Table of Contents
Marble Run Physics Experiment: Energy Accounting
A marble run physics experiment that turns a toy into an energy ledger: measure the drop, the rebound, and the share a rolling marble never gives back.
Release a marble from 60 cm up a foam track and it will climb the far side to about 45 cm. Never 60. That missing 15 cm is the whole article. Most marble run builds are judged on whether the marble reaches the end; this marble run physics experiment is judged on whether you can say where the energy went. By the time you finish, your kid will be able to predict the exact height of the tallest hill their marble can clear, which is a genuinely useful piece of physics and also an excellent way to win an argument with a sibling.
Key Takeaways
- You’ll tape a foam track to a wall, measure release height against rebound height, and use the ratio as an energy balance.
- The concept is that energy is conserved but not all of it stays useful. Height becomes speed becomes height again, minus what friction and rotation take.
- The prediction to make: the highest hill your marble can clear is roughly its measured rebound height, not its release height.
- Works from age 5 for the racing version, and from about 9 for the measuring version.
- The usual trap: a joint in the track with a 2 mm step. It eats energy invisibly and ruins every comparison.
What You’ll Need for the Marble Run Physics Experiment
| Item | How much | Where to get it | If you don’t have it |
|---|---|---|---|
| Foam pipe insulation, 1 m, split lengthwise | 2 lengths | Hardware shop | A pool noodle cut in half the long way, or three cardboard tubes split and taped |
| Glass marbles, 16 mm | 3 identical | Toy shop | Ball bearings, or a large metal nut |
| Masking tape | One roll | Hardware drawer | Painter’s tape; avoid anything that marks walls |
| Tape measure or metre rule | 1 | Toolbox | A length of string marked in 10 cm steps |
| Sticky notes | A pad | Desk drawer | Scraps of paper and tape |
| A smooth wall or a door | 1 | Any room | The side of a wardrobe |
| Phone with slow-motion video | 1 | Count out loud; less precise but workable |
Time: prep 15 min, experiment 30 min, cleanup 5 min Cost: a couple of foam pipe lengths, or free with a pool noodle you own Mess level: low, but marbles escape, so clear the floor
Do the Experiment
Step 1 — Build a valley, not a ramp
Tape one split foam length to the wall sloping down from 70 cm to the floor, then tape the second one rising back up on the other side. You want a V with a smooth, continuous bottom. Marble runs that end at the floor measure nothing; a run that climbs back out measures everything.
Step 2 — Fix the joint properly
At the bottom, overlap the two track pieces by 3 cm and tape the seam so the marble never crosses a step or a gap. Run your fingertip along the inside. If you can feel a lip, the marble can too, and your numbers will drift.
Step 3 — Mark heights up both sides
Stick a sticky note every 10 cm up both slopes, measuring vertical height from the floor rather than distance along the track. The vertical height is what matters, and confusing the two is the most common mistake in this experiment.
Step 4 — Measure the rebound ratio
Release the marble from exactly 60 cm, with no push at all, just letting go. Note the highest sticky note the marble reaches on the far side. Five releases, then average. Divide rebound height by release height to get your ratio.
Step 5 — Predict and test the tallest hill
Build a hump in the middle of the valley from a rolled sticky-note wedge or a small box, with its top at exactly your measured rebound height. Predict out loud whether the marble clears it. Then try hills 5 cm higher and 5 cm lower.
Step 6 — Test whether steepness changes the speed
Rebuild so the marble falls the same 60 cm by two different routes: one short and steep, one long and gentle. Film the bottom section with slow motion and compare how long the marble takes to cross a 20 cm marked stretch. Same drop, different steepness.
What you should see: On clean foam, the marble returns to between 60% and 85% of its release height, so a 60 cm release gives a 36 to 51 cm rebound. The hill test works: the marble clears a hump a centimetre or two below its rebound height and stalls on one above it. The steepness test surprises nearly everyone. Both routes give almost the same speed at the bottom, within 5% or so, even though the steep route gets there much sooner. The gentle route usually comes out marginally slower, because the marble spends longer rolling and loses a little more to friction.
What the Marble Run Physics Experiment Reveals
| Measurement | Typical value | What it tells you |
|---|---|---|
| Rebound height from 60 cm | 36–51 cm | Fraction of energy still usable after one trip |
| Second rebound | 22–40 cm | Losses compound; each pass takes its cut |
| Tallest hill cleared | Within 2 cm of rebound height | Rebound height is the real budget |
| Bottom speed, steep vs gentle | Within 5% | Drop height sets speed, slope sets timing |
| Share of energy in spin | 28.6% for a solid marble | Energy you cannot get back as height |
What’s Actually Going On
At the top of the track the marble holds gravitational potential energy, which is mass times gravity times height. Nothing else. Let it go and that store converts into motion. At the bottom, the energy is all kinetic, and here is the part that most explanations skip: a rolling marble’s kinetic energy is split between two kinds of motion. It is travelling, and it is also spinning.
