By Brolin Evans, July 10, 2009
- Middle School
- analyze the energy transformations, simple machines and motion of a marshmallow and a catapult (or any machine they choose to design) as the machine launches the object
- improve designs to maximize the distance the marshmallow travels
Junkyard Wars video
- large wooden craft sticks (about 4 per student)
- small wooden craft sticks (at least 10 per student)
- masking tape
- plastic spoon (flexible, not brittle or really hard)
- rubber bands of different sizes.
- large marshmallows
- measuring tape, or meter stick
- one shoe box per group (to put their materials in)
- acceleration: an increase in rate of change
- balanced force: two forces that are equal in size and opposite in direction
- catapult: an engine that provided medieval artillery used during sieges; a heavy war engine for hurling large stones and other missiles
- elastic energy: the energy which causes or is released by the elastic distortion of a solid or liquid
- kinetic energy: maximum kinetic energy is when an object is at the mean position and minimum at the extreme position
- mass: the property of a body that causes it to have weight in a gravitational field
- Newton’s Laws of Motion: three physical laws that form the basis for classical mechanics. They are: 1) In the absence of a net force, a body either is at rest or moves in a straight line with constant speed. 2) A body experiencing a force F experiences an acceleration a related to F by F = ma, where m is the mass of the body. Alternatively, force is equal to the time derivative of momentum. 3) Whenever a first body exerts a force F on a second body, the second body exerts a force -F on the first body. F and -F are equal in magnitude and opposite in direction.
- potential energy: maximum potential energy is when an object is at the extreme position and minimum at the mean position
- trebuchet: a trebuchet or trebucket is a siege engine that was employed in the Middle Ages either to smash masonry walls or to throw projectiles over them
- unbalanced force: an unopposed force that causes a change in motion
1. First, set up the scenario for the students by giving a brief history of ancient and medieval warfare. This can be accomplished through a brief discussion, or a brief PowerPoint presentation.
2. Show students the Junkyard Wars video.
3. Wrap Day 1 up by recapping major points in the video. (How do catapults work? What are the two types of catapults shown in the video? What elements of design did the contestants in the video use? How did the contestants improve upon their designs?)
1. Put students into small groups of three to four.
2. Pass a blank sheet of paper out to each group. Say: “Yesterday you all saw the design process in action. Today, it’s your turn. Before any group began building their catapults, what did they do?” (Accept any reasonable answer, but steer them in the right direction.)
3. Say, “Yes, they put their ideas on paper and made a sketch of what they were going to build. This is what you are going to do today. Remember, just because you sketch a particular design, doesn’t mean that you have to build or commit to that design. As you begin the process, you may find that you need to tweak certain elements of your design based on the materials you are given and functionality. Remember that you can build your device however you would like, but it must be free standing. Thinking outside of the box is a BIG PLUS!!! … Your materials list is written on the board. Use this to aide you in your design. You will have about fifteen minutes towards the end of class to begin working with your materials.”
4. Allow the students to work on their plans until there are 15 minutes left in class. Have materials manager come up to receive the materials for the project.
5. Five minutes before class is up, have students begin cleaning up. Have the groups place their materials into the shoe box with their names written on it.
Days 3 and 4:
These days are primarily intended for students to build their designs. They are then allowed to test them and make any adjustments needed until they get the maximum launch distance. The teacher’s pri mary role is a facilitator. Circulate the room and offer advice and help to the groups. I find it is most helpful to ask the students questions and allow them to come up with an answer; i.e. How do you think it would affect your distance if you increased the tension here? Why did you construct your frame the way you did? What do you think would happen if you changed this? Have an activity for any group that finishes early. (Note: Usually, it takes both periods or longer to actually build the designs.)
1. Before class: With masking tape, mark a start line and increments of a foot up to about 20 feet. (Note: You’ll be surprised how far some marshmallows go. The record for my classes was 17.5 feet!)
2. Have one group at a time come up and launch their marshmallows. Give each group three trials. You can either average the three trials or take the best distance of the three trials. Whichever group has the best distance, is the winner. Have some sort of reward on hand for the winners.
3. Do this for all of your classes and display the winning designs from each class.
4. Have students complete their handout on the marshmallow catapult.(Note: You could time each trial with a stop watch. This would allow your students to calculate speed and acceleration. As a result, they could graph their results explain the trends within the graph. This would also allow students to calculate the force exerted by the device, using Newton’s second law of motion (F=ma). You would also need a triple-beam balance to measure the mass of the marshmallow.)
Assessment of final prototype: Launching distance of at least one foot. Originality. Stability.Students will complete the handout, which asks students to analyze the science behind there device.