(EA) Trebuchet Lesson
Trebuchet Lesson
Introduction
Pictured to the right is Portchester Castle.
Before the development and use of gunpowder, ancient civilizations used devices to launch projectiles in anticipation of weakening the adversary’s defenses. This was one of several means to break into an opponent’s fortress or bomb the enclosure with large items, disease ridden materials, or other items. Once the wall was breached, then combat would ensue.
Ingenuity improved the earlier methods of siege machines for a safer distance from archers, improved accuracy, and greater payload, to more successfully break down the fortified walls of castles and other bastions of defense. These weapons were sophisticated and effectively designed for the purpose demanded by siege warfare. In this lesson you will learn about the development and use of the trebuchet as the most effective siege weapon of its time. At the conclusion of this lesson you will be tasked to build a model working trebuchet and test its effectiveness. You will analyze the results of your experiments and through evaluation of the data consider possible improvements to make your model more effective.
Explore
This exploration activity will guide you through the background for trebuchet development, and the applications of basics physical knowledge leading to trebuchet design and operation.
The concepts of the trebuchet and catapult are still used for other applications such as the aircraft carrier launch mechanism shown in the image to the right. The concept of the catapult extends into slingshot applications and other design and problem solution ideas generated from the concept.
The purpose of the hinged count weight is to ensure the weight can maintain a vertical position through the movement of the beam. This alignment reduces forces of motion spreading out and energy loss through the frame. Besides providing stability for the trebuchet, this is a more efficient transfer of energy into the beam and projectile.
See the following PBS video for additional information on trebuchets.
Physics and Design
Mathematical Relationship of a Lever
The beam acts like a lever with the pin/axle on the stand (pivot point which the arm swings) which is the fulcrum. From that aspect it is like a seesaw, which is a simple class 1 lever.
Refer to the image, Mathematical Relationship of a Lever. The beam is placed off center. This position increases the force of launch and extends the range of the launch. The recommended position is a 1 to a 3.25 to 4.0 ratio. So for every 1 inch on the side with the counterweight (see “a”) you would have 3.25+ inches on the side with the sling (see “b”).
Centripetal Force
Refer to the image, Centripetal Force. The shorter portion of the beam on the counterweight side will result in the payload end having a greater arc as the counterweight drops. This movement forces the payload end to reach a higher linear velocity as it undergoes centripetal acceleration. This is these forces that form basis for the tremendous power at launch.
Maximum range is achieved when the length of the end of the sling to pouch is equal to the length of the payload arm.
Challenge
Take Away
The trebuchet evolved to include two more major decision considerations. These are a guide chute to mount on bottom of base for the sling and a hinged counterweight. What are the technical benefits for each of these improvements?
The guide chute will control the movement of the sling through the base of the frame and stabilize the payload in the pouch until the acceleration is sufficient to hold it securely.
IMAGES CREATED BY GAVS