Mechanical Engineering

Design Your Way Middle School Curriculum Mechanical Engineering 231

Activity: Building a Catapult In this activity, youth learn how to design and build a catapult that uses a pivot arm (lever) and a fulcrum to launch a Ping-Pong ball across the room. They investigate how different catapults designs work, and then use that information to construct their own catapult. Finally, kids test their catapults and make improvements to their original designs.

Grade Level Grades 6 - 8

Activity Level Intermediate

Activity Time 1 hour 30 minutes

Preparation Time 10 minutes

Grouping Pairs

Key Scientific and Engineering Terms

Fulcrum Kinetic energy Pivot arm

Stop block Potential energy

Career Connection Mechanical engineer

Learning Targets I can construct, use, and present arguments to support the claim that when the kinetic energy of an

object changes, energy is transferred to or from the object I can use the Engineering Design Process to create a simple catapult design that can launch a Ping-

Pong ball, using limited materials and time I can plan and carry out tests in order to identify aspects of a model or prototype that can be improved

Connections to NGSS MS-PS3-2. Develop a model to describe that when the arrangement of objects interacting at a

distance changes, different amounts of potential energy are stored in the system. MS-ETS1-1. Define the criteria and constraints of a design problem with sufficient precision to ensure

a successful solution, taking into account relevant scientific principles and potential impacts on people and the natural environment that may limit possible solutions.

MS-ETS1-2. Evaluate competing design solutions using a systematic process to determine how well they meet the criteria and constraints of the problem.

Materials Per Pair:

1-2-3 Catapult handout 10 Rubber bands

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Engineering Design Process handout (located on Page 3)

Four Spools

Pencil Four wooden or bamboo skewers

Scratch paper Four Dixie cups or bottle caps

10 popsicle sticks Ping-Pong balls or other small projectiles

Per Class: Optional: Ping-Pong ball (for demonstration purposes) Optional: Rubber bands (one for each youth) Scissors Tin snips Hot glue guns and glue sticks Extension cords Instructor Notes: Catapult Design Tips

Advance Preparation Set up the testing station. The testing station should include markers to help youth keep track of the

distance their catapults throw the Ping-Pong balls. Provide at least ten feet of space in which to test. See Extensions for more testing ideas.

Introduction (15 minutes) 1. Describe the activity. Explain to youth that they will take on the role of mechanical engineers as they design

and build a catapult that can launch a Ping-Pong ball across the room. Their catapults will make use of potential and kinetic energy.

2. Optional: Introduce youth to potential and kinetic energy.  If  youth  haven’t  been  introduced  to  the  concepts of potential and kinetic energy during another activity (such as the Kinetic Cars or Marble Roller Coaster activities), ask them the following question:

Have you ever heard the terms kinetic energy or potential energy? If so, what do you think they might mean?

If necessary, explain that potential energy is stored energy. Objects can store energy because of their position (for example, objects that are lifted off the ground have more potential energy, and so do stretchy objects that are stretched out). Kinetic energy is the energy of an object in motion.

To demonstrate, lift a Ping-Pong ball above your head and explain that you have increased the potential energy of the object by raising it up. Then, let the ball drop to the ground, and explain that the potential energy in the moving ball has been transformed into kinetic energy.

3. Optional: Have youth experiment with potential and kinetic energy using a rubber band. If you are introducing kinetic and potential energy, give each youth a rubber band to play with. Have them discuss the following questions with partners, and then ask volunteers to share answers with the group:

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What happens when you stretch the rubber band? Does it have kinetic or potential energy?

Possible answers: The stretched-out rubber band has potential energy.

What happens when you release the rubber band? Does it have kinetic or potential energy?

Possible answers: The moving rubber band has kinetic energy.

With your partner, describe in your own words what kinetic and potential energy are. What other objects do you think have potential or kinetic energy?

Possible answers: A slingshot has potential energy when the pouch holding the rock is stretched back, and this energy is transformed into kinetic energy when the rock is released. Water held at the top of a dam has potential energy; when the water is released and flows down, it has kinetic energy.

4. Introduce the anatomy of a catapult. Distribute the 1-2-3 Catapult handout. Have kids work in pairs to identify the components listed in the box at the bottom of the handout on each of the catapults, and have volunteers share their answers with the group:

Basket Pivot arm (lever) Fulcrum Stop block Potential energy Base

Tip: See Science and Technology Background for an illustrated diagram identifying all components.

5. Discuss catapult designs. Ask students:

What are some similarities and differences between the catapult designs? How are potential and kinetic energy used in each catapult?

Brainstorm and Design (10 minutes) 1. Introduce or remind youth of the Engineering Design Process, and discuss criteria and constraints.

Distribute the Engineering Design Process handout. If youth have used the process before, ask volunteers to describe the different steps. If this is the first time youth are using the process, you can describe each step.

Explain that engineers often have to work with criteria (design requirements) and constraints (design limits). For this challenge, they will work with the following criteria and constraints:

Criteria:

The catapult must be able to launch a Ping-Pong ball

Constraints:

Pairs can  only  use  the  materials  that  they’ve  been  given Pairs have to work within the time limits you set for design and building

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2. Have pairs design and sketch their catapults. Make pairs and distribute materials. If you think your youth might need extra help you can share the information from Instructor Notes: Catapult Design Tips. Tell kids to follow the Engineering Design Process to do the following:

Brainstorm what they want their catapult to look like Sketch  out  designs,  being  sure  to  include  the  all  the  main  components  of  a  catapult  that  they’ve  learned  

about Share their designs with you for approval

Tip:  If  time  permits,  you  can  have  a  gallery  walk  of  everyone’s  designs.    

