CD Hovercraft Experiment: Build One in Ten Minutes

A CD pushed across a table stops in about 20 centimetres. The same CD with a balloon on top, riding on its own escaping air, crosses the whole table and keeps going. You will want something at the far end to catch it.

What makes this CD hovercraft experiment worth more than a nice glide is the number behind it. To lift a CD on a cushion of air you need a pressure of roughly 13 pascals above the room, which is about one ten-thousandth of atmospheric pressure. A party balloon has that to spare, and once you have calculated it, the whole thing stops looking like a toy.

Key Takeaways

• You will glue a pop-top bottle cap over a CD’s centre hole, add a balloon, and measure how far it glides.
• The concept is that a thin layer of air replaces surface contact, so sliding friction almost disappears.
• Best for ages 6 to 12, with an adult doing the hot-glue step.
• The build detail that decides everything: the glue seal must be airtight and fully cured.
• Compare it with the bare CD. Without that baseline, the glide is impressive and means nothing.

What You’ll Need for the CD Hovercraft Experiment

Item	How much	Where to get it	If you don’t have it
Old CD or DVD	1, flat and uncracked	A drawer of forgotten discs	A thin, rigid plastic disc with a centre hole
Pop-top bottle cap	1	A sports-drink or flavoured-water bottle	A plastic bottle cap with a 5 mm hole drilled by an adult
Hot glue gun	1	Hardware or craft shop	Strong all-purpose glue, left overnight to cure
Balloons	4	Supermarket	Round party balloons work best here
Smooth hard table or floor	1	Kitchen	Polished tile, laminate or glass. Carpet does not work at all
Ruler	1	Pencil case	Used as a consistent launcher, not just a measure
Tape measure	1	Toolbox	String marked at 10 cm intervals
Coins	8 identical	Pocket	Washers, for the added-mass test
Damp cloth	1	Kitchen	Essential. Dust ruins the cushion
Notebook	1	Anywhere	Five glides per condition

Latex note: party balloons are usually natural rubber latex, so skip this build if anyone in the house has a latex allergy.

Time: prep 10 min, glue curing 15 min, experiment 35 min, cleanup 5 min Cost: a glue stick and a bag of balloons Mess level: low

Do the Experiment

Step 1 — Glue the cap on, and take your time

An adult runs a continuous bead of hot glue around the CD’s centre hole and presses the closed pop-top cap down onto it, centred, with its opening over the hole. Keep the bead unbroken. A single 1 mm gap in the seal means no hover at all, and that is the commonest failure by a wide margin.

Step 2 — Let it cure, then test the seal

Wait 15 minutes. Then close the cap, blow the balloon up, stretch it over the cap and listen. You should hear nothing escaping around the base. Put a drop of washing-up liquid on the glue line and look for bubbles if you are not sure.

Step 3 — Measure the bare CD first

Before any balloon, take a second CD, or your own with the cap closed, and push it across the wiped table. Use the ruler as a launcher: lay it flat, put the CD against its end, and slide the ruler forward to a taped mark, which gives the same push every time. Measure how far the CD travels after it leaves the ruler. Five pushes.

Step 4 — Launch the hovercraft

Inflate the balloon to roughly the size of a grapefruit, pinch the neck, stretch it over the closed cap, then set the CD on the table and pull the cap open. The disc will lift. Give it the identical ruler push and measure the glide. Five launches at the same inflation.

Step 5 — Vary the inflation

Repeat with a small balloon, a medium one and a large one, five glides each, and also time how long each one hovers before settling. You now have two measurements per condition: glide distance and hover duration.

Step 6 — Add mass

Tape two coins to the top of the disc, then four, then eight. Five glides at each mass, plus the hover time. Predict, before you start, what more mass will do to each of the two numbers.

What you should see: the bare CD covers 10 to 25 cm on a wiped table. The hovercraft covers one to three metres from the same push, and on a long smooth floor it may simply keep going. Hover time runs from about 20 seconds with a small balloon to a minute with a big one. Adding mass shortens both numbers, and heavily loaded it stops hovering and starts scraping. You can hear the difference before you see it.

What’s Actually Going On With a CD Hovercraft

Air leaves the balloon, goes down through the cap and the CD’s hole, spreads out radially under the disc, and escapes around the rim. While it is doing that, the layer of air under the CD is at a slightly higher pressure than the room, and that pressure difference pushing up on the underside carries the disc’s weight. The CD is no longer touching the table. It is floating on a film of air a fraction of a millimetre thick.

