Catapult Experiment for Kids: Find the Best Launch Angle

Every physics textbook says a projectile goes furthest when launched at 45 degrees. Build a catapult, measure it properly, and you will find the peak somewhere around 30 to 40 degrees instead. The textbook is not lying. It is answering a question with two assumptions baked in, and your marshmallow violates both.

Finding that out is a better afternoon than winning a distance contest. This catapult experiment for kids is built around measuring angle against range, which means the build has to let you change the angle on purpose.

Read the safety section before you build. A catapult throws things at eye height.

Key Takeaways

• You will build a shoebox catapult whose launch angle can be set, then measure range at five different angles.
• The concept is projectile motion, and specifically that the ideal 45-degree answer assumes no air and a flat launch.
• Best for ages 8 to 14, with an adult present for every shot.
• The build detail that makes or breaks it: the base must be heavy, or it jumps and your numbers are noise.
• This is outdoors only, with eye protection, soft ammunition, and a named firing direction. No exceptions.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Shoebox or similar stiff box	1	Shoe cupboard	A cereal box stuffed with newspaper
Wooden paint stirrer or 30 cm ruler	1	Hardware shop, usually free	A wooden spoon handle, or a plywood strip
Pencils, round, unsharpened	3	Desk	Bamboo chopsticks
Rubber bands	6	Stationery drawer	Strips cut from an old bicycle inner tube
Plastic bottle cap	1	Recycling	A plastic spoon bowl, taped on
Strong tape	1 roll	Toolbox	Duct or electrical tape; sticky tape is too weak
Protractor	1	Pencil case	A phone angle app, often more accurate
Tape measure, 5 m or more	1	Toolbox	String pre-marked at 50 cm intervals
Safety glasses	1 pair per person	Hardware shop, cheap	None. No substitute
Mini marshmallows	1 bag	Supermarket	Pom-poms, or paper screwed into balls
Chalk or masking tape	1	Stationery drawer	Two pegs and a line of string
Books for ballast	4	Shelf	Bags of rice, bottles of water
Notebook	1	Anywhere	Required. The data is the experiment

Allergen note: marshmallows contain gelatin and may be made in facilities handling wheat or nuts. Nothing here gets eaten, but check if that matters at home.

Time: prep 10 min, build 30 min, experiment 45 min, cleanup 10 min Cost: a bag of marshmallows and a pair of safety glasses; the rest is scrap Mess level: do this outside

Do the Experiment

Step 1 — Build the pivot

Turn the box upside down. Push a pencil through both long sides, 5 cm from one end, so it spins freely. Tape the paint stirrer to the middle of that pencil at a right angle, so it stands up and swings forward and back. Tape the bottle cap to the free end, open side up.

Step 2 — Add the power and the stop

Hook two rubber bands from near the cap end of the arm to a paper clip pushed through the front face of the box, so the bands pull the arm forward and up. Then punch a vertical row of holes, 1 cm apart, through both sides of the box in front of the pivot, and push a second pencil through one pair. That pencil is the stop the arm slams into. Moving it up or down changes where the arm halts, which changes the launch angle. Wrap three layers of tape around it so it does not bounce.

Step 3 — Weight the base and set the firing line

Put four books on top of the box, or inside it if your box is the right way up. A light catapult recoils and the whole machine hops, which ruins repeatability more than anything else you will do. Then chalk a firing line on the ground at the front of the box and chalk a second line three metres behind it. Everyone who is not loading stands behind the back line.

Step 4 — Set the first angle and check it

Put the stop pencil in the lowest hole. Pull the arm back, let it swing forward until it hits the stop, and hold it there. Measure the angle the arm makes with the ground at that point with a protractor or a phone app. That is your launch angle, and it is a number you record, not an estimate.

Step 5 — Fire five, measure five

Safety glasses on. Face the catapult down the garden at a fence or a hedge, never at a window, a person or an animal. Load one marshmallow, pull the arm all the way back to the same marked position each time, release. Measure from the firing line to where each marshmallow first lands, not where it rolls to. Record all five numbers.

Step 6 — Repeat at five angles, then check for drift

Move the stop and repeat for roughly 20, 30, 45, 60 and 70 degrees, five shots each, measuring the actual angle every time. Then go back and fire five more at your first angle. If those shots now fall short, your rubber bands have fatigued during the session and you need to account for it.

What you should see: scatter, first of all. Five shots at the same angle spread over 20 to 30 percent of the range, and that is normal, not a mistake. Through the scatter, range rises from 20 degrees, peaks somewhere between 30 and 40, and falls again by 70. The headline result: your 30-degree shots beat your 60-degree shots, even though simple theory says they should match exactly. Typical ranges are one to three metres.

What’s Actually Going On

Once the marshmallow leaves the cup, nothing is pushing it. That sentence is worth saying out loud, because the belief that a moving object carries a force along with it is one of the most durable errors in mechanics (Hestenes, Wells & Swackhamer, 1992). In flight there is gravity pulling down and air pushing back. That is all.