3. Ask questions. As pairs work, ask open-ended questions about their designs, such as:

How does your design work? Why did you decide to make your design that way? What other designs did you consider, and why did you choose this one?

Build (20 minutes) 1. Build catapults. Give kids a set amount of time to build their catapults, using only the materials they have

been given.

2. Ask questions. Check in with pairs throughout the building phase with questions such as:

Can you describe the different parts of the catapult and their purpose? Predict how you think your catapult is going to work. When will your catapult have the most potential energy? How will that energy be transformed into kinetic

energy?

Test (15 minutes) Test Designs. Do the following:

Have pairs discuss how they will test their catapults and then create data sheets to record the distance their Ping-Pong balls travel.

Have each pair come up and test its catapult in the testing zone while the group watches, marking with a piece of tape how far the Ping-Pong ball flew.

After each pair launches its ball, have other pairs make suggestions for improvements.

Redesign and Retest (20 minutes) 1. Redesign. Give pairs additional time to make adjustments to their catapult designs.

2. Retest. Have a final test to see if pairs lengthened the distance the Ping-Pong ball traveled.

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Discussion and Reflection (10 minutes) Once youth have completed the challenge, lead a think-pair-share about the design process. For each question, give youth 30 seconds to think about the answer on their own, two minutes to discuss in pairs, and then have pairs share out their answers with the large group. This process will give all youth the opportunity to voice their thoughts about the activity.

The timings above are suggested guidelines. Feel free to adjust them based on the needs of your group and the nature of the questions you are asking them to reflect on. Some possible discussion questions include:

How did you use the Engineering Design Process to construct your catapult? How  many  of  your  final  designs  look  just  like  your  original  sketches?  If  they  don’t,  what  changes  did  you  

have to make to get your design to work? How is potential energy transformed into kinetic energy in your catapult? With more time, what additional materials would you need to improve your design? What simple machines can you identify in your catapult?

Extension An alternative to the distance test is to make testing a game by setting up targets around the room and letting youth try to launch Ping-Pong  balls  into  them.  Also,  you  could  hold  a  “Catapult  Olympics”  and  give  out  awards  for  accuracy, distance, height, power, etc.

Science and Engineering Connections Potential and Kinetic Energy

Potential energy is energy stored in an object because of the position of the object. Objects with potential energy have the potential to cause change—for example, a ball held above your head can drop, causing a change to occur, and a stretched rubber band can snap back and fly forward, causing a change to occur.

There are a few types of potential energy—the ones relevant to this activity are:

Gravitational potential energy: energy stored in an object because of its position relative to Earth. The higher an object is (father away from Earth), the more potential energy it contains.

Elastic potential energy: energy stored by objects that stretch or compress, like rubber bands or springs. The more a rubber band is stretched, the more potential energy it contains.

Kinetic energy is the energy of moving objects. The faster the moving object is, and the heavier the object is, the more kinetic energy it has.

Catapults

Catapults are devices that use potential energy to launch objects in a specific direction. All catapults use a lever and fulcrum. The lever is the pivot arm, and the fulcrum is the point where the arm pivots. When tension is applied to the arm (for example, with a stretched rubber band), the arm is forced to move from its original location and

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potential energy is stored in the rubber band. When the tension is released (by releasing the stretched rubber band), the arm springs forward (the energy is transformed from potential to kinetic) and is then abruptly stopped by the stop-block. This sudden stop of motion/energy is what hurls the Ping-Pong ball. The greater the tension on the arm, the further the Ping-Pong ball will travel.

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Anatomy of a catapult

The pivot arm is the lever that rotates around the fulcrum.

The fulcrum is the point where the arm pivots.

Potential energy can be stored in a rubber band, a spring, or anything that can add tension to the arm

The stop-block stops the arm at the right launching point.

The base supports the catapult.

The basket is where the ball will rest before it is launched.

Handout: 1-2-3 Catapult

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Examples of catapults:

Identify: Basket—where the ball rests Pivot arm (lever)—the part of the catapult that moves the ball forward Fulcrum—the point where the pivot arm moves Stop block—the part of the catapult that stops the arm after it launches Base—supports the catapult The potential energy—how is potential energy being stored?

Instructor Notes: Catapult Design Tips

Design Your Way Middle School Curriculum Mechanical Engineering 239

Here are some tips that you can use to help youth when they are designing and building their catapults.

Make sure each catapult has a base to support the pivot arm (launching device). Make sure youth identify where and how the arm will be attached to the fulcrum. Have youth identify how the fulcrum will be attached to the base of the catapult. Have youth play with the rubber bands. Ask them to identify how the rubber band will connect to the

arm and the catapult. Have youth identify where their stop-block will go. The placement of the stop-block will determine the

direction their object will travel. Make sure youth create a basket out of a Dixie cup or other small cup-like object and securely glue it to

the pivot arm. Suggest to youth that they can glue the pivot arm onto the spool and slide the spool onto a Popsicle

stick or skewer to work as the rotating device. Do not glue the spool because then it will not rotate. Notches can be cut into the popsicle sticks so that the rubber band will stay in place. Have youth assess the sturdiness of the catapult base.

Examples of catapults built by youth: