Turn your sound on, then press play.
This cannon shoots nothing but air. That sounds like it should not work, but air is real stuff. It takes up space, it has weight, and when it moves fast it can push things over. Wind is moving air, and a strong wind can knock down a tree.
When you pull back on the balloon, you stretch the rubber. Stretching takes work from your muscles, and that energy does not disappear. It gets saved in the stretched rubber. Scientists call it stored energy. A stretched rubber band holds stored energy too.
When you let go, the rubber snaps forward and shoves the air inside the cup. That air has only one way out, which is the opening at the front. A quick burst of air shoots out.
The energy starts in your muscles. It moves into the stretched balloon. The snap gives it to the air. The air carries it across the room and gives it to the target, and the target falls over. Energy moved from you to the target, and you never touched it.
If the balloon is loose or the tape has gaps, some of the air squirts out the back and the sides. Only the air that goes out the front can hit the target.
Structure is how something is built and shaped. The way a thing is built decides what it can do. Your cannon is only a cup and a balloon, but its shape turns one small snap into a blast that can cross a room. Keep an eye out for how each part of the shape matters.
The burst of air does something surprising when it leaves the cannon. It curls into a spinning ring shaped like a donut. This is called a vortex ring.
Here is how the ring forms. The air in the middle of the opening rushes out fast. The air at the edges drags against the rim and gets slowed down. The fast middle and the slow edges make the puff curl back on itself, and it rolls up into a ring.
The spinning is what makes the ring special. A plain puff of breath spreads out and fades in a foot or two. A vortex ring holds itself together, so it can travel across a room and still have enough push to tip a target.
You cannot see the ring from your cannon, because air is invisible. Vortex rings also show up in places where you can see them. Dolphins blow bubble rings under water. Some volcanoes puff out giant smoke rings.
This cannon is filled with fog, so the air shows up. You are looking at it from the side.
A longer pull stores more energy in the balloon, so the ring comes out faster and hits harder. A tiny pull makes a weak ring that can barely move a strip of tissue.
Do not overdo it. If you pull too far, the balloon can tear or snap off the cup. Pull until the rubber feels firm, then stop.
A vortex ring does not last forever. As it travels, it rubs against the still air around it and slowly loses energy. It gets slower and weaker the farther it goes. A target that falls at 3 feet might not even wobble at 10 feet.
A target falls when the push from the ring is stronger than whatever is holding the target up. Light targets are easy to tip. Heavy targets need a harder hit.
Shape matters too. A tall target with a narrow bottom tips over easily. A short target with a wide bottom is steady and hard to tip.
Good targets to try are a strip of tissue paper, a folded paper tent, and an empty paper cup turned upside down. Line them up from lightest to heaviest and see how far down the line your cannon can go.
Engineers measure a cannon by its reach. The reach is the farthest distance where the cannon can still knock over a target. Start close, then step back one foot at a time until the target stops falling.
Engineers test their ideas before they build. In this lab you fire a pretend cannon down a long table. There is a target at every foot. The ring knocks over each target it reaches with enough push, and the farthest one that falls shows the reach.
The fair test rule: change only one thing at a time. If you change two things at once, you cannot tell which one made the difference.
Pull back is how far you pull the knot. A pull of 1 is tiny. A pull of 5 is long.
No two shots are exactly alike, so the same cannon can have a slightly different reach each time. That is why engineers test more than once.
| Shot | Pull | Opening | Targets | Reach | How it went |
|---|
No shots yet. Press the green button to fire.
The building steps open when every mission is finished. Here is what you still need to do:
You earned it. Follow each step in order and check the box when a step is done.
Ask an adult to poke a starter hole in the bottom of the cup. Put your scissors in the hole and cut around the bottom until the whole bottom comes off. Go slowly and try to leave a smooth, round opening. This is where the air will come out.
Trim off any pointy bits. If the edge still feels sharp, fold small pieces of tape over it and press them flat. Keep the opening as round as you can.
Do not blow the balloon up. Tie a knot in the neck of the empty balloon. The knot will be your handle.
The top is the round end, across from the knot. Cut straight across, about one third of the way down from the top. Keep the big piece with the knot. Put the small round piece in the trash.
The open end is the wide end you would drink from. Stretch the cut balloon over it like the skin of a drum. The knot goes in the middle and points out. This is easier if a partner holds the cup still.
Wrap tape all the way around the cup, over the edge of the balloon. Go around twice and leave no gaps. A leak here lets air sneak out the back.
The balloon should be flat and snug, like a drum. The knot should sit in the middle. Look in through the cut end and make sure the opening is clear and round.
Cut a piece of paper about as big as your hand. Fold it in half so it stands up like a tent. Make five of them. Remember that light, tall targets fall the easiest.
Stand your targets in a row near the edge of a table. Mark a firing line on the floor 2 feet away from the targets. Make sure nobody is standing behind the targets.
Hold the cup in one hand and point the cut end at a target. Pinch the knot with your other hand. Pull it straight back, then let go. If the target falls, step back one foot and try again to find your reach.
Remember the fair test rule. Change only one thing between tests, and write down what you changed. Write the reach in feet.
| Test | What I changed | Feet |
|---|
| What happened | Why | Try this |
|---|---|---|
| The blast is very weak | Air is leaking, or the balloon is loose | Stretch the balloon tighter. Add tape where you feel air escape. |
| It keeps missing | The pull was sideways, or the cup moved | Pull straight back. Hold the cup still. |
| It works up close but not far away | The ring is breaking apart | Smooth the cut edge and make the opening round. |
| The balloon popped off | The tape let go | Use stronger tape and wrap it around twice. |
| The balloon tore | The pull was too long | Put on a new balloon and use a gentler pull. |
Write three or more sentences. What did you change on your cannon or your targets? What happened? Why do you think it happened? Use at least two of these words: energy, stored, air, vortex ring, reach, target.
Big idea question: how did the shape of your cannon change what the air could do?
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