How to Build a Trebuchet

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this project, you're going to be building a model trebuchet. Although you won't be able to storm cities, this project will teach you about kinetic and potential energy as well as the law of conservation of energy.


Goal: Build a model trebuchet to launch a marshmallow
Age: Middle school and up
Safety Concerns: Hot glue can burn you. Use caution and don't get any on your skin. Never launch a payload at anyone, they can get hurt even if the object is small.
Time: One hour

Have you ever read a story set in medieval times? Despite lacking the modern technology we have today, people in medieval times were able to create some serious weapons. The trebuchet is similar to a catapult, able to launch large objects such as stones over high castle walls to cause damage to the enemy.

A trebuchet relies on stored energy due to height, or gravitational potential energy (GPE). The trebuchet has an arm with a large counterweight on one side and the payload, or object to launch on the other. The payload is pulled down, hoisting the counterweight up. When the payload is released, the counterweight falls and its stored energy launches the payload.

Today, we're going to build a model trebuchet that will launch a marshmallow to study how GPE can be converted into kinetic energy, or energy due to movement. For a review of energy before we start, you can check out this lesson: Kinetic Energy to Potential Energy: Relationship in Different Energy Types


  • 8 popsicle sticks
  • One pencil
  • One marker
  • One foot square sheet of thick cardboard
  • Hot glue
  • Scissors
  • One paperclip
  • One foot of string
  • 3 small rubber bands
  • One large plastic straw
  • One or more mini-marshmallows
  • One AA battery or larger


Safety Tip!! Hot glue can burn your skin, use caution.

1. First, you need to construct your frame. Glue two popsicle sticks together in an 'X' shape where the top is slightly smaller than the bottom. Repeat to make two 'X's.

2. Next, use the scissors to cut a popsicle stick in half. Glue each half in the bottom part of the 'X's to make an A frame.

3. Now, you need to attach your A frames to the cardboard. Hold the A frame up and use the marker to mark where one popsicle sticks will insert into the cardboard. Cut two slices into the cardboard with scissors to create a stand for the first A frame.

4. Now, use the pencil to measure how far away the other A frame should be. The pencil should be able to rest in the small 'v' at the top of each A frame.

5. Mark where the second A frame should go, then cut slits into the cardboard like you did for the first A frame.

6. Glue your A frame into the slots.

7. Now, cut a piece of the plastic straw about 1'' long. Thread the pencil through the straw.

8. Next, place the pencil in the small 'v' shapes at the top of the A frame. Secure them to the A frame with a small rubber band on either side.

9. Using scissors, cut two notches on the sides of one end of another popsicle stick. This will help you attach the counterweight.

10. Attach this popsicle stick to the straw on the pencil using another rubber band. You can also add hot glue if you need more support.

11. Next, tie a string around the AA battery and hang it from the popsicle stick in the slits so that it just barely touches the ground when suspended.

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