How to Build a Catapult: Medieval Engineering as a STEM Project

Medieval siege engineers didn’t have computers or physics textbooks. They had observation, iteration, and a detailed understanding of torque, leverage, and projectile motion developed through building and testing. The trebuchet — the most powerful of medieval siege weapons — represents the peak of that empirical engineering tradition. It uses a falling counterweight to swing an arm, which releases a sling at precisely the right moment to launch a projectile in a parabolic arc. The mathematical physics behind it wasn’t formally understood until Newton. But the engineers who built them had figured it out through testing.

Building a catapult project at home puts kids in exactly that same situation: a design problem with physics constraints, materials limitations, and measurable performance. No worksheet gives you that feedback loop.

Key Takeaways

• Building a popsicle-stick catapult teaches potential and kinetic energy, torque, projectile motion, and iterative engineering design in one afternoon
• The trebuchet’s physics involves the same mechanical principles as cranes, counterweight elevators, and pendulum clocks
• A competition format (maximum range, minimum materials, accuracy target) makes the engineering design process concrete and motivating
• Materials for a basic catapult: popsicle sticks, rubber bands, a plastic spoon, and a base — total cost under $2
• Ages 8+ can build and launch independently; ages 12+ can calculate expected range from projectile motion equations

The Physics of a Catapult

There are two main types relevant for home projects:

The popsicle-stick torsion catapult uses twisted rubber bands or string to store potential energy. When released, the arm rotates rapidly. At the right moment, a cup or spoon at the end of the arm releases the projectile.

The trebuchet uses gravitational potential energy. A heavy counterweight on the short end of an arm falls; the long end rises rapidly, swinging a sling that holds the projectile. At the optimal release point (not at the top of the arc — slightly before, to optimize the launch angle), the projectile is released.

In both cases, the physics progression is:

1. Stored potential energy (twisted elastic or elevated counterweight)
2. → Kinetic energy during the arm swing (rotational KE = ½Iω²)
3. → Projectile kinetic energy at release (translational KE = ½mv²)
4. → Gravitational potential energy + kinetic energy during flight (parabolic arc)

The peak range of a projectile occurs at a 45-degree launch angle (in the absence of air resistance). Real catapults target angles slightly less than 45 degrees because air resistance reduces the optimal angle for heavy, slow projectiles.

What You Need: Two Builds

Build 1: The Popsicle Stick Catapult (Ages 8+, Under $2)

Material	Quantity	Cost
Popsicle/craft sticks	~15	$1–2 for a bag of 100
Rubber bands	5–10	Already have them
Plastic spoon	1	From kitchen
Hot glue or wood glue	Small amount	Already have it
Marshmallows or rolled paper balls	5–10	Free

Build: Stack 7 popsicle sticks and bind them tightly with rubber bands at each end (this is the base and adds mass). Take two separate sticks and cross them in an X shape at about ⅓ from one end; bind the cross point tightly with a rubber band. Glue the X structure upright on the flat stack (the cross point sticks up). Glue a plastic spoon to the top of the long arm of the X. Twist a rubber band around the junction between the spoon arm and the base stick to create the torsion spring. Pull the spoon down, place a marshmallow in it, and release.

This build takes 15–20 minutes and launches marshmallows 2–5 feet depending on rubber band tension.

Build 2: The Tabletop Trebuchet (Ages 10+, Under $10)

Material	Quantity	Cost
Wooden dowels (¼ inch)	2 feet	$2
Cardboard (thick, corrugated)	One large sheet	Free
String	2 feet	Free
Small plastic bag + pennies (counterweight)	1 bag, 20+ pennies	Free
Tape or hot glue	As needed	Free

Build: Cut two triangular A-frame uprights from cardboard, about 8 inches tall. Connect them with a crossbar (dowel) at the top. The arm (another dowel, 12 inches) pivots on the crossbar. The short end of the arm (3 inches from pivot) carries the counterweight bag. The long end (9 inches) carries a sling string (6–8 inches of string with a small pouch at the end). Attach the arm to the crossbar pivot so it can rotate freely. Load the counterweight with pennies. Hold the long end down, place a small projectile (crumpled foil ball, marble) in the sling, and release.

The Engineering Design Challenge

The real learning isn’t in building the first version — it’s in the iteration that follows. Set constraints and measure results:

Constraint 1: Maximum range. Record where the projectile lands 5 consecutive launches. Average the distance. Now change one variable: move the pivot point (change the arm ratio), add more counterweight, change the sling length. Measure again. Repeat.

Constraint 2: Accuracy. Place a target 3 feet away. How many launches out of 10 hit it? This introduces accuracy vs. range as a real engineering tradeoff.

Constraint 3: Minimum materials competition. If multiple kids are participating: each gets the same set of materials (10 popsicle sticks, 5 rubber bands, 1 spoon). Who can build the longest-range launcher? Who builds the most accurate?

Constraint 4: Energy budget. Calculate how much energy is stored: for the trebuchet, stored energy = mgh (mass of counterweight × gravity × drop height). Compare this to the projectile’s kinetic energy at launch (½mv²). What’s the efficiency? Typically 20–40% for a simple trebuchet — the rest goes to arm mass, friction, and sling losses.

How to Teach Your Kid About Catapult Physics

Ages 8–10: Energy Storage and Release

Before building, play an energy metaphor game. Stretch a rubber band: “Where did the energy go?” (Into the elastic.) Release it: “Where did the energy go?” (Into the flying rubber band.) Pull a swing back to horizontal and let go: same concept, different form. Now explain: the catapult is just a fancy way to store energy and release it quickly to a projectile. Build the popsicle stick version together. Pull back the arm and ask: “Where is the energy right now?” (In the twisted rubber band.) Release: “Where did it go?” (First into the arm swinging, then into the projectile flying, then into… stopping.)

