Build a Working Catapult

Overview

A catapult is a machine that stores energy and releases it all at once to launch a projectile. It is also one of the most satisfying things a child can build, because it does something dramatic: you push down, let go, and something flies through the air. That moment of launch is pure physics — energy transfer, projectile motion, and the lever principle — all visible in a single, thrilling second.

This is a multi-session woodworking project. The child will design, build, test, and refine a working catapult. They will use real tools, real wood, and real fasteners. The result will be a functional machine that they built with their own hands.

The Deliverable

A working catapult that can launch a marshmallow or ping pong ball at least 6 feet. The child can explain how it works (lever, fulcrum, stored energy) and has tested and adjusted it to improve performance.

Materials & Tools

Gather materials before the first session. A trip to the hardware store can be part of the project — let the child help select the wood and fasteners.

Wood options:

• Pre-cut craft wood from a hobby store (easiest)
• Scrap lumber from a garage or construction site (free, requires cutting)
• Thick wooden rulers or paint stir sticks (simplest, but less robust)

Tool note: This project requires drilling at least one hole and driving at least one screw. An adult operates the drill. The child can operate a hand screwdriver with supervision. See Safety Notes.

Project Phases

Phase 1: Design and Understand (Session 1, 45 minutes)

The lever lesson. Before building, understand the machine.

Place a ruler across a pencil (the fulcrum) on a table. Put a coin on one end. Push down on the other end. The coin launches.

"This is a lever. It has three parts: the arm (the ruler), the fulcrum (the pencil), and the load (the coin). When you push down on one side, the other side goes up. The farther the fulcrum is from the push side, the higher the load goes."

Experiment: Move the fulcrum closer to the middle, then closer to the push side, then closer to the load side. "Which position launches the coin the highest?" (Fulcrum closer to the push side = higher launch but requires more force. Fulcrum in the middle = balanced.)

Stored energy. "Our catapult needs power. Where does the energy come from? You could push down every time — but real catapults store energy. We are going to use rubber bands. When you stretch a rubber band, you store energy in it. When you let go, the energy releases all at once."

Demonstrate: Stretch a rubber band between two fingers. Let one end go. It snaps. "That snap is stored energy becoming motion. Our catapult will do the same thing, but in a controlled way."

Design the catapult. Sketch the catapult in the notebook. Label the parts:

• Base: The platform the catapult sits on
• Arm: The lever that swings
• Fulcrum/Pivot: Where the arm rotates
• Basket: The cup that holds the projectile
• Power source: Rubber bands that pull the arm and release it

Discuss: "Where should we put the pivot point? What should the basket look like? How will we attach the rubber bands?" Let the child make decisions. Guide, do not dictate.

Phase 2: Build (Session 2, 60 minutes)

Step 1: Prepare the base. If using a board, it needs to be stable. A flat piece of plywood (about 8x12 inches) works well. If you do not have plywood, attach two shorter boards perpendicular to the bottom of the base board as feet — like an upside-down T.

Step 2: Make the fulcrum. Attach the shorter block of wood upright to the base. This is the fulcrum. It should be about one-third of the way from one end of the base. Use wood screws driven from the bottom of the base up into the block, or use glue and let it set.

Step 3: Attach the arm. Drill a hole through the arm (the longer board) near its center point — slightly closer to one end than the other (the longer end will hold the basket). Drill a corresponding hole through the top of the fulcrum block. Attach the arm to the fulcrum with a bolt and nut — tight enough that it stays in place, loose enough that the arm can swing freely.

The child's role: Marking where holes go (measuring and penciling). Holding pieces in place during assembly. Driving screws with a hand screwdriver (parent pre-starts the screw). Sanding rough edges. Applying glue.

Step 4: Attach the basket. Glue or tape a plastic spoon, small cup, or bottle cap to the long end of the arm. This holds the projectile.

Step 5: Add the power source. Loop rubber bands from the short end of the arm down to a hook, screw, or nail on the base. When the arm is pulled down (basket end up), the rubber bands stretch. When released, they snap the arm forward, launching the projectile.

