Balloon Rockets

Objectives

• Explore and demonstrate the effects of action and reaction forces.

• Apply their understanding of forces and their effect on objects to manipulate the flight of toy rockets.

Materials

• Per Group or Pair of Students:
  a balloon
  3 m length of string (thin enough to thread through a straw – fishing line works well)
  1 straw
  tape

Key Questions

• What is the force causing the balloon to go forward? In what direction is that force?
• Can you suggest any ways that we can make the balloon travel further along the string? Try a few out!
• What do you predict would happen if you change the size or shape of the balloon?
• What makes the balloon stop?

What To Do

Put students in pairs or small groups and have them try the following:

1. Tie or tape one end of the string to a desk, a post, or the wall.
2. Thread the straw onto the string. Have one person hold up the free end of the string so that it runs parallel to the ground. They should hold the string taut.
3. Blow up a balloon and pinch the end shut to stop the air from escaping. Don’t tie it off!
4. Without letting any air out, tape the blown-up balloon to the straw with the mouth end facing you (and the person holding the string).
5. Release the balloon, and see how far it goes!

Teacher Tip: If you’re working with a younger groups, set up the strings with the straws before the start of the activity.

Extensions

• What do you predict will happen if you use a different kind of string (yarn, thread, etc.) or change the angle of the string?
• Add cargo to your balloon!
• Modify the balloon with fins or a nose cone to see if it affects the flight. The fins on the rocket act to steer it. As the rocket moves, the fins "slice" through the air. Since the air coming out of the balloon is pushing the rocket, the fins will try and go through the air in the easiest path possible. The easiest path is always straight. Generally speaking, larger fins will cause the rocket to fly straighter. However, if the fins are too large, they get wobbly, and will make the rocket go crooked. Real rockets actually have very small fins.
• What would happen if you were able to control the direction the bottom of the balloon was facing? In real rockets, the rocket engine (the part where the air comes out) can be tilted slightly. This allows the astronauts to steer the rocket by changing which way the "push" is.
• Run the fishing line vertically and challenge students to make their balloons reach the ceiling. Why might it be harder to move straight up? Discuss how rockets have to be extremely powerful to overcome gravity.

Other Resources

Follow up with Pop Bottle Rocket Part II:Projectile Motion