Objectives
Students study hot-air balloons to discover the principles of lighter-than-air flight.

• test different objects to see if they float in water
• relate whether or not an object floats to the density of its material
• discover that warm air is less dense than cool air
• launch a solar-heated hot-air balloon

Schedule
Session I—About 45 minutes
Session II—About 15 minutes, followed by observations every 10 minutes for an hour

Vocabulary
density
envelope
lift

Materials
For each student
1        Activity Sheet 3, Parts A and B

For each team of four
1        balloon
1        bottle, plastic, 16-oz
2        containers, plastic

For the class
1        balloon
1        container, plastic
1        *fan, electric, small
5 lb    *ice
1         marble, glass
1         marble, wooden
1 roll   *paper towels
           *pictures of hot-air balloons
1         *rock, heavy
2          rubber bands
1 pr     *scissors
1          solar balloon
            *water, tap (cold and hot)

*provided by the teacher

1. Make a copy of Activity Sheet 3, Part A, for each student.

2. Find pictures of hot-air balloons in magazines and books, and bring them to class to show students.

3. On the morning of the activity, purchase one 5-lb bag of ice and store it in a freezer until needed.

4. You will need a plastic container of water, a glass marble, and a wooden marble for a demonstration. Each team of four will need one balloon, one plastic soda bottle, and two containers of water—one filled with hot water and the other filled with ice water. Have paper towels on hand in case of spills.

1. Make a copy of Activity Sheet 3, Part B, for each student.

2. In order for the air in the solar balloon to heat up sufficiently, you will need to conduct this session on a sunny day. Choose an area outdoors that will receive direct, sustained sunlight while remaining shielded from the wind.

3. You will need the solar balloon, a small fan (or hairdryer) to inflate the balloon, two rubber bands, and a rock to tether the balloon to keep it from floating or blowing away.

Background Information

There are two main classifications of aircraft: lighter-than-air craft and heavier-than-air craft.

Both types of aircraft must achieve lift in order to fly. Lift is the upward force that enables an object to overcome Earth’s gravity. Heavier-than-air craft achieve lift by the movement of air molecules against their surfaces.

Lighter-than-air craft achieve lift by making their total density less than that of the air surrounding them. Once they do this, they essentially float in the air because objects float in materials whose density is higher than their overall density. (Corks float in water because cork is a less-dense material than water, that is, its molecules are packed together less tightly and so it weighs less per unit area.)

A hot-air balloon is an example of a lighter-than-air craft. It consists of a large, globe-shaped nylon bag, called an envelope, filled with air. At the mouth of the envelope is a propane burner that heats the air inside the envelope. Suspended from the envelope by cables is a wicker basket called a gondola, which carries the balloon’s passengers, fuel tanks, and supplies.

<Insert Figure 3-1 here.>
Figure 3-1. The parts of a hot-air balloon.

In order for a hot-air balloon to lift off the ground, the total density of the craft must be less than the density of the air surrounding it. But the balloon and all its cargo weigh hundreds of pounds. How can such a craft become less dense than air?

By heating the air inside the envelope, the pilot increases the energy of the air molecules and causes them to move farther apart. As the air in the envelope expands, it becomes less dense. When the air becomes so spread out that the density of the craft as a whole is less than the density of the air outside the balloon, the balloon floats.

To land a balloon, the air inside the envelope is allowed to cool. As the air inside the envelope cools, it becomes more dense and, combined with the weight of the balloon and its cargo, becomes more dense than the air around it. This causes the balloon to sink back down to Earth.

In this activity, students discover the relationship between density and flotation and observe how changing the temperature of air can increase or decrease its density. Then they launch a solar-heated hot-air balloon and explain how it floats.

Teaching Suggestions
Session I
1
Remind students of their experiments with parachutes in Activity 2. Ask, What other types of people-made craft have you seen in the air?
&
Students may name planes, helicopters, gliders, and hot-air balloons, among others.
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Draw students’ attention to hot-air balloons and point out that hot-air ballooning is a very early method of flight.

2
Distribute a copy of Activity Sheet 3, Part A, to each student. Then pass around the pictures of hot-air balloons for all students to look at.

Review the parts of a hot-air balloon. Write the word envelope on the board. Ask, Which part of the balloon is the envelope?
&
Students will probably guess that the large, colorful, inflatable part of the balloon is the envelope.
&

Review the other parts of the balloon: the burner beneath the mouth of the envelope, which heats the air inside the balloon; the suspension cables by which the wicker basket, or gondola, is attached to the balloon; and the fuel tanks in the basket.