For a solid sphere rolling without slipping, that split is fixed and calculable. Rotational energy works out to 2/7 of the total kinetic energy, so 28.6% of the marble’s motion energy is in spin and 71.4% in travel. The practical consequence is a number you can use: a rolling marble arrives at the bottom with speed equal to the square root of 10gh/7, which is 0.845 times the speed a frictionless sliding object would reach. From 60 cm, a slider would hit 3.43 metres per second and your marble manages about 2.90. HyperPhysics sets out the rotational work-energy relationship that this comes from.
That spin energy is not lost. It converts back into height on the way up, which is why the rebound ratio is governed by friction rather than by rotation. The real losses are rolling resistance, where the foam deforms slightly under the marble and does not give all the energy back, sliding friction at the walls when the marble wobbles off-centre, air drag, and sound. Every one of those is small. Together they take 15% to 40% of the energy per pass, and because the loss is a percentage, successive rebounds shrink geometrically. The Physics Classroom’s treatment of mechanical energy is a good reference for the conservation side of this.
Now the misconception, which is worth taking seriously. Ask a child why the marble goes fast and most will say “because the track is steep”. Steepness feels like the cause because it is what they can change. The truth is that the speed at the bottom depends only on the vertical drop, while steepness controls how quickly the marble gets there and how long friction has to act. This is a textbook example of what Vosniadou and Brewer documented in children’s models of the physical world: a coherent, intuitive theory that fits plenty of everyday evidence and has to be revised rather than simply corrected. A child who has measured two routes with the same drop has done the revising themselves.
Worth being honest about a limit of the demonstration. The rebound ratio depends on your track material, so two families will get different numbers. That is fine. What transfers is not the number but the method: measure the energy you put in, measure what comes back, account for the difference.
How to Explain It to Your Kid
Ages 5–8: it never gets back up as high
Let them release the marble and point at where it reaches on the other side, then at where it started. Ask them whether it got back up to the same place. Do it three times so they see it is not an accident. Then give them the hill to try: a hill lower than the marble’s rebound gets crossed, a higher one doesn’t. That is a prediction a six-year-old can make and win on.
Ages 9–12: the rebound is the budget
Introduce the ratio as pocket money. If the marble starts with 60 units of height and comes back with 45, it kept three-quarters and spent a quarter. Then the key move: the marble can only buy a hill it can afford. Have them calculate the tallest affordable hill before building it. When they get it right to within a couple of centimetres, the idea of an energy budget has landed properly.
Ages 13+: quantify the spin and the losses
Have them compute the theoretical bottom speed two ways, sliding and rolling, using the square root of 2gh and the square root of 10gh/7, then measure the real speed from slow-motion video over a marked 20 cm. The measured value should sit near the rolling figure, below the sliding one. The remaining gap is friction, and they can express it as a percentage per metre of track. That is a real experimental result with real uncertainty.
The question to ask: “If we used a marble twice as heavy on exactly the same track, would it climb higher, lower, or the same on the far side? Why?”
Why Teaching This Early Pays Off
Energy accounting is the single most portable idea in physics, and the marble track is the cheapest honest version of it. The habit being built is specific: when a quantity appears to vanish, look for where it went rather than concluding it was destroyed. That habit later carries electrical efficiency, metabolic energy, heat loss from a house and every sensible conversation about batteries.
It also quietly fixes a confusion that bites later. Students routinely mix up height with distance along a slope in mechanics problems, losing marks for years. A child who has stuck sticky notes at vertical heights and discovered that track length does not set the speed has pre-empted it with a physical memory.
The structure matters more than the activity. Ask for the prediction before each release, which is the Predict-Observe-Explain pattern set out in White and Gunstone’s Probing Understanding, and keep the explanation coming from the child. The National Research Council’s framework treats energy as a crosscutting concept for exactly this reason. One afternoon on a foam track will not teach mechanics, but the phrase “where did the energy go” becomes available, and that is a question a child can keep asking for a decade.
