Overview

Students first examine their balloon cars closely to explore how they work, and do some mini-experiments to see what role each part plays. Next, there are some focusing questions that help to review and develop energy concepts related to balloon cars. Finally, they write descriptions about how balloon cars work.

Advance Preparation

• Photo copy worksheet for “How a Balloon Car Works”. Download below.

Materials

• Balloon cars already made by students
• Materials for making balloon cars, for spare parts and for doing experiments

Procedure

1. Class meeting. Ask students what they have noticed about the way a balloon car works. Chart their observations. Here are some focusing questions that might help to structure the discussion:

• What is changing as you blow up the balloon? What does it “want” to do? How are you preventing it?
• Blow up the balloon, release the straw, but hold the car so it can’t go. What comes out of the straw? What direction does it go and why?
• Now blow it up again, but this time let the car go. What is making it go?

2. Balloon car experiments. Here are two guided experiments (or demonstrations) that reveal more about how balloon cars work:

Adjust the balloon and straw so the straw points sideways to the car (see a video) , perpendicular to the direction the car is supposed to go. Try the car.

• What happens?
• Why doesn’t it work?
• Where is the air going?

Replace the straw so it is again pointing to the rear. Adjust the front axle so it is at an angle with the rear axle, instead of parallel to it (see a video). Try the car.

• What happens now?
• What controls the direction of the car?

3. Class meeting. The class analyzes the information from these experiments, then breaks down what happens from the time they begin to blow the balloon up to when  the balloon car is traveling on its own. Help students describe each step in terms of what is happening with the energy.  These questions might help:

• You put energy into the wind-up and balloon car.  Where did you get your energy?
• How did the energy get into your food?
• A wind-up stores potential energy in the rubber band, when you wind it up. When and where does a balloon car store energy?
• What happens to this energy? When?
• A balloon car won’t work unless someone blows up the balloon. Where do we get the energy from that we need to blow up the balloon?

Extend the concept of energy to include the different forms of energy, the transformation of energy from one form to another, and the conservation of energy. Do this with reference to both wind-ups and balloon cars,  making connections with what students have done in this unit. Here are explanations of energy relationships, conservation of energy, and of situations where energy does not seem to be conserved: friction and the wind-up and friction and the balloon car.

4. Homework: Students complete the worksheet “How a Balloon Car Works”.

5. Outcomes:

• Students look at a balloon car closely to uncover its principles of operation.
• Students use the results of simple experiments to draw conclusions about how a balloon car works.
• Students develop a series of cause-and-effect relationships linked together in a causal chain.
• Students review concepts of potential and kinetic energy and apply them to balloon cars.

Unit Assessment