The Science behind the Balloon Powered Car

These are some things that can be discussed with the class regarding the science behind the whole project. Taking a closer look at something as simple-looking as a balloon powered car toy can reveal a lot about the interaction of forces and energy conservation laws that are at the core of fundamental science.

Our toy car consists of a balloon attached to a straw, fixed to a lightweight wheelbase. When someone blows up the balloon, the rubber material that makes up a deflated balloon is forced to expand to the volume of the entering air. It takes a constant force to keep the rubber material stretched- and this is represented by the pressure of the air inside the balloon. The balloon wants to deflate, and this can be represented by a certain amount of elastic potential energy being stored in the rubber material. The air is pushing on the inside of the inflated balloon- by Newton’s Third Law, the rubber material is pushing back on the air. Since the balloon is spherical and has no holes- the forces are balanced in all directions and nothing happens.

As soon as someone lets go of the straw, a leak is formed. This can be more easily explained by representing the leak as just a hole in the balloon. When a hole forms in the balloon wall- there is no longer anything pushing back on the air at that point. The rest of the material is still pushing against the air, which causes an imbalance in forces and produces motion (the air rushes out of the balloon). Newton’s Third Law still holds true, however, so the air still pushes on the balloon even as it is coming out. The only difference is now that the air can no longer push on the part of the balloon where the hole formed (there is no rubber material there). This causes an imbalance in forces on the balloon, since air is pushing on one side of the balloon but not the other, which results in the balloon traveling in the direction opposite the hole. The purpose of the balloon powered car is to harness this power and physically attach the balloon to a chassis. Instead of the balloon flying away, the toy car rolls away.