Tell students to label the diagram at the top of their activity sheet.

3
Ask, If gravity is constantly pulling down on objects in the sky, how can they go up and stay up?
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Students may guess that the objects must be exerting some sort of force to overcome gravity.
&

Write the word lift on the board. Tell students that lift is the upward force that enables an object to overcome gravity, and that all aircraft must achieve lift in order to fly.

Ask, How do you suppose hot-air balloons achieve lift?
&
Some students may suspect that the hot air inside a balloon has something to do with it. Accept all answers.
&

Write lighter than air on the board. Tell students that hot-air balloons are considered lighter-than-air craft. Lighter-than-air craft achieve lift by making themselves essentially “lighter than air.”

Blow up a balloon, tie it, and hold it up for students to see. Ask, What is this balloon filled with?
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air
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Drop the balloon. After it lands on the floor, ask, Is this inflated balloon lighter than air or heavier than air?
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The balloon fell to the floor, so it must be heavier than air.
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Ask, So how can a hot-air balloon, with all the equipment and passengers it carries, possibly be lighter than air?
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Again, some students may suggest that heating the air inside the balloon has something to do with it. Accept all answers.
&

4
Place a clear plastic container of water on a desk or table for all students to see. Hold up the glass marble and the wooden marble. Ask, What do you think will happen to these marbles if I drop them in water? Have students write their ideas on their activity sheets.
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Answers will vary.
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Drop the marbles in the water. Then ask, What happened to each marble? Have students record their observations on their activity sheet.
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Students should observe that the glass marble sank while the wooden marble floated.
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Ask, Which marble floated in the water?
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the wooden marble
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Ask, What makes some objects float and others sink?
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Some students may guess that it has something to do with the material that makes up an object. Accept all answers for now.
&

5
Write the word density on the board. Tell students that density is the amount of something in a given volume, or space.

Remind students that all matter is made up of molecules. Explain that the closer together the molecules in a substance are, the denser the substance. As a result, a given amount of a more-dense material is heavier than an equal amount of a less-dense material. Draw Figure 3-2 on the board.

<Insert Figure 3-2 here.>
Figure 3-2. The more packed together the molecules in a substance, the denser that substance is.

Next, tell students that whether or not an object floats depends on the density of the object and the density of the material around the object: An object that is less dense than the material around it will float in that material. An object that is more dense than the material around it will sink in that material.

Ask, What material were the marbles placed in for this demonstration?
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water
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Ask, Is the glass marble more dense or less dense than the water?
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The glass marble sank in the water, so it must be more dense than water.
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Ask, Is the wooden marble more dense or less dense than the water?
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The wooden marble floated, so it must be less dense than water.
&

6
Return students’ attention to the pictures of the hot-air balloons. Tell students that hot-air balloons float in the air much like a wooden marble floats in water.

Ask, What can you guess about the density of the air inside a hot-air balloon?
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Students should realize that for a hot-air balloon to float, the air inside the balloon must be less dense than the air outside the balloon.
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Ask, How could you make the air inside a balloon less dense than the air outside the balloon?
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Some students may suggest heating it.
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Tell students that the next experiment will show them what happens to a quantity of air when it is heated.

7
Distribute two plastic containers, a plastic bottle, and a balloon to each team of four. Fill one of each team’s containers with hot water and the other with ice water.

Hold up one of the bottles and ask, What is inside this bottle?
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air
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Tell students to fit the balloon over the mouth of the bottle, making sure that no additional air can leak into or out of the bottle.
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Point out to students that air cannot enter or leave the bottle now because the seal between the balloon and the bottle is tight.
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Instruct students to hold the bottle in the container of hot water and observe what happens to the balloon (see Figure 3-3).

<Insert Figure 3-3 here.>
Figure 3-3. Heating the air inside the bottle causes the air to expand and inflate the balloon.

After about one minute, ask, What happened to the balloon?
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The balloon began to fill with air.
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Ask, Where did the air come from?
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It must have come from inside the bottle because no air was able to enter the bottle from the room.
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Remind students that air is made up of molecules that move around in all directions. Explain that when air is heated, its molecules begin to move around faster and spread out. As the molecules move farther apart, the air expands.

In this experiment, heating the bottle caused the air inside the bottle to heat up and expand. As the air expanded, it move into the balloon and caused the balloon to inflate.

