Zip Line Experiment for Kids: Angle, Speed, Safe Stop

Read this part first: nothing in this experiment carries a child. A zip line experiment for kids, done at the kitchen table, is a 3 metre string with a drinking-straw trolley and a 30 gram toy hanging off it. No child is ever tied to a line, and no child hangs from one. What you get instead of a thrill ride is something better: a measurement. Lower the far end of the line a few centimetres at a time and the trolley eventually stops partway across. The angle at which that happens is a direct measurement of friction, and the tangent of it gives you a real coefficient you can then use to predict the speed at every other angle.

Key Takeaways

• You’ll string a 3 m line between a door handle and a chair, send a straw trolley carrying a light toy down it, and time it over a marked section.
• This is a toy-carrying model, not a rideable zip line. The line stays low, the payload stays under 100 g, and nothing is attached to a person.
• Two measurements: average speed against drop height, and the minimum drop at which the trolley just completes the run.
• The quantitative payoff: at the stall point, tangent of the slope angle equals the coefficient of friction.
• Best for ages 8 to 14 with an adult setting up anchors. The usual snag is a sagging line, which makes the second half of the run uphill.

What You’ll Need for the Zip Line Experiment for Kids

Item	How much	Where to get it	If you don’t have it
Smooth nylon fishing line or kite string	4 m	Hardware shop	Dental floss works; wool does not, it grips
Drinking straw	1 (cut a 3 cm piece)	Kitchen	A short length of plastic tube, or a bead with a wide hole
Paper clips	3	Desk drawer	Bent wire
Payload: small plastic toy or bag of coins, 20–50 g	1	Toy box	A film canister with coins in it, weighed first
Rolled sock or foam offcut	1	Laundry	A folded tea towel, taped in place
Masking tape	One roll	Hardware drawer	Any tape
Tape measure	1	Toolbox	String marked in 10 cm steps
Protractor, printed	1	Print or trace	Work the angle out from the drop and the length instead
Phone with slow-motion video	1	Pocket	A stopwatch, with a longer measured section
Kitchen scale	1	Kitchen	Skip it and use coins of known mass

Time: prep 15 min, experiment 30 min, cleanup 5 min Cost: a reel of line, if you don’t already own one Mess level: low

Do the Experiment

Step 1 — Choose anchors an adult has checked

Upper anchor: a door handle or the top rail of a sturdy chair, at about 140 cm. Lower anchor: a table leg or a second chair, at 30 cm above the floor. Both must be too heavy to pull over. Run the line along a wall, never across a doorway or a route anyone walks.

Step 2 — Thread the trolley before you tie the second knot

Slide the 3 cm straw onto the line first. Tie off the lower end, pull the line properly taut, and hang the payload from the straw with a paper clip. If you forget the straw, you will be untying knots, which is how most families start this experiment.

Step 3 — Tape the bumper at the bottom

Tape the rolled sock to the lower anchor so the trolley hits something soft and stops. A zip line with no stop at the end throws its payload across the floor, and on a real one the stopping arrangement is the part engineers worry about most.

Step 4 — Mark a measurement zone

Put tape markers on the line 50 cm after the start and 250 cm after the start, giving a 2 m timed section that excludes the launch and the stop. Measure the vertical drop between the two anchors with the tape measure and write it down.

Step 5 — Run the drop-height sweep

With the upper anchor fixed, set the lower anchor to give drops of 70, 50, 30 and 20 cm, keeping the line length the same. Three runs at each drop. Time the trolley across the 2 m zone, then divide to get average speed in metres per second.

Step 6 — Find the stall point and compute friction

Keep lowering the drop in 2 cm steps until the trolley no longer reaches the bumper. Record the largest drop at which it stalls. The slope is that drop divided by the horizontal distance, and that ratio, the tangent of the angle, is your coefficient of friction.

What you should see: With a 50 cm drop over 3 m, average speed usually comes out between 1 and 2 metres per second, well below the 3.1 m/s that a frictionless slide from 50 cm would reach. As you reduce the drop, speed falls much faster than the drop does, and then the trolley stops completing the run altogether. For a straw on nylon line, the stall usually appears somewhere between a 25 and a 50 cm drop over 3 m, which puts the coefficient of friction between about 0.08 and 0.17.

What the Zip Line Experiment for Kids Measures

Drop over 3 m	Slope angle	Typical average speed	What’s happening
70 cm	13.5°	1.6–2.4 m/s	Comfortably above the friction threshold
50 cm	9.6°	1.0–1.8 m/s	Running, but friction takes a large share
30 cm	5.7°	0.3–0.9 m/s, often stalls	Close to the threshold, very inconsistent
20 cm	3.8°	Stalls	Friction force exceeds the driving force

The jump from “slow” to “doesn’t arrive” happens over just a few centimetres of drop, and that sharpness is the most interesting thing in the table.

