After this activity, students should be able to:

• Explain Newton's third law of motion using words or pictures.
• Explain why a rocket must work harder to move up as opposed to across.
• List at least two different types of engineers and how they make rockets possible.

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How does a rocket fly? Can it fly in space and in Earth's atmosphere? What comes out of the back of a rocket? Rockets fly because of Newton's third law of motion. Newton's third law states that for every action there is an equal and opposite reaction. For example: if you are standing on a skateboard and you push against a wall, what happens? The answer is that you and the skateboard would move in the opposite direction. Here the action is pushing against the wall, while the equal and opposite reaction is the movement in the opposite direction.

Let's say you're in outer space wearing a spacesuit and holding a bowling ball. What do you think would happen if you threw the bowling ball? Would anything happen to you? Think about Newton's third law of motion. What might it tell us about what would happen? The answer is that you would move away from the bowling ball in the opposite direction. In this case, throwing the bowling ball is the action and you moving in the opposite direction is the equal and opposite reaction. This is similar to how a rocket flies, but instead of throwing bowling balls, a rocket pushes lots of gas in one direction, causing the rocket to move in the opposite direction.

So, how does a rocket push this gas? Rockets combust (burn) fuels such as liquid hydrogen. The combustion process causes the fuel to get incredibly hot and expand (just like an explosion). This combustion happens inside the rocket, but the hot gas does not fit very well in the rocket and it creates a lot of pressure. Pressure is when something is pushing very hard on something else. In our case, the gas is pushing very hard on the inside of the rocket. Even though a rocket is very strong, too much pressure will cause it to explode. Since we want our rocket to move and not to explode, we let the gas escape. We put a hole in the bottom of the rocket and use a device called a nozzle to direct the gas behind us. This escaping gas is known as exhaust and it is the action in our rocket. The equal and opposite reaction is our rocket moving in the opposite direction. How much power the rocket provides is called thrust.

Do you think it is easier for the rocket to move straight up (away from Earth) or straight across the surface of the Earth? Why might it be harder to move straight up? Gravity is what makes it harder to move up. Gravity pulls objects down toward the center of the Earth. If we want our rocket to reach space, it has to move up and away from the Earth. This means that rockets have to be extremely powerful to overcome gravity. Has anyone ever seen a rocket on television, in a photograph, or in real life? Do you know what makes up most of a rocket? Do you think its suitcases or cargo or scientific equipment or crew quarters for the astronauts? In reality, it is none of these. Amazingly, most of a rocket is fuel. It takes so much work (energy) to get to outer space that most of a rocket is fuel.

So, who is responsible for designing and building rockets? The answer is engineers. Rockets are complicated and usually very large objects. It takes many different types of engineers to get a rocket to work. Mechanical engineers create the structure of the rocket and make sure it all holds together. Electrical and computer engineers make sure that all the electrical and computer systems on the rocket function. Chemical engineers design the fuels and make sure the rocket gets all the power it needs. Materials engineers design heat shielding material for the outside of the rocket, as well as the astronauts' spacesuits. Aerospace engineers make sure the rocket is shaped in the best way to cut through the atmosphere and they also make sure all the systems work together.

Figure 1. Using a balloon to demonstrate action and reaction. click for copyright

Do all rockets require combustion? The only rockets that can make it into space right now require combustion, but some simple rockets do not require combustion. Have you ever blown up a balloon and then let it go without tying it shut? What happens? The balloon acts like a rocket. In fact a balloon is very similar to a real rocket. When you inflate a balloon you are forcing air into a small space. This creates pressure just like a space rocket creates pressure by burning fuel. Just like a rocket, a balloon lets the high pressure escape out a hole in the back. The gas escaping out of the back of the balloon is the action and the balloon moving in the opposite direction is the equal and opposite reaction (see Figure 1). Today, we are going to make balloon rockets and observe Newton's third law of motion in action. We will also see that overcoming gravity is more difficult than flying horizontally.

Before the Activity

With the Students

1. Divide the class into teams of two students each. Pass out supplies and worksheets.
2. Have students cut the straw in half, making two shorter straws to share with other teams.
3. Place an equal number of teams at each rocket course. The teams take turns using the course.
4. Have one student from each team inflate the balloon. Twist the open end of the balloon and secure it closed temporarily using a paperclip.
5. Next, have the other teammate tape the straw to the balloon so that it is in the center of the balloon with one end facing the balloon top and the other end facing the balloon opening (nozzle). See Figure 1.

Figure 2. A balloon rocket ready for launch. click for copyright

6. Next, have students thread the string at the unattached end of the course through the straw so that the twisted end of the balloon faces away from the end of the line that is attached to the wall (see Figure 2).
7. Instruct one team member to hold the end of the line so that it is taut and level. The other team member makes sure the balloon hangs below the line.
8. With the class counting aloud down from 10, have students release the twisted ends of the balloons at zero.
9. Give the students a minute to determine how far their rocket flew by measuring how far it went on the fishing line.
10. Record the distance traveled on the worksheets.
11. Have the teams take turns using the course until each team has launched their rocket three times, and recorded the distances traveled on their worksheets.

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1. Detach the lines from the far end of each rocket course (from the wall or a chair), and move them up higher, so that each is between 6 and 10 feet (2 or 3 meters) above the floor (as high as you can reach).
2. Next, with the lines positioned at an incline and taut, have each group launch their rocket three more times and record the length traveled on their worksheets. Have the students hold the line at the starting end near the ground so the balloons must travel up equal inclines.
3. Have students finish their rocket trials and complete their worksheets.
4. Conclude by holding a class discussion, comparing results, and conducting the post-activity assessment activities described in the Assessment section. See if students understand Newton's third law of motion. Talk about test results in both horizontal and incline conditions and how the expelled air created thrust.

• Advise students not to over-inflate their balloons.

Pre-Activity Assessment

Activity Embedded Assessment

Post-Activity Assessment

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• For younger students, conduct the activity as a teacher demonstration.
• For upper grades, have students experiment with different balloon shapes (rockets) as well as the opening size (straw diameter).Ask students to come up with different prototypes and work through the engineering design process to see which combination of factors produce the best balloon rocket. Design process steps: brainstorm many different design ideas; select the best design; draw, create and test your design; review your design; improve your design based on what you learned.
• For more advanced students, have them calculate (and graph) the average rocket distance traveled for their horizontal and incline data collected. Have them hypothesize what other factors contribute to the different distances achieved.