All Resources

Balloon Rockets

Students learn about Newton’s Third Law by making balloon rockets.

Using a balloon, we can create a good demonstration for the type of propulsion that moves a rocket. When we inflate a balloon, we fill it with a pressurized gas (air). When we let go of the end, the air rushes out and pushes against the air around the balloon to move it in the opposite direction. 

Newton describes this effect in his third law of motion: for every action, there is always an equal and opposite reaction. The gas rushing out of the rocket or balloon is the action and the movement of the object in the opposite direction is the reaction.


Because balloon openings are wobbly, the flight path of a balloon is wobbly, too. The air escaping the balloon (the action) pushes out in every which way and the reaction of the balloon is to move in every which way, too.

One way to stabilize the direction of the balloon is to attach it to a simple track made of string. Once attached to a straight path, it’s easier to see the direct relationship between the size of the action (the amount of air escaping the balloon at once) and the resulting size of the reaction (the distance the balloon travels).

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 pair or group of students:
    a balloon

  • 3 m length of string (thin enough to thread through a straw – fishing line works well)

  • one straw

  • some 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. Follow up with Pop Bottle Rocket Part II: Projectile Motion.

Other Resources

Science World Resources | Full Unit | Forces

Science World Resources | Full Unit  | Balloons

Science World Resources | Full Unit  | Air

Science World Resources | Unit | The Air Up There (an air unit for kindergarteners)

Science World | School Programs | Roller Coaster