Put the number on it, because the number is the surprise. A CD weighs about 15 g, so its weight is around 0.15 newtons. Its area is roughly 0.011 square metres. Pressure is force divided by area, so you need about 13 pascals. Atmospheric pressure is about 101,000 pascals (NASA Glenn on gas pressure). Your cushion is one ten-thousandth of that. This is why a balloon, which is a feeble pressure source by any industrial standard, is more than enough.

What the air film buys you is the removal of solid contact. The standard model of friction says the frictional force is the coefficient of friction multiplied by the force pressing the surfaces together (HyperPhysics on friction). Lift the surfaces apart and that normal contact force between solids goes to nearly zero, so the friction force does too. What remains is viscous drag in the thin air layer plus ordinary air resistance on the moving disc, and both are tiny at these speeds (The Physics Classroom on types of forces).

You can quantify the improvement from your own glide distances. For an object sliding to a stop, the effective coefficient of friction is the launch speed squared divided by twice gravity times the distance. Because you used the same launcher, the launch speed is the same in both conditions, so the ratio of the two distances is the inverse ratio of the two coefficients. A hovercraft going ten times further than the bare CD has about a tenth of the friction. That is a measured result from a ruler and a tape measure, and it is more convincing than any demonstration.

Two further points matter. The first is that the hovercraft never speeds up on its own. All the air goes downward and then outward in every direction equally, so there is no net sideways push. It coasts on whatever you gave it. That is worth comparing directly with the balloon rocket on a string, which uses the same balloon and the same air but points the exhaust out of one side and therefore produces thrust. Same energy source, different nozzle geometry, completely different machine.

The second is Newton’s first law doing quiet work. Once the friction is nearly gone, the disc keeps its velocity for a long time, which is exactly what the law predicts and almost never what a child’s experience of the world looks like (The Physics Classroom on Newton’s first law). Most things stop quickly, and most children conclude that stopping is what things do. A hovercraft is a small window onto the frictionless case.

Real air-cushion vehicles solve the same problem with a flexible skirt around the edge. The skirt narrows the gap air has to escape through, which means far less air flow is needed to hold the cushion up, which is why a full-size hovercraft can lift tonnes rather than grams.

How to Explain It to Your Kid

Ages 5–8: It is riding on its own air

Have them press a hand flat on the table and slide it, then do the same thing while blowing hard at their palm from underneath, which obviously will not work, and laugh about it. Then show them the CD. Say the balloon is pushing a thin layer of air under the disc so the disc is not really touching the table, and if it is not touching, it cannot rub.

Ages 9–12: Pressure times area holds up the weight

Introduce pressure as force spread over an area, and do the arithmetic together: 0.15 newtons over 0.011 square metres is about 13 pascals. Then tell them atmospheric pressure and let them work out the ratio. The fact that such a tiny pressure lifts the disc is the memorable part, and it is a calculation a ten-year-old can follow.

Ages 13+: Effective friction from glide distance

Derive the stopping distance for an object decelerating under constant friction, then invert it to get the coefficient from measured distance. Apply it to both conditions and report two coefficients with an uncertainty from your five trials. Then reason about where the remaining resistance comes from, and predict what a skirt would do before you build one.

The question to ask: “If we doubled the diameter of the disc without changing its weight, what would happen to the pressure we need?” (It would fall to a quarter, because area goes with the square of diameter while weight is unchanged. It tests whether they are thinking in pressure rather than in force, which is the whole conceptual step here.)

Why Teaching This Early Pays Off

Friction is usually taught as a thing that exists and slows objects down, which is true and unhelpful. Treating it as something you can engineer away, by separating the surfaces, puts a child in the position of a designer rather than an observer. That framing carries straight through to bearings, lubricants, air tables, maglev trains and the hard drive heads that once flew over spinning platters on exactly this principle.

The pressure calculation is the part I would keep if I could only keep one thing. Children meet pressure as a word long before they meet it as force divided by area, and a result as counterintuitive as “thirteen pascals lifts a CD” is a far better introduction than a definition. The National Research Council’s framework for school science puts using mathematics and computational thinking among the eight practices precisely because numbers like that one do work no explanation can (NRC, 2012).

Be realistic about the scale of the effect. One build does not change a child’s relationship with physics. What it can do is leave them with a measured number, a before-and-after comparison and a mechanism they can explain, which is three more things than most ten-minute projects deliver. If it lands, our friction ramp measurements and our guide to pneumatics and air pressure projects are the natural next steps.

