Balloon rocket — size vs distance

1. Pick a balloon size. The number is how wide the balloon is before you blow it up, in inches.
2. Use the Blown up to slider to choose how full the balloon is. 100% means all the way full.
3. Choose how tight the String is. Taut means pulled tight. Slack means it sags.
4. Press Release. The balloon flies to the left along the string. The readouts at the top show its speed and distance.
5. When it stops, a tape flag marks the spot. The trial is added to the Trials table and the graph.
6. Read the graph. Each dot is one trial. The color shows the string. A hollow dot means the balloon was not full.
7. Press Copy data to paste your trials into a spreadsheet. Press Clear trials to start over.

This lab shows Newton's third law. It says forces come in pairs. When one thing pushes on another, the second thing pushes back just as hard, in the opposite direction.

The balloon squeezes the air out of its neck to the right. The air pushes back on the balloon to the left. That push is called thrust. It is what moves the balloon. The two pushes happen at the same time, and they are the same size.

Watch the two arrows while the balloon flies. They are the same length because the forces are equal.

Every balloon uses the same straw. So every balloon pushes about the same amount. A big balloon does not push harder. It pushes for longer, because it holds more air.

Air also pushes against the balloon as it moves. This is called drag. A big balloon has more drag, so it flies a little slower. But it keeps pushing for so long that it still travels much farther.

Because of this, the graph of distance vs. size is a curve, not a straight line.

A saggy string slows the balloon down. The straw rubs and sticks where the string bends. This rubbing is called friction. The balloon also has to drag the loose string along.

A very slack string can cut the distance of the biggest balloon about in half. You can see the sag in the long view of the string.

You can change three things: balloon size, how full it is, and the string. In a fair test, you change only one thing at a time. Everything else stays the same.

• To test size: keep the slider at 100% and the string taut. Try all five balloons.
• To test how full: pick one balloon. Move the slider down from 100%.
• To test the string: keep size and fullness the same. Change only the string.

The graph helps you check. Each string setting has its own color. Balloons that are not full show as hollow dots.

• Every balloon has the same straw. So what is different about a big balloon's push?
• Which balloon reached the highest speed? Did that same balloon go the farthest? How can both be true?
• Turn on Plot against air volume. Is that graph straighter? Why might air volume be a better thing to measure?
• No balloon reaches the far stand, 30 m away. What could you change to get one there? Would it still be a fair test?
• A classmate blew up a 10 inch balloon all the way and a 16 inch balloon about half way. They say size does not matter. What went wrong?

Thrust: T = 2 × p × A_straw. Here p is the air pressure inside the balloon. It drops as the balloon empties.

Drag: D = ½ × ρ × C_d × A_front × v². Drag grows fast as the balloon speeds up.

Friction: F = µ × m × g, plus extra friction that grows as the string sags.

The pressure inside is 2.5 kPa. The straw is 9 mm wide. The straw and tape add 1.5 g every time. The sim works out the motion in tiny steps, so the distance it records always matches what you see.

The balloons are drawn a little bigger than real size, so they are easy to tell apart.