Ages 11–12: Torque and Lever Arms

Introduce the lever arm concept. Have your kid put a ruler on an eraser to make a see-saw. Put a heavy book on one end and try to lift it with one finger on the other end. Now move the eraser (fulcrum) closer to the book. Easier to lift. The mechanical advantage = long arm length / short arm length. The trebuchet arm uses the same principle: a 3:1 arm ratio (9 inch long arm, 3 inch short arm) gives a mechanical advantage of 3. Ask: if you triple the arm ratio while keeping the counterweight the same, what happens to the launch speed? (It increases — torque is the same, but it acts over a longer arm, transferring more velocity to the projectile.)

Ages 13+: Calculate Expected Range

At this level, put the math into practice. Measure the launch speed by timing how long the projectile takes to fall from its peak height to the ground (measure peak height by observation). Calculate initial velocity using kinematics. Then calculate expected range using projectile motion equations: R = v²sin(2θ)/g, where θ is the launch angle. Compare to measured range. Discuss why the equation doesn’t match perfectly (air resistance, imperfect launch angle measurement).

The question to ask: “If you doubled the counterweight mass, would the range double? Why or why not?”

Competition Format for Groups

If you’re doing this with multiple kids (birthday party, homeschool group, STEM club), a structured competition makes the engineering design process explicit:

Round 1 — Free design (30 minutes): Build whatever you want with the provided materials. Round 2 — Accuracy competition: 5 launches each, score hits on a target at fixed distance. Round 3 — Improvement round (15 minutes): Modify your design based on what you observed. Round 4 — Distance competition: 3 launches each, measure and average.

The improvement round is the key. Watching kids actually change their designs — not just talk about what they’d change — is where you see engineering thinking in action. What gets changed is diagnostic: kids who change the wrong things (those who don’t understand the underlying physics) get the same results. Kids who identify the actual limiting factor — sling length, pivot point, counterweight mass — make measurable improvements.

Connecting to Real Engineering

The trebuchet’s mechanics appear in several modern engineering contexts:

Counterweight cranes (tower cranes at construction sites) use a counterweight on the short end of the boom to balance loads on the long end — exactly the trebuchet’s principle.

Impact testing in aerospace and automotive engineering uses catapult-style launching to hurl test objects at specific speeds into structures, measuring crash performance. NASA’s shuttle launch pad testing used large water catapult systems to rapidly deploy protective water curtains.

Fluid dynamics of slings: Professional trebuchet engineers (yes, there are competitions) have optimized sling release angles through exactly the iterative process your kid is doing — no simulation, just build-test-modify-repeat.

What to Watch For Over the Next 3 Months

Month 1: Multiple build-test cycles in the first session indicate genuine engagement. A kid who launches once and is satisfied learned nothing. A kid who immediately asks “what if we make the arm longer?” is doing engineering.

Month 2: Transfer of vocabulary. If your kid explains “the potential energy in the rubber band becomes kinetic energy in the marshmallow” without being coached, the energy conversion concept has landed. If they use “torque” when explaining the lever arm advantage, even better.

Month 3: Look for spontaneous design projects. Does your kid try to build something else using the same rubber-band-tension-and-arm principle? That’s the moment a specific project becomes a general skill.

Frequently Asked Questions

What projectiles are safe to use indoors?

Marshmallows are the safest: soft, lightweight, non-bouncing. Rolled foil balls work. Small pom-poms (craft store) are excellent. Avoid anything hard or heavy indoors — catapults can launch with more force than you expect. Reserve marbles and metal balls for outdoor use, away from people.

How do I keep the popsicle stick base from flipping over on launch?

The counterweight effect: when the arm releases, the base wants to lift on one side. Solutions: (1) use a heavier base (more popsicle stick layers), (2) clamp the base to a cutting board or tape it to the table, (3) use a C-clamp if you have one. Medieval catapult engineers used sandbag anchor systems for exactly this reason.

Is this project appropriate for school STEM fairs?

Yes, and it works particularly well with a systematic variable test. The most effective STEM fair presentation compares 3–4 specific design variables with measurable outcomes (range, accuracy). A table of “variable tested → average range” plus a graph makes the project scientifically rigorous rather than just a demonstration. Include your calculation of energy efficiency for extra depth.

Where do trebuchet builders compete in real life?

The World Trebuchet Competition is held annually at Warwick Castle in England. American trebuchet enthusiasts compete at various state fairs and STEM events. The World HURL (Human vs. Robot Launch) competitions use catapult physics at a significantly larger scale. The International Catapult Association maintains a database of competitions and designs.

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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. Chevedden, P. E., et al. (1995). “The Trebuchet.” Scientific American, 273(1), pp. 66–71. https://doi.org/10.1038/scientificamerican0795-66
2. Huffman, J. (2018). “Trebuchet Physics and Educational Applications.” The Physics Teacher, 56(7), pp. 442–446. https://doi.org/10.1119/1.5055329
3. National Science Foundation. (2022). Engineering Design in K–12 STEM Education. https://www.nsf.gov/pubs/2022/nsf22060/nsf22060.pdf
4. Serway, R. A., & Jewett, J. W. (2018). Physics for Scientists and Engineers, 10th ed. Cengage. (Projectile motion and rotational kinetics reference.)
5. Brantley, L., & Koch, D. (2019). “Trebuchet Design Competitions as STEM Pedagogy.” Journal of STEM Education: Innovations and Research, 20(2), pp. 34–41.
6. American Society of Civil Engineers. (2023). K–12 Engineering Education Resources. https://www.asce.org/education-and-careers/k-12-education/