Resource: Air Power: Experimenting with Balloons

Nearly everyone has blown up a balloon and released it just to watch it zigzag across the room and fly out of sight. It's the balloon's erratic behavior that makes this activity so entertaining. Airplanes and rockets are carefully designed to avoid such unpredictability. Streamlined to move smoothly through air, these aircraft have features which can return the machine to its proper path should it deviate, even slightly, in one direction or another.

Balloons, jet airplanes, and rockets all fly according to principles described in one of the most important laws of physics: Newton's third law of motion. This law states that every action has an equal and opposite reaction. In other words, when we push against something, no matter how inanimate that something is, it pushes back on us with an equal amount of force.

Balloons move because of the force applied by the air inside them. Before it's inflated, a balloon exerts no force on the few molecules of air inside it. As it's inflated, though, more and more air molecules are forced into it, increasing the balloon's internal pressure and causing it to expand. As the rubber of the balloon stretches, it applies greater force on the air inside, while that same air pushes back in all directions. When the balloon is released, the air at the opening escapes; at the same time, the air that's farther inside continues to push on the remainder of the balloon's interior. This unbalanced push propels the balloon forward.

A balloon's erratic movement is the result of resistance that the balloon encounters as it flies through the air. Unlike airplanes and rockets, balloons aren't shaped well for fast flight. They can't fly straight because they lack an aerodynamic point with which to cut through the air in a single direction. The straw that the ZOOM cast members attach to the balloon in this video segment serves to direct the jet of air in a consistent direction. Because resistance to the balloon's spinning motion is low, the jet's direction is maintained.

Why does the balloon move?

If the straw were sticking straight out, how would the balloon move?

What if the straw were bent more?

Can you get more than one straw to come out of a single balloon? If so, can you point different straws in different directions? Make a prediction as to how the balloon will behave when air is expelled from it.