What’s Actually Going On

Start with the energy ledger. The payload begins with gravitational potential energy equal to mass times gravity times the drop. By the bottom, some of that has become kinetic energy and the rest has been spent pushing the straw along the line against friction.

Friction’s work is the friction force multiplied by the distance travelled. On a shallow slope, the friction force is the coefficient of friction times the weight pressing the straw against the line, which is almost exactly the payload’s weight. So the energy spent on friction is roughly the coefficient times the weight times the horizontal distance. That gives a clean equation: final speed squared equals 2g times the quantity (drop minus coefficient times horizontal distance).

Now look at what that equation says. The bracket goes to zero when the drop equals the coefficient times the horizontal distance, in other words when the drop divided by the run equals the coefficient of friction. Drop divided by run is the tangent of the slope angle. So the angle at which the trolley just barely fails to arrive gives you the coefficient of friction directly, with no force meter required. HyperPhysics’ treatment of friction sets out the underlying force relationship, and the energy half of the argument follows the standard mechanical energy picture.

That same equation explains the sharp cliff in your table. Just above the threshold, the bracket is a tiny difference between two larger numbers, so a 2 cm change in drop can halve the energy available. Engineers call this operating close to a limit, and it is why real systems are specified with margin rather than at the edge of working.

Two honest corrections. First, a real zip line’s slope is not a straight line, because any cable sags into a curve, which means the first part is steeper than average and the last part is shallower or even uphill. That is why a sagging string stalls the trolley near the end and a taut one does not, and it is why real zip lines need careful tensioning. Second, speed at the bottom is set by the drop, not by the length of the line. A longer, shallower line gives a slower arrival because friction has acted over a greater distance, which is the opposite of what most kids predict.

One thing this model does not capture: real zip lines carrying people also deal with air drag, which rises with the square of speed, and with the rider’s position. At a toy’s speed, drag is negligible, so this experiment is a clean friction measurement rather than an aerodynamic one.

How to Explain It to Your Kid

Ages 5–8: too flat and it gets stuck

Let them send the toy down a steep line, then flatten it until it stops halfway. Ask them to find the flattest line that still works, by moving the chair themselves. They are performing a threshold experiment without the vocabulary, and the answer they find is a real measurement.

Ages 9–12: drop divided by run

Measure the drop and the horizontal distance and write the fraction. At the stall point that fraction is the friction number for straw on string. Then have them predict whether a heavier payload stalls at a different angle. It doesn’t, because both the driving force and the friction force scale with weight, and discovering that is a genuinely surprising result at this age.

Ages 13+: solve it, then test the solution

Have them write final speed squared equals 2g times the drop minus the coefficient times the run, plug in the coefficient they measured at the stall, and predict the arrival speed for a drop they have not tried. Then measure it with slow-motion video. Agreement within 20% is good going for a straw and a piece of string, and the discrepancy is worth discussing: the measured average over 2 m is not the final speed, and the line sags.

The question to ask: “If we doubled the weight of the toy, would it arrive faster, slower, or the same? And would the flattest line that still works change?”

Why Teaching This Early Pays Off

Friction is the most under-taught force in primary science. It usually appears as a word meaning “things slow down”, with no number attached. Here it gets a number, measured by the child, obtained from nothing more than a ratio of two lengths. That is a real physics measurement technique, and the same stall-angle method is how coefficients of friction are measured in a laboratory.

The second payoff is the idea of a threshold. Many systems do nothing at all until a condition is met, then behave normally afterwards, and the transition can be alarmingly sharp. Thresholds turn up in electronics, in biology, in materials that suddenly fracture, and in anything with a tipping point. Meeting one in a form you can walk over and adjust by hand is a good first encounter.

Keep the explaining with the child, not with you. Chi and Wylie’s ICAP framework places constructive and interactive engagement, where the learner generates explanations, above merely doing the activity. The National Research Council’s framework similarly treats planning and carrying out investigations as the practice to train, and sweeping one variable while holding the rest fixed is precisely that. For a second friction measurement with a different method, friction ramps tackle the same coefficient from another angle.

