Objective: To design a working model of a trebuchet and demonstrate the power of a Class 1 lever.
King Edward has asked you to design and build a mighty siege machine, called a trebuchet, that will fling a grape across a far distance. You have been provided with some materials to build your trebuchet: You must use plastic soda straws for all long construction pieces and a straightened paper clip for the axle. You missile will be a grape. Study the diagram to the right before designing your grape-throwing trebuchet.
You will need to construct the following parts of a working trebuchet:
To connect the straws together, pinch the end of one straw and slide it into the end of another straw. Wrap a band of tape around the joint to secure the connection.
Consider these and other methods as you design and build you trebuchet. As you work, keep a journal describing your design successes and failures. Include detailed, labeled drawings and descriptions in your journal. Tell what you discovered as you worked.
Materials for each student:
Students will use a Class 1 lever to raise the brick and a Class 2 lever to turn or move it. They will also use a Class 1 lever in designing their trebuchets.
If students are unfamiliar with classes of levers, run a mini-lesson with the following information:
When describing levers you need these four terms: lever, fulcrum, effort, and load. The lever itself is long and stiff. The fulcrum is the resisting point where the lever turns or pivots. Effort is the force you apply and load is what you move. When you apply effort, the lever pivots around the fulcrum moving the load.
The job the lever must do determines how the load, effort, and fulcrum are arranged. This arrangement determines the class of lever. Look at the illustrations on the right:
Once students understand the three different classes of levers, they will recognize them all around. Here's a quick method to classify levers.
Ask students to identify the class of lever for the following:
A trebuchet is a Class 1 lever. The counterweight provides the effort. The load is the lighter boulder or missile. Between them on the machine carriage is an axle that serves as the fulcrum.
Student designs will vary. They will discover how to best connect straws together and how to brace the frame. They will experiment with varying the position of the axle along the throwing arm, the design of the sling, and methods of attaching the sling and counterweight to the throwing arm.
History records that Archimedes, an ancient mathematician and physicist, said, "Give me a lever long enough and a place to stand, and I will move the earth." His exaggeration proclaims the power of the simple lever. With levers, ancient Egyptians raised huge obelisks and the people of Rapa Nui raised massive moai. Because of their utility, levers became part of many other machines from trebuchets to modern devices.
This activity will help students understand the difficulties ancients faced in raising the obelisk or moai, including the instability of the rock pile and the problem of creating adequate fulcrums as the brick rises higher. For a follow-up exercise, students may want to raise a brick to the vertical. But as the ancients discovered, students will find that this will take many more stones and much more time.
Students may choose to find their own weight comparisons. To get them started, you may want to give them the following weights of some common objects: sport utility vehicle = 4,500 pounds (2,025 kilograms); blue whale = 150 tons (135 metric tonnes); bowling ball = 16 pounds (7.2 kilograms); refrigerator = 200 pounds (90 kilograms).
In an arch, the top stones distribute their weight to the blocks on either side and will not fall unless they can push the stones beneath them sideways. Stable arches, therefore, require that side stones be firmly set in place. The riverbanks of Chinese rainbow bridges provided this sideways support. The multiple arches in Roman aqueducts and the double arches of the Roman baths had similar support.
Arches are unstable during construction until the two sides meet in the middle. To experience the instability, have students stand back to back with a classmate with their shoulders touching. Have them slowly step away from each other, but keep their shoulders in contact. The two students maintain stability because the weight of their bodies is distributed down and sideways through each other's legs. If either were to move away suddenly, both would crash to the floor.
Raising the arch requires some dexterity. Remind students to apply inward pressure on the cookies to keep them in line. Tell them that early engineers built scaffolding to hold the stones in place until the arch achieved its own stability.
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