Ask, What do you think happens to the density of air as it expands?
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Accept all answers for now.
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8
Next, have students hold the bottle in the container of ice water and observe what happens. After about a minute, ask, What has happened to the balloon?
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The balloon has deflated and collapsed.
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Ask, What do you think caused the balloon to deflate?
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Guide students to understand that as the air in the bottle cooled, it contracted.
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Ask, What do you think happens to the density of the air as it contracts?
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Accept all answers for now.
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9
Have students look at the two drawings at the bottom of their activity sheets. The one on the left represents the bottle in cold water. The one on the right represents the bottle in warm water.

Tell students to imagine that there are 20 air molecules inside their bottle, even though there are really many more than that. Have them draw the molecules evenly spread out within each bottle.

Ask, In which bottle are the molecules more-densely packed? In which bottle are they less-densely packed?
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The molecules are more-densely packed in the bottle placed in cold water, and less-densely packed in the bottle placed in warm water.
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Ask, Which is less dense: hot air or cold air?
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hot air
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Ask, Based on what you just observed, why do you think the air inside a hot-air balloon is heated? Ask students to record their answer at the bottom of the activity sheet.
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The air is heated to make it less dense than the air outside the balloon. Decreasing the density of the air inside the balloon allows the balloon to rise and float in the air.
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10
Tell students that in the next session they are going to launch a solar-heated hot-air balloon and observe firsthand the principles of lighter-than-air flight.

Empty the containers of water in the sink. Collect the bottles and balloons and return them, along with the plastic containers and marbles, to the kit.

Session II
11
Distribute a copy of Activity Sheet 3, Part B, to each student. Tell students that today they are going to launch a hot-air balloon.

Ask several student volunteers to hold the solar balloon while you fill it with air from a fan. When the balloon is about 90% full of air, tie each end off using a rubber band.

With the help of student volunteers, hold the balloon up in the air. At the count of three, let go of the balloon and let it fall to the floor.

After the balloon has fallen to the floor, ask, Is this balloon lighter than air or heavier than air?
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The balloon did not float, so it must be heavier than air.
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Ask, What do we need to do to make it lighter than air and get it to float?
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Heat the air inside the balloon, to make the air inside less dense than the air outside
&

12
Ask, How is the air inside a hot-air balloon heated?
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Students should recall that the air is heated with burners located beneath the opening of the envelope.
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Tell students that they will not be using open flames to heat this balloon. Ask, What are some other heat sources that you can think of?
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Students may suggest heating the balloon with light bulbs, immersing it in a tub of hot water, or even sticking it in the oven.
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Tell students that they are going to place their balloon in sunlight. The dark color of the balloon will absorb the sunrays and heat the air inside the balloon.

13
Set up the balloon in a sunny spot outdoors, sheltered from the wind. Tie one end of the line around the knot in the balloon. Tie the other end of the line around a rock. This string will act like a tether line to keep the balloon from floating or blowing away.

Tell students to record the time on their activity sheet as well as the position of the balloon in relation to the ground. Tell them to also record whether the balloon is currently heavier than air or lighter than air.

Check on the balloon every ten minutes for an hour. Each time, have students record the time and their observations.

14
After an hour, retrieve the balloon and discuss the results. Ask, What made this balloon float? Tell students to write their explanation at the bottom of the activity sheet.
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The sunlight warmed the air inside the balloon, causing the air to expand and become less dense. When the air inside the balloon became less dense than the air that surrounded the balloon, the craft became lighter than air and floated.
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Ask, What do you think would happen if you left the balloon outdoors overnight?
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Students should realize that as the sun set, the air inside the balloon would cool, making it more dense. At night, when there is no sunlight to warm the air inside the balloon, the balloon would rest on the ground.
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15
Tell students that in the next activity, they are going to learn about heavier-than-air flight, beginning with kites.

Reinforcement
Have students measure the circumference of the solar balloon before and during flight. Does the size of the balloon change? What does this suggest about the density of the air inside the balloon?

Cleanup
Retrieve the solar balloon, deflate and fold it, and return it to the kit.

Activity Sheet 3: Hot-Air Balloons

<Insert Figure AS3-1>

1. Label the envelope, burner, suspension cables, wicker basket, and fuel tanks in the diagram of the hot-air balloon below.

2. Predict what will happen to the marbles when they are dropped in water.
____________________________________________________________

What happened to the marbles? ____________________________________

3. Draw 20 air molecules evenly spread out in each bottle below.

Why is the air inside a hot-air balloon heated? __________________________
_____________________________________________________________
_____________________________________________________________

What heat source will you use to heat the air inside your hot-air balloon?
_____________________________________________________________

4. Set the balloon outside in the sun. Check on it every ten minutes for an hour and record your observations in the chart below.

What made the solar-heated balloon float? ___________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________