Test the mechanism before proceeding. Does the arm swing? Do the rubber bands pull it? Does the basket hold a marshmallow? Adjust as needed.

Phase 3: Test and Improve (Session 3, 45 minutes)

First launch. Place a marshmallow in the basket. Pull the arm down. Let go.

Did it launch? How far? Mark the landing spot with tape or a coin. Measure the distance.

Record in the notebook: Launch 1 — distance.

Iterate. Now improve the design. Change one variable at a time and test:

• Rubber band tension: Add more rubber bands. Does it launch farther? (More stored energy = more launch force.)
• Arm length: If possible, try a longer arm. Does it change the launch? (Longer arm = wider swing = more speed at the tip.)
• Fulcrum position: Move the pivot point. What happens? (Pivot closer to the power end = more leverage but less speed. Pivot closer to the basket = less leverage but more speed.)
• Launch angle: Prop the catapult up at the front to change the launch angle. Does a steeper or shallower angle send the projectile farther?

Record each change and its result. "Change: added 2 more rubber bands. Result: launched 3 feet farther."

The competition: Set targets at different distances (cups, boxes, stuffed animals). Can the child adjust the catapult to hit specific targets? This requires understanding the relationship between power, angle, and distance — which is exactly what ballistics engineers do.

Phase 4: Present (End of Session 3, 15 minutes)

The child demonstrates the catapult for a family member. They explain:

1. How a lever works
2. Where the energy comes from (stored in rubber bands)
3. What changes they made and why
4. Their best launch distance

Take a video of the best launch. This is the portfolio moment.

Success Criteria

• The catapult launches a projectile at least 6 feet
• The child can name the three parts of a lever (arm, fulcrum, load)
• The child made at least one design change based on testing data
• The child used at least one real tool during construction (screwdriver, sandpaper, drill with supervision)

Common Pitfalls

• Pivot too tight or too loose: If the arm does not swing freely, the bolt is too tight. If it flops around, it is too loose. Adjust the nut. A small washer between the arm and fulcrum can help.
• Rubber bands breaking: Use thick rubber bands, not thin office ones. If bands break during launch, they snap painfully. Safety goggles during testing.
• Base not stable: If the catapult slides or flips backward on launch, weigh down the base. Clamp it to a table, or add weight (a brick, a heavy book) to the base.
• Parent takes over building: The most common pitfall. The child must hold, mark, sand, and screw. It will be slower and less precise. That is the point.

Extensions

• Marshmallow war: Build two catapults and take turns trying to land marshmallows in each other's target zones. Accuracy over distance.
• Trebuchet upgrade: A trebuchet uses a hanging counterweight instead of rubber bands. Build one from the same base. Compare performance. The trebuchet is more consistent because the energy source (gravity) is constant.
• History connection: Research how catapults were used in ancient warfare. Romans, Greeks, and medieval armies all used them. What were they launching? How big were the real ones?
• Physics measurement: For 7-8 year olds, introduce the concept of measuring launch angle with a protractor and graphing distance vs. angle. Which angle produces the farthest flight? (About 45 degrees, in theory.)

Safety Notes

• Safety goggles mandatory during all testing. Rubber bands can snap, and projectiles can ricochet. Everyone in the testing area wears goggles.
• Tool safety: The adult operates the drill and saw. The child may use a hand screwdriver and sandpaper. Review proper grip and technique before they start. Never rush a child with a tool.
• Launch direction: Always launch away from people, pets, and breakable objects. Establish a "launch zone" with a clear downrange area. No one stands downrange during testing.
• Projectile selection: Marshmallows and ping pong balls only. No rocks, no marbles, no hard objects. A catapult is a weapon — treat projectile selection with seriousness.
• Splinters: Raw wood has rough edges. Sand all surfaces the child will touch before assembly. If a splinter occurs, remove with tweezers and clean with antiseptic.
• Pinch points: Where the arm meets the fulcrum, fingers can be pinched during rapid swing. Keep fingers clear of the pivot when launching. Demonstrate the "pull down, hands clear, release" sequence.