How Does the Volume of Air Affect How Far a Balloon Rocket Travels?

Talk It Over

A famous scientist once showed that "every action has an equal and opposite reaction." You can prove that by jumping off the front of a skateboard. Which way does the board travel? Backward. . .and with a force that equals your weight! The same principle propels rockets. How can you show the effect of force on the distance a rocket travels?

Get

• A large, outdoor space with 2 poles,trees,or fence posts that are 5 or more meters apart
• Scissors
• Fishing line, 5 meters or more in length
• Flexible, metal measuring tape (the kind carpenters use), 5 meters or longer
• Fabric measuring tape (the kind people who sew use), marked in centimeters
• Masking tape
• Round balloon
• A helper (It takes more than two hands to do this experiment!)
• Plastic straw

Go

1. Find a spot outdoors where you can work. You need two trees, poles, or posts that are 5 or more meters apart. Measure the distance between the supports with your metal tape, like this:



2. Tie one end of the fishing line to one of the supports, 1 meter off the ground. Stretch the fishing line to the other support, but do not tie it. Cut the line, leaving about 50 cm extra.
3. On the second support, measure up 1 meter from the ground. Place a piece of masking tape at that height. It will serve as your starting point, like this:



4. Have your helper blow up the balloon slightly. Do not tie the balloon. While your helper holds the balloon shut so no air escapes, measure the distance around the balloon at its widest point. Use the sewing measuring tape, like this:



5. Record that number, which is the circumference of the balloon.
6. While your helper continues to hold the balloon, attach a straw to the balloon with tape, like this:



7. Your helper is still holding. (Helpers need patience and dedication.) Thread the fishing line through the straw. Raise the line to the starting point on the support. Pull the line taut and position the balloon on the line, like this:



8. When you say go, your helper will release the balloon, and your model rocket will fly along the fishing line.
9. While you continue to hold the line, have your helper run out and put a finger on the line at the point where the rocket stops. Use your flexible metal tape to measure the distance that your rocket travels in centimeters.
10. Repeat this many more times, each time adding a little more air to the balloon. (The more air in your balloon, the greater its circumference.)

Stay Safe

Don't stand in front of the balloon when you launch it. It could smack you a wallop.

Go Easy

The "Go" procedure will work for you. Get an adult's help with the setup, measuring, and data collection.

Go Far

Use volume, not circumference, in your data tables. Although circumference is proportional to the volume of air in the balloon, we did not actually measure volume in the "Go" procedure. (Volume is the total amount of space an object fills.) You can, however, calculate the balloon's volume from its circumference (assuming that the balloon is a sphere, which it isn't, but it is close) in two steps:

1. Find the radius, which is the distance from the center of the sphere to its outer edge:



radius = circumference ÷ 6.28

2. Use the radius to find the volume.

volume = 4.18 × radius × radius × radius

For example, if the circumference of your balloon is 21 cm, its radius is 21 ÷ 6.28 = 3.34 cm. Its volume is 4.18 × 3.34 cm × 3.34 cm × 3.34 cm = 156 cm³.

You may also want to add time and speed to your experiment. Use a stopwatch (more helpers' hands are needed) to measure the time of your balloon's flight. Divide distance by time to obtain speed.Compare speed to the balloon's volume. What is the relationship?

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