Newton's Third Law of Motion Experiment

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this experiment, you'll be studying Newton's third law of motion and how it applies to rocket propulsion. Using a balloon as a model of a rocket, you'll be able to understand how action-reaction pairs cause rockets to travel through the atmosphere to the moon.

Introduction

 Research Question: How do action-reaction pairs cause motion? Age: Middle school and up Safety concerns: None Time: 1 hour Independent variable: Size of balloon Dependent variable: Distance traveled Control variable: Type of balloon

Picture a rocket zooming off into space. At 4.4 million pounds, it seems unlikely that this beast will leave the atmosphere. Why does keeps it going? You might think, 'Obviously, the rocket fuel.' This is true, but the rocket fuel only provides a force pushing into the ground. What pushes the rocket up?

To figure out this problem, we need to address Newton's third law of motion, which states that for every action, there is an equal but opposite reaction. So, if I push on the wall with 100N of force, it pushes back on me with 100N of force.

So why don't I move when the wall pushes back? Friction is also pushing on me with a greater force than the wall pushes me back, so overall I don't move. To review Newton's third law before we get started, take a look at this overview: Overview of Newton's Third Law of Motion.

The action and reaction forces are called an action-reaction pair. To understand how these work in a rocket, we're going to use balloons as a model. The size of the balloon will represent how much fuel the rocket has, and thus how big the action force is.

We'll examine how this affects motion by letting the balloon deflate and push the air out, then measure the distance the balloon travels. If you're interested in rockets and like to know more about their structure, you can read up on this lesson: Parts of a Rocket: Lesson for Kids

Materials

• 2 balloons of the same type
• 1m of string to measure the balloon
• 5m of string for experiment
• 4 2-inch pieces of masking tape
• 2 straws
• Meter stick
• Data table:

Circumference Distance Traveled

Steps

1. Slide the 5m piece of string through the straw to create a track for the balloon.

2. Blow up the first balloon as large as it will go. Do not tie it off.

3. Attach the straw to the balloon. Put the straw on top of the balloon and use 2 pieces of masking tape to secure it.

4. Now, measure the circumference of the balloon by wrapping the string around it. Measure the string with the meter stick. Record this in your data table.

5. Now, it's time to let your balloon fly. Pull the balloon to one side of the string and either secure the other side to an object or have a friend hold it. Let your balloon open and watch it fly on the track.

6. Measure the distance traveled using the meter stick and record this in your data table.

7. Repeat step 1-6 for the second balloon, but make it a smaller size.

Troubleshooting

You may need a friend to help you measure the circumference while keeping the balloon fully inflated. If the straw isn't staying attached to your balloon, you're not going to get accurate results.

Discussion Questions

Which balloon traveled farther?

How did the action-reaction pair influence the balloons traveled distance?

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