8.6 Density and Buoyancy in Action Key Question: How do density and buoyancy affect our lives? Density and buoyancy affect the fluids all around us. Here are a few situations in which density and buoyancy are important. Oil Spills Oil is usually less dense than water. It floats on the water s surface. During an oil spill, this property can be both helpful and harmful. Many animals live near the water s surface. The oil floating on the water s surface during a spill can harm animals. Oil may get into their food or make it difficult for them to breathe. Oil may get on animals fur or feathers. This can make it difficult for the animals to keep warm or swim. However, floating oil can be controlled. Floating booms, like the one in Figure 1, keep the oil from spreading across the water s surface. The oil can be scooped or sucked out of the water. This makes cleaning up the oil much easier. Figure 1 This floating boom has trapped most of the spilled oil, but some oil has escaped. Chapter 8 Worksheet 8.6-1 179 TURN
Balloons, Airships, and Blimps How does a balloon float in the air? The helium gas inside the balloon is less dense than the air outside the balloon. Many airships use differences in density to move through the air. The Hindenburg was a famous German airship that was filled with hydrogen gas. Hydrogen gas is less dense and less expensive than helium. Hydrogen gas is also much more explosive than helium. On May 6, 1937, the Hindenburg caught fire. Thirty-six people died in the fire. Today, lighter-than-air craft are used mainly for advertising and recreation. Blimps today are not filled with hydrogen gas. Instead, they are filled with helium just like helium balloons! Hot air balloons are another type of craft. They are filled with hot air (Figure 2). Figure 2 These hot air balloons can rise and sink by changing the density of the air inside. A hot air balloon is open at the bottom. A burner under the balloon heats up the air inside. The cooler air is forced out of the balloon. The hot air inside the balloon is less dense than the cool air outside the balloon. Therefore, the balloon rises. 180 Chapter 8 Worksheet 8.6-1
The Importance of Buoyancy The flower in Figure 3 is a water hyacinth. These plants float on water. Figure 3 The round parts of the water hyacinth s stem hold air. The extra air makes the plant less dense than water, so it floats. Water hyacinths have air chambers in their stems, which make the plant less dense than water. Humans use flotation chambers in objects such as life preservers and pool chairs. swim bladder a controllable, balloonlike chamber that allows fish to alter their buoyancy Fish and Submarines Most bony fish use a swim bladder to control how high they float in the water. A swim bladder is a sac in a fish s body that contains oxygen (Figure 4). The fish takes in dissolved oxygen from the water to fill its swim bladder. When there is more oxygen in the bladder, the fish floats higher. When there is less oxygen in the bladder, the fish sinks lower. That way, a fish can change position in the water without using a lot of energy. swim bladder Figure 4 The oxygen in a fish s swim bladder comes from the dissolved oxygen in the water. Chapter 8 Worksheet 8.6-1 181 TURN
Name: Date: Submarines can also control how high they float in the water. They use ballast tanks, which work in a similar way to swim bladders. ballast tanks compartments in a ship or submarine that take in water to keep the ship stable or help a submarine dive below the surface Ballast tanks are parts of a submarine that can be filled with air or water. Valves on the ballast tank can be opened to let air and or water in and out of the ballast tank. When a submarine lets water into the ballast tank, the submarine sinks (Figure 5a). When it adds air to the ballast tank and pushes out the extra water, the submarine rises (Figure 5b). compressed air valves open air forced into ballast tank air valves open air expelled from ballast tank sinking rising water valves open water enters ballast tank water valves open water expelled from ballast tank Figure 5 (a) When there is more water in the ballast tank, the submarine becomes denser and sinks. (b) When there is more air in the ballast tank, the submarine becomes less dense and rises. Fish and submarines can change their densities by controlling the balance of water and air inside them. If the fish or submarine has positive buoyancy, it will rise. If it has negative buoyancy, it will sink. Neutral buoyancy is when the fish or submarine remains in one place, without rising or sinking. CHECK YOUR UNDERSTANDING 1. When there is an oil spill, how is the density of oil helpful? How is it harmful? 182 Chapter 8 Worksheet 8.6-1
Name: Date: 2. Describe an instance when humans might have used an example from nature to design a device that uses density and buoyancy. 3. (a) Hydrogen is less dense than helium. Why did this make it a good choice for the owners of the Hindenburg to use? (b) Why was hydrogen a poor choice? 4. Think back to the Key Question at the beginning of the section. How does density affect your daily life? Give one example. Chapter 8 Worksheet 8.6-1 183 END