Before You Start: What Parents Should Know

Best age	6 to 12. A six-year-old can do everything except the gluing, and will enjoy the launching most
Supervision	An adult operates the hot glue gun and handles the disc until the glue has cured. Keep the launch path clear
Real hazards	Hot-glue burns, which are the real risk here and are worse than they look. Balloon fragments, which are a choking hazard for the under-threes. Sharp edges if a CD is cracked
Don’t	Don’t use a cracked or chipped CD. Cracked polycarbonate throws small sharp pieces, and a disc that is already damaged can break apart when it hits something at the end of a long glide

Hot glue is around 180 °C and sticks to skin while it cools, which is why it burns more than its temperature suggests. The adult does that step, alone, with the child watching from the other side of the table. After that the whole project is harmless. Put a cushion or a box at the far end of your test run so the disc stops against something soft, and keep balloons and their fragments away from small children, counting them as you go.

When It Doesn’t Work

No hover at all, just a wheeze. The seal has a gap. Go round the glue line with soapy water and look for bubbles while the balloon is inflated. Re-glue the whole circumference rather than patching one spot, because a patched bead usually leaks somewhere else too.

It hovers for two seconds and dies. Either the balloon was barely inflated or your gap around the rim is large because the CD is warped. Hold the disc up to a light against a flat edge; a visible bow means the cushion is escaping on one side. Use a flatter disc.

It hovers but will not glide far. Dust. A wiped table and a dusty table give completely different results, because grit bridges the air film and makes contact. Wipe with a damp cloth and let it dry, every few runs, not just at the start.

It works on one table and not another. Surface texture. Polished tile, glass and laminate are excellent; varnished wood with visible grain is mediocre; anything with a woven texture is hopeless. Pick one surface and do all your comparisons on it.

The balloon pops off the cap mid-run. The cap is too smooth or too small for the balloon neck. Wrap two turns of tape around the cap to thicken it, or use a cap with a lip. A balloon that leaves the disc at speed is the one part of this that can hit someone.

Take It Further

Measure the effective coefficient of friction. Use the same launcher for the bare disc and the hovercraft, five runs each, and compute the ratio of median distances. Then convert it into two coefficients using the stopping-distance relation. Reporting “friction fell by a factor of 12, plus or minus 3” is a real result.

Build a skirt. Tape a 2 cm band of thin plastic, cut from a freezer bag, around the rim so it hangs just short of the table. This narrows the escape gap. Measure hover time before and after at the same inflation. If your skirt is well made the hover should last noticeably longer, which is exactly why real hovercraft have them.

Load it until it fails. Add coins two at a time, recording hover duration and glide distance each time, until it stops hovering. The failure point tells you the maximum weight your cushion can support, and dividing that by the disc area gives you the maximum pressure your balloon and gap can sustain. That is a performance envelope, which is how engineers describe machines.

What to Watch For Over the Next 3 Months

• Week 4: Your kid points out something that works by floating on air, usually an air-hockey table, and explains it correctly without being asked.
• Month 2: The word “pressure” starts appearing with “area” attached to it rather than on its own.
• Month 3: They propose a thrust version, with a second balloon pointing sideways. Say yes. It is the right next question and it connects this build to the balloon rocket in one step.

Frequently Asked Questions

Why does so little pressure lift the disc?

Because pressure acts over the whole underside. The CD weighs about 0.15 newtons and has an area of about 0.011 square metres, so you only need about 13 pascals, which is around one ten-thousandth of atmospheric pressure. Spreading a small pressure over a large area produces a usable force.

Does the hovercraft push itself along?

No. The air leaves symmetrically in all directions around the rim, so there is no net sideways force. It only coasts on the push you give it. To get thrust you would have to send the air out of one side, which is what a balloon rocket does.

What is the best surface to use?

Polished tile, glass or laminate, wiped clean and dry. Varnished wood works less well, and carpet or any woven surface does not work at all. Dust matters more than people expect, so wipe between runs.

How long should it hover?

Twenty seconds with a small balloon up to about a minute with a large one, assuming a good seal. Much less than that means either a leak at the glue line or a warped disc letting air escape on one side.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. Nave, R. “Friction.” HyperPhysics, Georgia State University. http://hyperphysics.phy-astr.gsu.edu/hbase/frict.html
2. NASA Glenn Research Center. “Gas Pressure.” Beginner’s Guide to Aeronautics. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/gas-pressure/
3. NASA Glenn Research Center. “Newton’s Laws of Motion.” Beginner’s Guide to Aeronautics. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/newtons-laws-of-motion/
4. The Physics Classroom. “Types of Forces.” https://www.physicsclassroom.com/class/newtlaws/Lesson-2/Types-of-Forces
5. The Physics Classroom. “Newton’s First Law.” https://www.physicsclassroom.com/class/newtlaws/Lesson-1/Newton-s-First-Law
6. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. https://doi.org/10.17226/13165