Trebuchet Launch

Engineers solve real-world problems. In fact, all engineers, including mechanical, electrical and chemical engineers, are faced with challenges that they solve via engineering designs, whether through the development of new technologies or improvements on existing functionality. In essence, engineers design and analyze solutions to generate new technology that is sophisticated and user friendly. Launching an object with precision is an ongoing challenge for people who use trebuchets; in this activity, students act as engineers to find solutions to this problem.

After this activity, students should be able to:

• Design a LEGO MINDSTORMS EV3 robot that can be used to launch objects.
• Program a robot using LEGO MINDSTORMS EV3 software.
• Determine the distance an object travels and relate it to its respective weight.
• Design and carry out a controlled engineering experiment.

Basic concepts of a how a trebuchet functions.

Who loves watching objects fly through the air? (Give students times to discuss among themselves.) The term launch is often used to describe how far an object can travel. Trebuchets are devices that can launch object through the air. They were designed during medieval times to throw payloads (heavy objects) at enemies. Believe it or not, friendly competitions exist in some areas today in which trebuchets are used to launch pumpkins and even pianos. During this type of competition, a team wins when their trebuchet launches a pumpkin the farthest. Can you imagine how far a trebuchet would have to launch a pumpkin in order to win? (Give students time to guess a distance.)

In this activity, you design and test a trebuchet—actually a robot—to determine how far an object will travel based on its weight. Then, you adjust the angle and the length of the robot arm and determine how far different objects will travel, given modified variables. Basically, through experimentation, you learn how to design the best type of trebuchet to launch a variety of objects.

Why are we learning about trebuchets? I'm glad you asked! Trebuchets are an important educational tool because they demonstrate several principles encountered in basic physics—a branch of science that studies matter and its motion. Today, we will use computers and robots to model some of these physics concepts.

So, in this engineering activity, you apply math and science to predict how objects move through air as you design, build and program your own robotic creation to propel objects through the air. You also learn the importance of a variable, which is something that can be changed. By changing one variable at a time, you can determine what makes an object travel farther. Let's start launching!

Background

During the first century, the Romans figured out different ways to make projectiles reach a certain location. During battle, they had to reiterate their designs in real time so that objects they hurled reached their opponents. The Romans applied a great deal of math and science, as well as trial and error, to solve this problem; the result of their efforts was the design of a crude, but effective, trebuchet—a type of catapult. Today, we use the trebuchet for both advanced solutions, as well as for entertaining purposes, such as catapulting pumpkins in friendly competition.

Before the Activity

• Gather materials and make copies of the How Far Does It Go? Worksheet (PDF), one per student.
• Load the EV3 software onto the computer.
• Divide the class into groups of four students each and assign each group member one of the following tasks: Launcher Driver (launches the object); Launcher Passenger (prepares the launcher); Measurer (measures how far the object travels); and Recorder (records the data observed from launches).

With the Students

Part 1: Preparing the Robot

1. Have groups design their own robot launchers. Since many ways exist to design robots, tell students that this is an open-ended design project. (See some examples of student-designed projects in Figure 1.)Figure 1. Example trebuchet robots.CopyrightCopyright 2013 Jennifer S. Haghpanah, Polytechnic Institute of NYU
2. Have students build their own robots before moving on to the programming stage. Since no set of instructions are provided for building, encourage students to be creative and work together to come up with viable designs.
3. Following Figure 2, instruct students to program the robot to launch objects. If the program does not look like Figure 2, then instruct students to look in their pallet on the left side of the window for the program and make sure the motor, direction and power agree with Figure 2. The arrow in Figure 2 represents the direction, and the semi-circle filled with the orange color represents the power.
4. Once programmed, have students plug the cable into motor A, located on the EV3 brick.
5. Open the MINDSTORMS program and ask students to drag the motor block to the orange square; select the motor, power and direction. It is important that students understand they must have one motor do one rotation forward in order for the launcher to work.

Part 2: Preparing for the Launch

1. Review correct measurement skills with students and teach them how to measure distance.
2. Show students the variouis objects available to be launched, and ask them to predict which will travel the farthest.Figure 2. The MINDSTORMS program image.CopyrightCopyright 2013 Jennifer S. Haghpanah, Polytechnic Institute of NYU
3. Ask students: What do you think will happen to the object's travel distance when the arm angle is changed?
4. Ask students: What do you think will happen if you increase/decrease the angle or increase/decrease the length of the arm?

Part 3: The Launch

1. Show students the objects to be launched again. Have them use a scale to determine and record the weight of each object.
2. Direct students to take turns within their groups to launch objects. To do this, take an object and place it at the end of the arm for launching. (Note: Watch that students do not launch objects at each other.)
3. Once launched, have groups measure the distance traveled (that is, the distance from the robot to the object); be sure students know the weight of each object before they launch it.
4. Instruct students to record the object's distance traveled on their worksheets. Make sure they measure where the object first hits the ground, not where it rolls or slides to.)
5. Once all groups have obtained data for their first trials, change the variables, repeating steps #2-4 for each variable change. First, change the angle of the arm and next, return the arm angle to its original setting but change the arm length).
6. Direct students to complete the follow-up questions on their worksheets.

Pre-Activity Assessment

Guessing Game: Have students weigh the different objects. Ask them to predict which object will go the farthest, giving reasons for their answers.

Activity Embedded Assessment

Design a Robot: Direct students to design robots that will be able to launch objects. Explain that this is called open-ended design since there is no one right way. Ask them to explain to you how their designed launchers function to launch objects.

Post-Activity Assessment

Worksheet: Have students use the How Far Does It Go? Worksheet (PDF) to record their trebuchet experiment data and results. Review their data, graphs and answers to gauge their level of comprehension.

Tuning the Equation: Ask students what happens when both the arm length and arm angle are changed? Ask them how far the object goes when these two variables are manipulated together.

Re-Design: Ask students to think about how changing variables (that is, the arm length and arm angle) affected their robot performances. As engineers, how would they re-design their existing robots to launch an object the farthest? How would they modify the design if accuracy was more important than distance?

• Remind students to only launch towards unoccupied space, never towards other people.

Remind students to test their launchers before they begin to launch the objects.

Remind students that all launches should be precise; they should not estimate their launches.

The rubber bands on their robot launchers should have some sort of tension; students may need to adjust the tension after they have launched an object.

For upper grades, have students experiment with other materials, and launch them with different angles and arm lengths. Specifically, challenge students to create robots with long arms and find out what length impairs the function of the arm. Additionally, experiment to find out what arm length is not strong enough to launch an object and what length provides the best launch.

Hewitt, Paul. Conceptual Physics. Upper Saddle River, NJ: Prentice Hall, 2002.

Zitzewitz, Paul. Physics Principles and Problems. Columbus, OH: McGraw-Hill, 2002.