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Secrets of Lost Empires II -- Medieval Siege

Classroom Activity

Objective
To design a working model of a trebuchet and demonstrate the power of a Class 1 lever.

• copy of "Fling It!" student handout (HTML)
• plastic soda straws
• large and small paper clips
• short lengths of sturdy wire
• tape
• yarn or string
• pennies, bolts, or other small, heavy objects for counterweights
• ring or washer
• grapes
• tongue depressor
• small strips of cloth (such as cotton or muslin)

1. Make sure students understand how a Class 1 lever works. (See Activity Answer.) Organize students into teams and distribute a copy of the "Fling It!" student handout to each team.

2. Discuss the guidelines for trebuchet design. They are minimal to allow for maximum student creativity.

3. As a class, have students decide on a protocol for running the experiment, such as any constraints on how the materials will be used or how the data will be collected. Also have students decide how to determine each trebuchet's effectiveness, i.e., will it be based on which machine throws a grape farthest, which throws the farthest with the least amount of effort, or some other criteria?

4. Set a reasonable deadline for the models to be built depending on whether the students can work at home or only during class time.

5. Supervise the launchings on the day students demonstrate their models, making sure that students wear safety goggles during the procedure. (Note that eye injury or other accidents could occur if safety rules aren't followed.)

About Levers
Students will use levers in two of these four activities. They 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 following illustrations:

Once students understand the three different classes of levers, they will recognize them all around. Here's a quick method to classify levers.

1. Find the fulcrum. If it's in the middle, it's a Class 1. On the end, it's a Class 2 or 3.

2. To determine whether it's 2 or 3, find the load. If it's in the middle, it's a Class 2. On the end, it's a Class 3.

Ask students to identify the class of lever for the following:

• A claw hammer pulling a nail (Answer: Class 1. A hammer pivots on the middle of its head.)

• A wheel barrow (Answer: Class 2. The wheel is the fulcrum and the barrow is the load.)

• An oar rowing a boat (Answer: Class 1. The oarlock is the fulcrum.)

• A paddle paddling a canoe (Answer: Class 3. The top hand is the fulcrum and the blade is the load.)

• A bottle opener (Answer: Class 2. The fulcrum is on the end and the load is in the middle.)

Fling It!
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.

National Science Education Standards
Grades 5-8/9-12
Standard B: Physical Science—Motions and Forces