Trebuchet Design & Build Challenge

The trebuchet is a classic example of ancient engineering design. Not only is it a compound machine (a device that uses a number of simple machines), it relies on one primary simple machine, the lever, to launch a projectile. Trebuchets were engineered due to an enhanced understanding of gravity, potential energy, and acceleration—each of which is a key concept in engineering today. This design project allows students to use the engineering design process to research, design, and build a trebuchet.  They also rely on their knowledge of algebra and physics to explain their results.

The steps of the engineering design process.CopyrightCopyright TeachEngineering.org

After this activity, students should be able to:

• Design and build a trebuchet with constraints.
• Test their designs and analyze the results of their tests.
• Communicate their design process and results.
• Create instructions on replicating their design.

Students should have an understanding of the key concepts of quadratic equations, including vertices, minimum and maximum, and zeros; and how to use Google SketchUp (see Additional Multimedia Resources for a link to a tutorial on this software).

In order to understand how modern engineers design, build, and test their projects, let’s take a step back in history. Has anyone ever heard of a trebuchet? (Field answers from the students.) Ok, has anyone ever heard of a catapult? (Again, field answers from the students; this term is probably more familiar.) A trebuchet is an ancient type catapult that uses a swinging arm to throw a projectile. Trebuchets are examples of compound machines, which are made up of two of more simple machines to operate. A simple machine is a mechanical device that changes the direction of a force, and you’ve probably heard of these classical types: level, wheel and axle, pulley, inclined plane, wedge, and screw.

A trebuchet uses a lever for its throwing arm, which is attached to a sturdy base by an axle. Trebuchets throw projectiles (these usually sit inside a sling at one end of the throwing arm) by quickly rotating the arm along its axle. Since the sling is attached to the longer end of the arm and opposite to where the force is applied at the fulcrum of the lever, the trebuchet takes advantage of leverage—also called mechanical advantage—to throw a projectile.

Along with using simple machines, trebuchets rely on gravity, potential energy, and rotational acceleration to loosen projectiles from a sling. To see these concepts in action, let’s watch a few videos that can help us visualize how a trebuchet operates. 

(Show a Punkin’ Chunkin’ video from one of the links below or from YouTube and ask students to pay close attention to the different types of machines used to launch the pumpkins.) 

The World Championship Punkin’ Chunkin’ Association (WCPCA) hosts a signature pumpkin launching event each year, fueling innovative engineering and science-based ideas that draw spectators from all over. (www.punkinchunkin.com/) Punkin’ Chunkin’ cultivates the odd, challenging, and competitive quest for distance that inspires creativity, ingenuity, teamwork, and passion.

• You can have students watch Physics of “Punkin Chunkin” (3:46 long). Also, show some other videos of Punkin’ Chunkin’ launches on YouTube. For example: Punkin Chunkin Trebuchet // Homemade Science with Bruce Yeany (4:25 long) or Punkin Chunkin' Festival 2015 (3:17 long).
•  Ask students to compare and contrast the different launch engines they observed in the video. 
• • How is a trebuchet different from the other siege engines?
  • • Sample answer: Uses a counterweight, works by gravity instead of tension.  
  • Are there any benefits to building a trebuchet over the other machines?
  • • Sample answer: More powerful, more accurate, etc.
  • How could trebuchets and quadratic functions be related?
  • • Sample answer: Kinematic equations that describe motion of a projectile is a quadratic equation; you can use quadratic equation to predict the motion of objects launched from a trebuchet.

For homework (or in class) do independent research on different trebuchet designs. You should come to class with a rough sketch that includes key parts of the trebuchet: frame, pivot beam, counterweight, and sling. You should also have an idea of what types of materials you would use to build your trebuchet. Remember to cite the sources of the websites you visit in your research!

Before the Activity

• Access the Punkin’ Chunkin’ All-Stars episode on Science Channel’s webpage or bring up the free alternative video Physics of Punkin' Chunkin' and a few extra videos of the event.
• Make copies of worksheets, one per group: Trebuchet Supply Order (XLSX), Instruction Manual Rubric (PDF), Trebuchet and Quadratics Data Collection Sheet (PDF), PowerPoint Presentation Rubric (PDF).

With the Students

Day 1: Introduction and Exploration

1. Present the Introduction/Motivation section to students, including the Punkin’ Chunkin’ video(s).
2. Assign students to research (either as homework or in class) as detailed in the Introduction/Motivation section.