Projectile motion splits cleanly into two independent problems. Horizontally, with no air, the speed never changes. Vertically, gravity removes about 9.8 m/s of upward speed every second (The Physics Classroom on projectiles). The launch angle decides how the launch speed is shared between the two. Shallow angles give lots of horizontal speed but little flight time; steep angles give a long flight but little forward speed. Range is the product of the two, which is why a best angle exists in the middle at all.

Work that through and, for launch and landing at the same height with no air resistance, the range comes out proportional to the sine of twice the launch angle, which peaks at 45 degrees. It also means 30 and 60 degrees should give identical ranges, because sine of 60 equals sine of 120 (The Physics Classroom on trajectory characteristics).

Your catapult breaks both assumptions. Drag grows with the square of speed and depends on frontal area and drag coefficient divided by mass (NASA Glenn on the drag equation). A marshmallow is almost the worst case: wide and nearly weightless, so its drag-to-weight ratio is enormous. Drag opposes motion for the whole flight, and a high-angle shot spends much longer in the air, so it loses far more speed. That pushes the best angle down and breaks the 30-to-60 symmetry in a specific direction: the shallower of any matched pair wins.

The second broken assumption is launch height. Your cap sits perhaps 25 cm up and the marshmallow lands at ground level. A launch above the landing plane always lowers the optimum angle, because extra flight time is partly free and forward speed becomes relatively more valuable.

So 45 degrees is the right answer to a cleaner question than the one you asked, and the gap is the physics. Launch a steel ball bearing instead and the peak moves back toward 45, because the drag-to-weight ratio collapses. Don’t, for reasons in the safety table.

How to Explain It to Your Kid

Ages 5–8: Flat and fast, or high and slow

Have them throw a ball three ways: almost flat along the ground, straight up, and halfway between. Ask which one went furthest. Flat ones hit the ground too soon; high ones come straight back down; the in-between one wins. That is the whole idea without a single number, and they discovered it with their arm.

Ages 9–12: The angle splits the speed in two

Draw an arrow at 45 degrees, then its sideways part and its upward part. Say the catapult gives a fixed total speed and the angle decides the split. Then make the prediction concrete: ask whether 30 degrees and 60 degrees will match. Textbook physics says yes. Get the prediction in the notebook before firing, because that disagreement is what makes the measurement worth doing.

Ages 13+: Where the symmetry comes from and why it breaks

Derive the range as launch speed squared times the sine of twice the angle, over g, and show why that is symmetric about 45. Then introduce drag as a force proportional to speed squared acting along the whole path, and reason out which of 30 and 60 suffers more. Finally compare the drag-to-weight ratio of a marshmallow and a stone and predict how the optimum shifts. Then check it against your own data, which is the part most courses never reach.

The question to ask: “If we fired this on the Moon, what angle would win?” (45 degrees, because there is no air. It tests whether they have separated the ideal result from the correction, which is the actual skill here.)

Why Teaching This Early Pays Off

The lesson is not that 45 degrees is wrong. It is that a clean model and a messy measurement can both be right, and the interesting work lives in the gap between them. That is roughly what engineering is. A kid who measured a 34-degree peak and can explain why it is not 45 has done something different in kind from a kid who memorised the number.

It attacks a specific misconception too. The Force Concept Inventory, given to tens of thousands of students, finds many arrive believing a projectile needs a continuing force in the direction of travel (Hestenes, Wells & Swackhamer, 1992). Watching a marshmallow leave the cup and arc with nothing touching it is the cleanest counterexample there is, especially if you say that sentence out loud while it is in the air.

The National Research Council’s framework names analysing and interpreting data among the practices school science should build (NRC, 2012). Five shots at five angles with a recorded spread is a genuine dataset with genuine uncertainty, and that matters more than the peak itself. Nobody should claim one afternoon shifts a trajectory. If this clicks, read our guides to catapults and medieval engineering and to the simple machines already in your house.

Before You Start: What Parents Should Know

Best age	8 to 14. Younger kids can load and measure, but an adult operates the arm
Supervision	An adult present for every shot, without exception. One person fires, everyone else stands behind the back line
Real hazards	Eye injury. A catapult launches at eye height, and a projectile in the eye is a permanent injury, not a bruise. Also rubber bands snapping back, and pencil ends
Don’t	Don’t ever load anything hard. No stones, marbles, coins, dried beans, batteries, pencils, screws or bottle caps as ammunition. Soft only: mini marshmallows, pom-poms or crumpled paper. Never aim at a person, a pet, a window or a road, and never fire indoors

Four rules, stated as rules rather than suggestions. Outdoors only, with a clear 10-metre run. Safety glasses on every person present, the kind sold for building work, which cost little and meet a real impact standard; OSHA’s eye and face protection guidance explains why that standard exists. One named firing direction, chosen before the first shot, pointing at a fence or hedge. Everyone not loading stands behind a chalked line three metres back, never beside the arm, because the arm swings in an arc and a hand in that arc gets hit.

