Fluids. How do fluids exert pressure? What causes objects to float? What happens when pressure in a fluid changes? What affects the speed of a fluid?

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1 CHAPTER 3 SECTION 3 States of Matter Fluids KEY IDEAS As you read this section, keep these questions in mind: How do fluids exert pressure? What causes objects to float? What happens when pressure in a fluid changes? What affects the speed of a fluid? What Are Fluids? Recall that liquids and gases are examples of fluids. The particles in a fluid can move past each other. Fluids exert pressure, or push, evenly in all directions. The properties of fluids allow ships to float, divers to explore the ocean, and jets to soar across the sky. What Is Pressure? Pressure is the amount of force exerted on a given area of surface. For example, when you add air to a bicycle tire, you push air into the tire. Inside the tire, the air molecules push against each other and also against the walls of the tire. As you pump more air into the tire, the pressure inside the tire increases. Why? More air particles are pushing against the inside of the tire and against each other. The force of air particles inside the tire creates pressure. Pressure keeps the tire firm. READING TOOLBOX Summarize As you read, write down each scientific law or principle described in this section. Write each one in your own words. Include a diagram or equation that describes the principle or law. 1. Predict What would happen to pressure in the tire if some of the air were removed? Explain your answer. CALCULATING PRESSURE You can calculate pressure by dividing the force by the area over which the force acts. The equation for calculating pressure is on the next page. Interactive Reader 55 States of Matter

2 Pressure pressure = force area P = F A READING CHECK 2. Identify What is the SI unit of pressure? Discuss According to legend, Archimedes helped a king determine if his crown was made of pure gold or of silver. With a partner, discuss ways to use Archimedes principle to help the king learn if his crown was made of gold. (Hint: The density of silver is less than the density of gold.) The SI unit of pressure is the pascal. One pascal (1 Pa) is the force of one newton exerted over an area of one square meter (1 N/m 2 ). The newton is the SI unit of force. What Causes an Object to Float? What happens if you push a rubber duck to the bottom of a tub and release it? It does not sink to the bottom of the tub. It pops up to the surface. Why does this happen? A force called the buoyant force pushes the rubber duck upward. All fluids exert an upward buoyant force on matter. You may feel this force when you float on your back in a pool or lake. DETERMINING BUOYANT FORCE A Greek mathematician named Archimedes discovered a method to determine buoyant force. Archimedes Principle The buoyant force on an object equals the weight of the fluid that the object displaces. The figure below shows how to find the buoyant force on an object. 3. Identify Relationships What does the volume of displaced water equal? Lower an object into a container of water. The object displaces water. The displaced water flows into a smaller container. When water covers the object completely, you can measure the volume of the displaced water. The volume of the water equals the volume of the object. Once you have collected the displaced water, you can find its weight. The weight of the displaced water equals the buoyant force. Interactive Reader 56 States of Matter

3 DETERMINING IF AN OBJECT WILL SINK OR FLOAT You can predict if an object will sink or float by comparing its weight with the buoyant force on it. An object floats when the buoyant force is greater than or equal to the object s weight. 4. Analyze If an object s weight is less than the buoyant force, will it sink or float? You can also compare the densities of a fluid and an object to predict if the object will float. If the object is denser than the fluid, it will sink. For example, the density of a brick is 2 g/cm 3. The density of water is 1.00 g/cm 3. The brick is denser than water and thus will sink in water. Steel is almost eight times denser than water. However, large ships made of steel can float in the ocean. Why? The shape of the boat allows it to float. Because a large part of the boat is hollow, the same mass of steel takes up a larger volume. Increasing volume decreases overall density and the boat floats. 5. Explain Why does a block of steel have a larger density than a boat made of steel? A large part of the boat is hollow. Although shape does not change the mass of the boat, the volume of the hollow boat is larger. A larger volume for a particular mass reduces the density of the boat. A block of steel is denser than water, so the block sinks. Interactive Reader 57 States of Matter