• • This research should include design elements of a trebuchet as well as common materials used to build one.

Day 2: Research and Design

1. Divide students into groups of 3-4.
2. Give students time to review their research as a group and discuss the merit of each of their designs.  Discuss how materials can affect design and build time.
3. Give students the constraints for the project.
4. Provide more time for group research, if necessary.

Group Constraints:

• • • The base of your trebuchet can be no more than 1 m x 1 m (about 3 ft. x 3 ft.).
    • The height of your trebuchet can be no more than 1.2 m tall (about 4 ft.).
    • Final product must collapse to fit a 38 cm x 30 cm x 25 cm (15 in. x 12 in. x 10 in.) cardboard box. This can adjusted based on any available cardboard box.
    • You must not spend more than $45 on supplies at a hardware store.

5. Have students begin the first iteration of their group design on graph paper based on the above constraints. Their schematic should include:

• • Side view, front view, and rear view
  • Measurements in meters (they may also include feet and inches)
  • Initial materials list                 

6. Have students test their design on Virtual Trebuchet 2.0, (http://www.virtualtrebuchet.com/), a free web-based simulator.
7. Circulate during this time answering student questions, offering suggestions, and helping students stay within the constraints provided.

An example of a first iteration sketch of a trebuchet design.CopyrightCopyright 2017 Kimberly Collins, University of Texas RET

Day 3: Design and Order

1. Instruct groups to assign team members to complete different duties.

• • Google Sketchup Team
  • Materials Team

2. Have one or two team members use Google SketchUp to create their first design.

• • The design should include a side view, front view, rear view with measurements.

3. Have one or two team members research materials online (such as on a hardware store website) and create an order list based on their design iteration. Have students complete the Trebuchet Supply Order (XLSX) with their list. Note: the teacher can pick up supplies from a local hardware store or have them shipped to the school.

• • A sample list may look like this:
  • • 7 buckets with lids, 3.78 L (~1 gallon)
    • 42 m of PVC pipe, 2.5 cm (~1 inch) in diameter
    • 70 PVC 45° connectors, 2.5 cm (~1 inch) in diameter
    • 70 PVC three-way corner connectors, 2.5 cm (~1 inch) in diameter
    • 7 PVC four-way connectors, 3.8 cm (~1.5 inch) in diameter
    • 14 PVC endcaps, 2.5 cm (~1 inch) in diameter
    • 10 boxes
    • 1 multi-material screw
    • duct tape, 5 rolls
    • mason twine, 5 rolls, 76 m (250 ft.) in length, neon color
  • Use the preceding list as a guide only! Students should come up with creative solutions of how to build trebuchets with hardware store materials on their own.

An example using Google SketchUp that shows a side view of an initial design of their trebuchet based on their first iteration.CopyrightCopyright 2017 Kimberly Collins, University of Texas RET

A list of materials that a group needs to design and build their trebuchet.CopyrightCopyright 2017 Kimberly Collins, University of Texas RET

Days 4 and 5: Build and Create Instructions

1. Once the materials arrive, students can begin cutting and assembling their trebuchets according to their designs. Remind students to document any changes to their original design and provide a justification for those changes. 
2. Ask students the following key questions:

• • Did you create a detailed step-by-step process for constructing the trebuchet?
  • Did you include pictures for each assembly step?

3. Once students have assembled their trebuchets, have them begin creating an instruction manual for assembly following the Instruction Manual Rubric (PDF). Students will iterate on their instruction manuals as they test their trebuchet and make adjustments to their design.

• • Optional: If you want the manuals to be more accessible and realistic, require students to have warnings and parts labeled in English and Spanish. This is an especially good task if the classroom has a number of ESL (English as a Second Language) students.
  • See the Instruction Manual Rubric (PDF) for grading.
  • If the students get stuck on how to write instructions, allow more time for students to research different types of schematics online. The furniture retailer IKEA has lots of simple instructions that show how to build their products, many of which could be adapted for this activity.  

An example of an instruction manual on how to build a trebuchet.CopyrightCopyright 2017 Kimberly Collins, University of Texas RET

An example of a student group using English and Spanish as their two languages in writing their instruction manual.CopyrightCopyright 2017 Kimberly Collins, University of Texas RET

Days 6 and 7: Test & Refine Design

1. Students will begin testing their trebuchets. Use the Trebuchet and Quadratics Data Collection Sheet (PDF) to record data.
2. Remind them of safety procedures when launching their payload.