Before You Start: What Parents Should Know
| Best age | 5–12. Racing and the hill test from 5. Ratios from about 9. The video analysis from about 13 |
| Supervision | Light. The main job is keeping track of marbles |
| Real hazards | Marbles are a choking hazard for under-3s, so keep them out of the room and count the marbles back into the bag at the end. Marbles on a hard floor are a slip risk. Glass marbles chip if dropped on tile |
| Don’t | Don’t substitute a bouncy ball for the marble. It stores energy in its squash and rebounds differently, which breaks the accounting this experiment depends on |
When It Doesn’t Work
- The rebound ratio changes every run. Check the joint at the bottom and the release technique. Any push at all adds energy you did not measure. Release by opening your fingers, not by letting the marble roll off a finger.
- The marble jumps the track on the inside of the curve. The valley is too tight. Widen the bottom curve so the radius is at least 15 cm, or the marble climbs the wall and loses energy sideways.
- The marble stops halfway up and rolls back repeatedly. That is the correct behaviour, and the oscillation is useful data. Measure the first three peaks and see the geometric decline.
- Rebound heights are wildly low, under 40%. Usually a sagging track. Tape the foam every 15 cm so it cannot flex. A flexing track absorbs energy exactly like a trampoline.
- The steepness test shows a big difference. Check that both routes really drop the same vertical height. Measuring along the track instead of vertically is the classic error here.
Take It Further
- Marble against ice cube. Release a marble and a small ice cube together on a steep smooth track. The ice cube slides without spinning, so none of its energy goes into rotation, and it should arrive faster. This is the 2/7 result made visible with items from the freezer.
- Mass sweep. Run marbles of three different masses, same track, same release height. The rebound ratio barely changes, because potential and kinetic energy both scale with mass. Measuring that non-effect is a real result and it surprises most kids.
- Loss per metre. Build tracks of 1 m, 2 m and 3 m with identical total drop, then plot rebound ratio against track length. The line should slope down, letting you quote a friction loss per metre for your own foam. Pair it with friction ramps for a second method, or with marble collisions to follow the energy into a second object.
What to Watch For Over the Next 3 Months
- Week 4: Predicting before releasing. The habit shows up as a declared guess rather than a question.
- Month 2: Transfer to the playground. A swing that never gets back to the height it was pushed from, a skateboard ramp, a slide. Comments about getting back up to the same place are the marker.
- Month 3: The question arriving unprompted about where energy went in a different system: a phone battery, a car braking, a bouncing ball that stops. That generalisation is what you were building. Pendulums are the natural next step, since a pendulum is the same energy trade with a rope instead of a track.
Frequently Asked Questions
Why doesn’t the marble come back to the same height?
Because some energy left the system on the trip. Rolling resistance in the foam, sliding friction when the marble wobbles, air drag and sound all take a share. Nothing is destroyed; it has turned into tiny amounts of heat and noise that you cannot recover as height.
Does a heavier marble go faster?
No, and this is worth testing rather than asserting. Both the potential energy at the top and the kinetic energy at the bottom are proportional to mass, so mass cancels out of the speed calculation. A heavier marble carries more energy and arrives at the same speed.
Why does the steep track feel so much faster?
Because it is faster in the sense of taking less time. Its speed at the bottom is nearly identical, since that depends on the vertical drop, but it covers the drop in less time and with a greater sense of urgency. Time and speed are different quantities, and this is a good place to separate them.
Can we do this with cardboard tubes instead of foam?
Yes. Split kitchen-roll and wrapping-paper tubes lengthwise and tape them end to end. Expect lower rebound ratios, around 50% to 70%, because cardboard is rougher and flexes more. Keep the material the same across every comparison and the conclusions hold.
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.
Sources
- Nave, R. Rotation and Work-Energy Principle. HyperPhysics, Georgia State University. http://hyperphysics.phy-astr.gsu.edu/hbase/rotwe.html
- The Physics Classroom. Mechanical Energy. https://www.physicsclassroom.com/class/energy/Lesson-1/Mechanical-Energy
- Vosniadou, S., & Brewer, W. F. (1992). “Mental models of the earth: A study of conceptual change in childhood.” Cognitive Psychology, 24(4), 535–585. https://doi.org/10.1016/0010-0285(92)90018-W
- National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. https://doi.org/10.17226/13165
- White, R., & Gunstone, R. (1992). Probing Understanding. Falmer Press.
- Alfieri, L., Brooks, P. J., Aldrich, N. J., & Tenenbaum, H. R. (2011). “Does discovery-based instruction enhance learning?” Journal of Educational Psychology, 103(1), 1–18. https://doi.org/10.1037/a0021017
