What in the World is a Trebuchet?

Okay… that’s your first goal… research. The basic design of a trebuchet involves a counterweight. You raise the weight on one side of a lever, load the object to be flung, release, and let gravity do the rest. More sophisticated designs also employ a sling that adds a whip-cracking effect. This project will reinforce the concepts of acceleration, velocity, projectile motion, force, energy transfer, and torque.

Basic Information

Your team’s goal is to build a device, using any materials, which will launch a water balloon as far as possible. The basic principles of the trebuchet must provide the launching power (i.e. gravitational potential energy). Do not use an elastic material such as rubber bands or springs to power your machine.

The device will be tested outdoors on the ground slightly south of the new weight room. It may not be weighted or otherwise restrained. Any device considered unsafe will not be allowed to be fired. The launching device may not exceed the dimension restrictions described below.

You will be launching water balloons that you provide. An interval of time will be allowed for verification that your device is working (these launches do not count). After that, each launch team will be allowed to launch 3 balloons, trying to obtain an accurate launch. The target will be a standard hula-hoop, with its center ten meters away from the starting line. To perform well in this area, you must have conducted some testing of your machine, so that you can make adjustments accordingly.

After that, each launch team will be allowed to launch 5 balloons, trying to obtain the farthest positive horizontal displacement possible. The front of the launch device will be set behind the starting line and must remain behind the line before, during, and after the launch. All measurements will be made with respect to the starting line, and you must get the attention of the instructor before launching.

More Specifics

• Trebuchet size: Maximum height at the pivot will be 4 ft. Maximum length of the lever arm will be 6 ft. You must be able to transport the trebuchet to and from the school with a standard vehicle.
• Counter balance mass: Must remain attached to the launch vehicle and may not exceed 25 lbs.
• Launch mechanism: The machine should be designed so that it can be loaded and a trigger can be set. Then only when the trigger is released will the machine be set in motion. This is for safety reasons, and so that we can keep the testing day moving smoothly. The “trigger” should NOT be someone’s hand of foot holding a machine part in place!
• Balloon: Must be filled with water only. Can be any size you choose.
• Construction log: Each individual student must keep a dated, detailed journal of your group’s progress. You should include a summary of every stage of the design and construction: research, design planning, gathering of materials, construction, testing, problems and their solutions. It is to be kept current and may be checked without notice. Point will be deducted if it is found to unavailable, inaccurate, or not current.
• Modifications: Modifications may be made on the launch site (bring your tools).

What does my group turn in on Launch Day?

• Your trebuchet. (Depending on the weather, we may or may not test that day.)

• A discussion of the applicable physics principles (at least one typed page for a passing grade). This is where the research comes in. If your discussion centers on logic and intuition, and doesn’t include any serious mention of rotation and torque, do not expect a high grade on this part. You can also earn some bonus credit by taking time and distance measurements, and calculating important aspects of projectile motion.

• A list of materials and tools.

• A written procedure that could be used (by someone not in your group) to build your trebuchet. You should include diagrams to help explain, and a good procedure will involve multiple diagrams throughout the description. All tools used should be included in the list above. So if you mention something in the procedure, it also needs to be in the list.

• A neat, labeled diagram of your actual trebuchet. Important dimensions should be included. This diagram should be large enough to see some detail; a full page.

What do I turn in the day after launch day?

You INDIVIDUAL construction log. Detailed and dated. Include an entry for the launch day, summarizing your results and thoughts.

Grading

• Individual construction log (15%): If any logs appear very similar in organization or content, expect to share the grade. This part should not be a collaborative effort. If you have questions about the format or expectations, then talk to me, not your group members.

• Design of the machine (10%): Average grades will be awarded to average machines.

• Construction of the machine (10%): Sloppy construction will earn a lower grade. Failure to follow restrictions may result in loss of all of these points.

• Physics principles (20%) This is discussed above. I expect to see evidence of research, including sources consulted.

• Construction and design description (15%): This includes the list of materials and tools, the procedure, and the final diagram.

• Distance Performance (15%): To earn a passing grade for the distance measurement, you must launch the balloon at least 10 meters. Launches of 20 meters will receive full credit.
• Distance launched grade (out of 15 points) %
• 20 m 15 100
• 18 13.5 90
• 16 12 80
• 14 10.5 70
• 10 9 60

• Accuracy Performance (15%): This grade will be determined by taking the distance from the center of the target away from 10 and multiplying by 15%.