No garden? Take it to a park and set up facing away from paths. Not a balcony. Not a shared corridor. Not an internal courtyard with windows.

When It Doesn’t Work

The whole catapult jumps when it fires. Not enough ballast. Every shot pushes the box backwards, and if the box moves, your launch speed and angle both change mid-shot. Pile more books on and check it does not slide on wet grass.

Ranges fall steadily through the session. Rubber band fatigue, and it is real. Latex weakens with repeated stretching and also with temperature. This is why Step 6 has you re-fire the first angle at the end. If the drift is large, change all the bands and start again, or randomise the order you test angles in.

Shots scatter far more than 30 percent. Usually the pull-back is inconsistent. Mark the full-back position on the box with tape and pull to that mark every single time. Second most common cause is the marshmallow sticking in the cap, which happens on humid days.

The arm bounces off the stop and the shot goes nowhere. An unpadded stop rebounds, and the arm catches the projectile on the way back. Add more tape to the stop, or fit a folded cloth pad.

All your angles give nearly the same range. Your spread of angles is too narrow. Check with the protractor rather than trusting the hole spacing: a 1 cm move of the stop may change the angle by only a couple of degrees. Spread the stop positions wider.

Take It Further With This Catapult Experiment for Kids

Test the symmetry prediction directly. Fire ten shots at 30 degrees and ten at 60. Write down the prediction from theory before you start, which is that they should match. Then compare the two sets of numbers. The size of the gap is a measure of how much the air is doing, and it is the single best result this build can produce.

Change the projectile, keep the angle. Fix the angle at your measured peak and fire a single marshmallow, then two taped together, then a pom-pom of the same size. Record ranges. You are varying the drag-to-weight ratio while holding the launch energy roughly constant, which is the cleanest way to show that the 45-degree answer depends on the thing you throw.

Double the power and check the squares. Fire with two rubber bands, then four, then six, at a fixed angle. Range scales roughly with the square of launch speed, so a quadrupled range means you doubled the speed. The number you get is testable against a slow-motion video.

What to Watch For Over the Next 3 Months

• Week 4: Your kid uses the phrase “in the air nothing is pushing it” about something unrelated, like a kicked football.
• Month 2: They notice a sport where the best angle is visibly below 45 degrees, which is most of them. A long baseball throw is released at well under 45, and so is a shot put, for the same reason.
• Month 3: They ask what happens with a heavier projectile. Right question, right answer: the peak moves back toward 45. Satisfy it with a slightly heavier soft object, never a hard one.

Frequently Asked Questions

Why is the best angle not 45 degrees?

Because 45 assumes no air resistance and a launch at the same height as the landing. A marshmallow has a huge drag-to-weight ratio, so a high, slow shot loses most of its speed to the air, and the launch cup sits above the ground. Both effects push the optimum lower, typically to 30 to 40 degrees.

What can we safely use as ammunition?

Mini marshmallows, pom-poms and crumpled paper balls. That is the complete list. Anything hard, dense or eye-sized is off limits, which rules out stones, marbles, coins, dried beans and bottle caps. The soft options also happen to make better physics, because the drag effect is bigger and easier to see.

Do we really need safety glasses for marshmallows?

Yes, for two reasons. A marshmallow at launch speed can abrade an open eye, and more importantly a rubber band or the arm itself can fail and come back at your face. The glasses also build the habit for the day the ammunition is less forgiving.

How many shots do we need for a real result?

Five per angle is the practical minimum, ten is better. Scatter on a homemade catapult is large, so one shot per angle gives a graph that looks convincing and means nothing. Report the median, not the best shot.

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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. The Physics Classroom. “What Is a Projectile?” https://www.physicsclassroom.com/class/vectors/Lesson-2/What-is-a-Projectile
2. The Physics Classroom. “Characteristics of a Projectile’s Trajectory.” https://www.physicsclassroom.com/class/vectors/Lesson-2/Characteristics-of-a-Projectile-s-Trajectory
3. NASA Glenn Research Center. “The Drag Equation.” Beginner’s Guide to Aeronautics. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/drag-equation/
4. NASA Glenn Research Center. “Factors That Affect Drag.” Beginner’s Guide to Aeronautics. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/factors-that-affect-drag/
5. Hestenes, D., Wells, M., & Swackhamer, G. (1992). “Force Concept Inventory.” The Physics Teacher, 30(3), 141–158. https://doi.org/10.1119/1.2343497
6. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. https://doi.org/10.17226/13165
7. Occupational Safety and Health Administration. “Eye and Face Protection.” U.S. Department of Labor. https://www.osha.gov/eye-face-protection