• • Where should they stand and why?
  • • Students should stand to the left or right of the trebuchet, never in the front or back.  
  • Did they clear their line of sight?
  • • Make sure no one is standing behind or in front of the trebuchet, up to 50 feet cleared distance to the front and back.
  • Is everyone wearing safety goggles?
  • • Wearing safety goggles is a best practice during many kinds of product tests.

3. Circulate during this time answering questions and offering suggestions. Ask the Key Questions:

• • How do you determine the trebuchet’s effectiveness?
  • What were the variables of your experiment? How did changing them change your results?
  • Is your explanation detailed and clear, showing complete understanding of the mathematical, science, and engineering concepts used to solve the problem(s)?
  • Did you include error analysis in your results?
  • How did your initial testing of the trebuchet change the original design?

4. Have students document each attempted launch and discuss the successes and failures. They may want to record each launch in slow motion video so they can make some visual analyses after each attempt. Students will need at least two launch videos for the Day 8 graphing activity.
5. As students adjust or change their designs, ask them how that affects their launch.
6. Remind students that changes must be reflected in their instruction manual.

Day 8: Graph Your Trebuchet Launch

1. ·Groups will decide on two of their best launches to graph.

• • Each group will get one (1) sheet of large graph paper.
  • Teacher will project the video of each launch onto the board or wall.
  • • Tape the graph paper to the wall or board using blue tape.
    • Adjust the size of the projected video to fit the graph paper.
    • Students will create an x- and y-axis using the trebuchet at the point of origin.
    • Play the video and stop it at key points to allow the students to plot the path of the tennis ball.
    • Turn over the graph paper and repeat the process for the second launch.
  • Students will use their collected data from each launch in order to set the scale of their x- and y-axis.
  • Students will label key attributes of each parabola: x-intercept, vertex, axis of symmetry.
  • Have students discuss the differences in each graph and how it relates to the two launches.

Days 9 and 10: Prepare and Present Findings

1. Have student groups prepare and present their findings in a PowerPoint presentation. Provide them with a copy of the PowerPoint Presentation Rubric (PDF). You may also show students the Trebuchet Example Presentation (PPTX) to give students an idea of how to set up their presentation.
2. Students may also write an individual reflection on their experiences during this project. See the Student Reflection Example (PDF) for reference.

Pre-Activity Assessment

Research: Students engage in research on how to build a trebuchet; they also learn how to document their research using a bibliography.

Activity Embedded Assessment

Google SketchUp: Students create a Google SketchUp design of their trebuchet that includes a front, side, back view, and accurate measurements.

Instruction Manual: Students write an instruction manual for the assembly of their trebuchet. See Instruction Manual Rubric (PDF).

Post-Activity Assessment

Presentations: Student groups will present their designs in a PowerPoint Presentation, detailing their success/failures during the testing phase, and discussing the connection to quadratics. See the PowerPoint Presentation Rubric (PDF).

Reflection: Students will write an individual reflection on their experiences during this project. See an example here: Student Reflection Example (PDF).

• Digital safety: since this is a research-intense project, discuss digital footprints and information gathering that happen when students perform searches online. Resources at Common Sense Media: https://www.commonsense.org/education/digital-citizenship
• Adhere to safety protocols when using shop equipment.
• Discuss proper trebuchet testing procedures including, where to stand, ensuring a clear line of sight, and using proper safety equipment.

• Ask your school librarian to discuss possible internet sites that can help with research and review common research techniques.
• Ask for help from parents with expertise in using the tools required for this project.
• Allow flexibility for time extensions for groups that struggle based on constraints and materials chosen.
• If there are many groups, consider imposing a time limit on the PowerPoint presentations so all groups can present on Day 10 (within a 60-minute class period).

For math or science teachers, create a lesson plan that would help a teacher use this trebuchet in an elementary classroom.

How could you make the instructions more user-friendly? Create translations in another language for your instructions.

Google SketchUp: watch the following video for a tutorial: https://youtu.be/ST-ay0TwH_4

Virtual Trebuchet 2.0, http://www.virtualtrebuchet.com/