Before You Start: What Parents Should Know

Best age	8–14. Younger kids can do the find-the-flattest-slope version. An adult chooses and checks both anchors every time
Supervision	Adult sets up, adult takes it down. The line comes down the moment the experiment ends, with no exceptions
Real hazards	A line strung across a room is a trip hazard, and a cord at neck height is a strangulation hazard for small children. Keep the line below adult chest height, along a wall, never across a doorway or a walking route, and never where a toddler or a pet can reach it. Keep the payload under 100 g and use nothing hard, heavy or glass
Don’t	This carries a toy and only a toy. Never tie a line to a child, never let a child hang from it, and never build a rideable version from household rope or string. Don’t anchor to a TV, a curtain rail, a bookshelf or anything that can tip. Don’t use elastic or bungee cord, which stores energy and can snap back

When It Doesn’t Work

• The trolley stops two-thirds of the way along. The line is sagging, so the last section is flatter than the average or even uphill. Pull it tighter and re-tie, or accept the sag and measure the local slope where it stops.
• Speeds vary wildly between runs. The payload is swinging, which means the trolley twists and binds. Shorten the paper-clip hanger so the mass sits close under the straw, and release without a push.
• The straw binds immediately. The line is too thick for the straw or there’s a knot in the running section. Use thinner line, and keep all knots outside the measured zone.
• The trolley flies off the end. Your bumper is too far past the lower anchor, or it isn’t taped on. Fix the bumper directly at the anchor, and keep the lower anchor low so a dropped payload falls only a short distance.
• Timing is impossible by hand. At these speeds a 2 m zone passes in about a second. Film at the highest frame rate your phone offers and count frames, or lengthen the measured zone.

Take It Further

1. Mass sweep. Run 20 g, 50 g and 100 g payloads at a fixed drop. Speeds should barely change, and the stall angle should not move at all. That is a direct test of the fact that friction and weight scale together, and it contradicts almost every child’s first prediction.
2. Line material comparison. Repeat the stall-angle measurement with fishing line, cotton string and dental floss. Each gives a different coefficient, so you end up with a small table of measured friction numbers, which is what a materials engineer would produce.
3. Trolley design. Swap the straw for a bead, a paper clip hook, or a short piece of plastic tube, and measure the coefficient of each. Rolling trolleys beat sliding ones, which is why real zip lines use pulleys. Pair it with marble runs as energy accounting to compare rolling against sliding losses.

What to Watch For Over the Next 3 Months

• Week 4: Naming friction as a quantity rather than an effect. Comments like “that one’s grippier” with a reason attached.
• Month 2: Noticing slope thresholds in the world. A ball that won’t roll on a gentle lawn, a suitcase that needs a push on a shallow ramp, a slide that’s too flat to slide on.
• Month 3: Asking about margin. Why engineers build things to work comfortably rather than just barely. If your child starts saying something is “too close to not working”, that’s the threshold idea generalising.

Frequently Asked Questions

Can we make a version my child can ride?

No, and not with household materials. A rideable zip line involves loads of several times a child’s body weight, anchor forces that can pull a tree branch down, a harness, and a stopping system that fails safely. Line, trees and hardware for rideable installations are engineered and inspected, and improvising one is how serious injuries happen. Keep this one carrying a toy.

Why does my trolley stop even though the line clearly slopes down?

Because the slope has to beat friction, not merely exist. If the drop divided by the horizontal run is less than the coefficient of friction, the trolley cannot accelerate at all. That threshold is the whole point of the experiment, and finding it is the measurement.

Does a heavier toy go faster?

Essentially no. The force pulling it down the slope and the friction force both grow in proportion to weight, so they cancel out of the speed calculation. It is worth testing, because nearly every child predicts otherwise.

Why is our measured speed so much lower than the calculation?

Two reasons, both legitimate. The frictionless calculation ignores the friction you are measuring, and your measured number is an average over 2 m rather than the final speed. Expect the average to be roughly half to two-thirds of the final speed on a short run.

---

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

1. Nave, R. Friction. HyperPhysics, Georgia State University. http://hyperphysics.phy-astr.gsu.edu/hbase/frict.html
2. The Physics Classroom. Mechanical Energy. https://www.physicsclassroom.com/class/energy/Lesson-1/Mechanical-Energy
3. Chi, M. T. H., & Wylie, R. (2014). “The ICAP Framework: Linking Cognitive Engagement to Active Learning Outcomes.” Educational Psychologist, 49(4), 219–243. https://doi.org/10.1080/00461520.2014.965823
4. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. https://doi.org/10.17226/13165
5. Smith, J. P., diSessa, A. A., & Roschelle, J. (1994). “Misconceptions Reconceived: A Constructivist Analysis of Knowledge in Transition.” Journal of the Learning Sciences, 3(2), 115–163. https://doi.org/10.1207/s15327809jls0302_1
6. White, R., & Gunstone, R. (1992). Probing Understanding. Falmer Press.