4 What Happens When Pressure Changes in a Fluid? What happens when you squeeze one end of a tube of toothpaste? The pressure you apply at one end is passed along throughout the entire tube. This causes toothpaste to come out the other end. In the 17 th century, a French scientist named Blaise Pascal explained this kind of observation. Pascal s Principle If the pressure in a container is increased at any point, the pressure increases at all points by the same amount. 6. Identify What do the variables P, F, and A represent? Pascal s principle can be stated by the following equations: Pascal s Principle P 1 = P 2 P = F A Therefore, F 1 = F 2 A 1 A 2 Math Skills 7. Calculate A hydraulic lift uses Pascal s principle to lift a 19,000 N car. The area of the small piston equals 10.5 cm 2 and the area of the large piston equals 400 cm 2. What force must you exert on the small piston to lift the car? HYDRAULIC DEVICES Hydraulic devices are based on Pascal s principle. Hydraulic devices use liquids to transmit, or pass, pressure from one point to another. They can be used to multiply force. A small downward force is applied to a small area. This force exerts pressure on the fluid in the device. Force 1 Area 1 Area 2 Force 2 The pressure is transmitted to a larger area, where the pressure creates a larger force. According to Pascal s principle, the pressure is the same on both sides of the fluid in the device. Therefore, a small force on a smaller area produces a larger force on the larger area. What Are the Properties of Fluids in Motion? Examples of moving fluids include liquds flowing through pipes, air moving as wind, and honey dripping. Fluids can flow at different rates. However, fluids in motion have some properties in common. Interactive Reader 58 States of Matter

5 FLUIDS AND AREA If the flow rate stays the same, fluids move faster through small areas than through large areas. For example, if you place your thumb over the end of a garden hose, your thumb blocks part of the opening. The water must flow through a small area. Because the area is small, the water leaves the hose at a faster speed. FLUID PRESSURE AND SPEED Imagine water carrying a leaf through a pipe. The water moves faster through the narrow part of the pipe than through the wider part. Therefore, the leaf carried by the water moves more quickly in the narrow part of the pipe. The water pressure behind the leaf is greater than the pressure in front of the leaf. The pressure difference causes the leaf to speed up, or accelerate, as it enters the narrow tube. READING CHECK 8. Describe How does area affect the speed of a fluid? More pressure Less pressure Pressure is lower in the narrow part of the pipe than in the wide part. As the leaf enters the narrow part, the pressure in front of the leaf is less than the pressure behind it. Thus, the leaf speeds up when it enters the narrow part of the pipe. 9. Identify On the diagram, write the words slower and faster to show how speed differs in different parts of the tube. A Swiss mathematician named Daniel Bernoulli described this property of a moving fluid. Bernoulli s Principle As the speed of a moving fluid increases, the pressure of the moving fluid decreases. VISCOSITY Some fluids move much more slowly than others. For example, it would take you longer to pour a cup of honey than a cup of water. Another way to say this is that different fluids have different viscosities. Viscosity is a fluid s resistance to flow. A viscous fluid does not flow as easily as a less viscous fluid. In general, the stronger the attraction between the particles of a fluid, the more viscous the fluid is. Interactive Reader 59 States of Matter

6 Section 3 Review SECTION VOCABULARY buoyant force the upward force that keeps an object immersed in or floating on a fluid pascal the SI unit of pressure; equal to the force of 1 N exerted over an area of 1 m 2 (symbol, Pa) pressure the amount of force exerted per unit area of a surface viscosity the resistance of a gas or liquid to flow 1. Describe How can you use density to determine if an object will sink or float in a fluid? 2. Analyze You put a small object into a cup of water and weigh the displaced water. The displaced water weighs 235 N. What is the buoyant force on the object? Explain your answer. 3. Explain How does a hydraulic device multiply force? 4. Infer A balloon filled with helium floats in the air. What does this tell you about the density of helium? 5. Explain How are speed and pressure of a fluid related? 6. Explain How do the attractions between particles in a fluid determine viscosity? Interactive Reader 60 States of Matter