°C fresh water exhales a 1.0-cmdiameter bubble. What is the bubble’s diameter just as it reaches the surface of the lake, where the water temperature is 20°C? Hint: Assume that the air bubble is always in thermal equilibrium with the surrounding water.

Problem 48GP A 6.0-cm-tall cylinder floats in water with its axis perpendicular to the surface. The length of the cylinder above water is 2.0 cm. What is the cylinder’s mass density?

Problem 49GP A sphere completely submerged in water is tethered to the bottom with a string. The tension in the string is one-third the weight of the sphere. What is the density of the sphere?

Problem 50GP You need to determine the density of a ceramic statue. If you suspend it from a spring scale, the scale reads 28.4 N. If you then lower the statue into a tub of water so that it is completely submerged, the scale reads 17.0 N. What is the density?

Problem 51GP A 5.0 kg rock whose density is is suspended by a string such that half of the rock’s volume is under water. What is the tension in the string?

Problem 52GP A flat slab of styrofoam, with a density of , floats on a lake. What minimum volume must the slab have so that a 40 kg boy can sit on the slab without it sinking?

Problem 53GP A 2.0 mL syringe has an inner diameter of 6.0 mm, a needle inner diameter of 0.25 mm, and a plunger pad diameter (where you place your finger) of 1.2 cm. A nurse uses the syringe to inject medicine into a patient whose blood pressure is 140/100. Assume the liquid is an ideal fluid. a. What is the minimum force the nurse needs to apply to the syringe? b. The nurse empties the syringe in 2.0 s. What is the flow speed of the medicine through the needle?

Problem 54GP A child’s water pistol shoots water through a 1.0-mm-diameter hole. If the pistol is fired horizontally 70 cm above the ground, a squirt hits the ground 1.2 m away. What is the volume flow rate during the squirt? Ignore air resistance.

Problem 56GP A hurricane wind blows across a 6.00 m × 15.0 m flat roof at a speed of 130 km/h. a. Is the air pressure above the roof higher or lower than the pressure inside the house? Explain. b. What is the pressure difference? c. How much force is exerted on the roof? If the roof cannot withstand this much force, will it “blow in” or “blow out”?

Problem 55GP The leaves of a tree lose water to the atmosphere via the process of transpiration. A particular tree loses water at the rate of ; this water is replenished by the upward flow of sap through vessels in the trunk. This tree’s trunk contains about 2000 vessels, each 100 mm in diameter. What is the speed of the sap flowing in the vessels?

Problem 57GP Water flows from the pipe shown in Figure P13.60 with a speed of 4.0 m/s. a. What is the water pressure as it exits into the air? b. What is the height h of the standing column of water?

Problem 58GP Air flows through the tube shown in Figure. Assume that air is an ideal fluid. a. What are the air speeds v1 and v2 at points 1 and 2? b. What is the volume flow rate? FIGURE

Problem 59GP Air flows through the tube shown in Figure at a rate of 1200 cm3/s. Assume that air is an ideal fluid. What is the height h of mercury in the right side of the U-tube? FIGURE

Problem 60GP ||| Water flows at 5.0 L/s through a horizontal pipe that narrows smoothly from 10 cm diameter to 5.0 cm diameter. A pressure gauge in the narrow section reads 50 kPa. What is the reading of a pressure gauge in the wide section?

Problem 61GP The mercury manometer shown in Figure is attached to a gas cell. The mercury height h is 120 mm when the cell is placed in an ice-water mixture. The mercury height drops to 30 mm when the device is carried into an industrial freezer. What is the freezer temperature? Hint: The right tube of the manometer is much narrower than the left tube. What reasonable assumption can you make about the gas volume? FIGURE

Problem 62GP Figure P13.65 shows a section of a long tube that narrows near its open end to a diameter of 1.0 mm. Water at 20°C flows out of the open end at 0.020 L/s. What is the gauge pressure at point P, where the diameter is 4.0 mm?

Problem 63GP Smoking tobacco is bad for your circulatory health. In an attempt to maintain the blood’s capacity to deliver oxygen, the body increases its red blood cell production, and this increases the viscosity of the blood. In addition, nicotine from tobacco causes arteries to constrict. For a nonsmoker, with blood viscosity of , and the arterial diameter would constrict to 90% of its normal value. What pressure difference would be needed to maintain the same blood flow

Problem 64GP A stiff, 10-cm-long tube with an inner diameter of 3.0 mm is attached to a small hole in the side of a tall beaker. The tube sticks out horizontally. The beaker is filled with 20°C water to a level 45 cm above the hole, and it is continually topped off to maintain that level. What is the volume flow rate through the tube?

