Density and Buoyancy in Action - PBworksDensity+and+Buoyancy.pdf · Density and buoyancy affect the fluids all around us. Here are a few situations in which density and buoyancy are

Density and Buoyancy in Action8.6

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 andbuoyancy are important.

Oil SpillsOil is usually less dense than water. It floats on thewater’s surface. During an oil spill, this property can beboth helpful and harmful.

Many animals live near the water’s surface. The oilfloating on the water’s surface during a spill can harmanimals. Oil may get into their food or make it difficultfor them to breathe. Oil may get on animals’ fur orfeathers. This can make it difficult for the animals tokeep warm or swim.

However, floating oil can be controlled. Floatingbooms, like the one in Figure 1, keep the oil fromspreading across the water’s surface. The oil can bescooped or sucked out of the water. This makescleaning 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 179Copyright © 2010 by Nelson Education Ltd.

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Balloons, Airships, and BlimpsHow does a balloon float in the air? The helium gasinside the balloon is less dense than the air outsidethe balloon. Many airships use differences in densityto move through the air.

The Hindenburg was a famous German airship thatwas filled with hydrogen gas. Hydrogen gas is lessdense and less expensive than helium. Hydrogen gasis also much more explosive than helium. On May 6,1937, the Hindenburg caught fire. Thirty-six peopledied in the fire.

Today, “lighter-than-air” craft are used mainly foradvertising and recreation. Blimps today are not filledwith hydrogen gas. Instead, they are filled withhelium—just like helium balloons!

Hot air balloons are another type of craft. They arefilled 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 underthe balloon heats up the air inside. The cooler air isforced out of the balloon.

The hot air inside the balloon is less dense than the coolair outside the balloon. Therefore, the balloon rises.

180 Chapter 8 Worksheet 8.6-1 Copyright © 2010 by Nelson Education Ltd.

Figure 3 The round parts of the water hyacinth’s stem hold air. The extra air makesthe plant less dense than water, so it floats.

Figure 4 The oxygen in a fish’s swim bladder comes from the dissolved oxygen inthe water.

swim bladder

Water hyacinths have air chambers in their stems,which make the plant less dense than water. Humansuse flotation chambers in objects such as lifepreservers and pool chairs.

Fish and SubmarinesMost bony fish use a swim bladder to control howhigh they float in the water. A swim bladder is a sac ina fish’s body that contains oxygen (Figure 4). The fishtakes in dissolved oxygen from the water to fill itsswim bladder.

When there is more oxygen in the bladder, the fishfloats higher. When there is less oxygen in the bladder,the fish sinks lower. That way, a fish can changeposition in the water without using a lot of energy.

swim bladder

a controllable, balloon-like chamber thatallows fish to altertheir buoyancy

Chapter 8 Worksheet 8.6-1 181Copyright © 2010 by Nelson Education Ltd.

TURN

The Importance of BuoyancyThe flower in Figure 3 is a water hyacinth. Theseplants float on water.

Submarines can also control how high they float in thewater. They use ballast tanks, which work in a similarway to swim bladders.

Ballast tanks are parts of a submarine that can befilled with air or water. Valves on the ballast tank canbe opened to let air and or water in and out of theballast tank.

When a submarine lets water into the ballast tank, thesubmarine sinks (Figure 5a). When it adds air to theballast tank and pushes out the extra water, thesubmarine rises (Figure 5b).

ballast tanks

compartments in aship or submarine thattake in water to keepthe ship stable or helpa submarine divebelow the surface

Figure 5 (a) When there is more water in the ballast tank, the submarine becomes denser and sinks. (b) When there ismore air in the ballast tank, the submarine becomes less dense and rises.

water valves openwater enters ballast tank

air valves openair expelled from ballast tank

sinking

water valves openwater expelled from ballast tank

compressed air valves openair forced into ballast tank

rising

Fish and submarines can change their densities bycontrolling the balance of water and air inside them.

If the fish or submarine has positive buoyancy, it willrise. If it has negative buoyancy, it will sink. Neutralbuoyancy is when the fish or submarine remains inone place, without rising or sinking.

CHECK YOUR UNDERSTANDING

182 Chapter 8 Worksheet 8.6-1 Copyright © 2010 by Nelson Education Ltd.

1. When there is an oil spill, how is the density of oil helpful? How is it harmful?

Name: _________________________________________ Date: ________________________

2. Describe an instance when humans might have used an example from natureto design a device that uses density and buoyancy.

3. (a) Hydrogen is less dense than helium. Why did this make it a good choicefor 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 doesdensity affect your daily life? Give one example.

Chapter 8 Worksheet 8.6-1 183Copyright © 2010 by Nelson Education Ltd.

END

Name: _________________________________________ Date: ________________________