Problem 65PP Blood Pressure and Blood Flow The blood pressure at your heart is approximately 100 mm Hg. As blood is pumped from the left ventricle of your heart, it flows through the aorta, a single large blood vessel with a diameter of about 2.5 cm. The speed of blood flow in the aorta is about 60 cm/s. Any change in pressure as blood flows in the aorta is due to the change in height: the vessel is large enough that viscous drag is not a major factor. As the blood moves through the circulatory system, it flows into successively smaller and smaller blood vessels until it reaches the capillaries. Blood flows in the capillaries at the much lower speed of approximately 0.7 mm/s. The diameter of capillaries and other small blood vessels is so small that viscous drag is a major factor. There is a limit to how long your neck can be. If your neck were too long, no blood would reach your brain! What is the maximum height a person’s brain could be above his heart, given the noted pressure and assuming that there are no valves or supplementary pumping mechanisms in the neck? The density of blood is . A. 0.97 m B. 1.3 m C. 9.7 m D. 13 m

Problem 66PP Blood Pressure and Blood Flow The blood pressure at your heart is approximately 100 mm Hg. As blood is pumped from the left ventricle of your heart, it flows through the aorta, a single large blood vessel with a diameter of about 2.5 cm. The speed of blood flow in the aorta is about 60 cm/s. Any change in pressure as blood flows in the aorta is due to the change in height: the vessel is large enough that viscous drag is not a major factor. As the blood moves through the circulatory system, it flows into successively smaller and smaller blood vessels until it reaches the capillaries. Blood flows in the capillaries at the much lower speed of approximately 0.7 mm/s. The diameter of capillaries and other small blood vessels is so small that viscous drag is a major factor. Because the flow speed in your capillaries is much less than in the aorta, the total cross-section area of the capillaries considered together must be much larger than that of the aorta. Given the flow speeds noted, the total area of the capillaries considered together is equivalent to the cross-section area of a single vessel of approximately what diameter? A. 25 cm B. 50 cm C. 75 cm D. 100 cm

Problem 67PP Blood Pressure and Blood Flow The blood pressure at your heart is approximately 100 mm Hg. As blood is pumped from the left ventricle of your heart, it flows through the aorta, a single large blood vessel with a diameter of about 2.5 cm. The speed of blood flow in the aorta is about 60 cm/s. Any change in pressure as blood flows in the aorta is due to the change in height: the vessel is large enough that viscous drag is not a major factor. As the blood moves through the circulatory system, it flows into successively smaller and smaller blood vessels until it reaches the capillaries. Blood flows in the capillaries at the much lower speed of approximately 0.7 mm/s. The diameter of capillaries and other small blood vessels is so small that viscous drag is a major factor. Suppose that in response to some stimulus a small blood vessel narrows to 90% of its original diameter. If there is no change in the pressure across the vessel, what is the ratio of the new volume flow rate to the original flow rate? A. 0.66 B. 0.73 C. 0.81 D. 0.90

Problem 68PP Blood Pressure and Blood Flow The blood pressure at your heart is approximately 100 mm Hg. As blood is pumped from the left ventricle of your heart, it flows through the aorta, a single large blood vessel with a diameter of about 2.5 cm. The speed of blood flow in the aorta is about 60 cm/s. Any change in pressure as blood flows in the aorta is due to the change in height: the vessel is large enough that viscous drag is not a major factor. As the blood moves through the circulatory system, it flows into successively smaller and smaller blood vessels until it reaches the capillaries. Blood flows in the capillaries at the much lower speed of approximately 0.7 mm/s. The diameter of capillaries and other small blood vessels is so small that viscous drag is a major factor. Sustained exercise can increase the blood flow rate of the heart by a factor of 5 with only a modest increase in blood pressure. This is a large change in flow. Although several factors come into play, which of the following physiological changes would most plausibly account for such a large increase in flow with a small change in pressure? A. A decrease in the viscosity of the blood B. Dilation of the smaller blood vessels to larger diameters C. Dilation of the aorta to larger diameter D. An increase in the oxygen carried by the blood