Exploring Variables While Testing & Improving Mint-Mobiles (for High School)

When developing a new product or device, engineers go through many iterations of the engineering design process. First, they imagine lots of ideas and share them with their teammates. The team then decides on one best design and plans its final materials, specifications, functionality and aesthetics. Before building a full-scale prototype, they create a small model on which they perform many tests. This process often involves balancing tradeoffs between optimized performance, safety, and cost. Once a model has been perfected, they build at full scale.

After this activity, students should be able to:

• Explain the steps of the engineering design process.
• Define criteria and constraints for a specific design problem.
• Defend design choices considering tradeoffs.
• Describe how Newton’s Laws of Motion apply to their mint-mobile’s movement, including the effects of mass, force, and acceleration.
• Explain the role of energy transfer (potential to kinetic) and friction in determining the distance a mint-mobile travels.
• Analyze real-world data by determining the equation of a line of best fit and use it to predict performance under different conditions.

Ability to make measurements using a meter stick. Knowledge of independent, dependent, and control variables. Knowledge on how to create a scatterplot, plot a (linear) line of best fit, and determine the equation of this line from data.

(Write Figure 1 on the board as each step is discussed. Start by drawing an arrow in the shape of a circle.)

The steps of the Engineering Design Process.CopyrightCopyright TeachEngineering.org

What do you think engineers do first when they begin to create a new product, say a new type of race car, a new style of backpack, or a new type of skis? That's right, the first thing engineers do is ask; that is, they ask what the product will do, what need they are addressing and what others have done. This also requires conducting research of the problem. Then, an engineer imagines all the possible features and qualities of the product. This takes a lot of creative brainstorming! After an engineer has made a list and some sketches of her/his ideas, all of the engineers in a team get together and share what they have come up with. As a team, they plan on one design by working with the best ideas. Once the details of the design have been decided, the team creates the product; then they get to test it out!

Tests are made on models of the product or device, which are often at a smaller than full-scale size. In the testing phase, the team pays attention to what needs to be changed to the model to make the product work better. The last step of the engineering design process is to improve the design.

While the engineering team designs the product, they first think about the criteria and constraints of their design problem. The criteria are all the things required of their product, or what the product must look like or be able to do in order to be considered successful. The constraints are all the things that limit the design. Engineers also balance tradeoffs, or they choose to include one element of a design while sacrificing another of equal importance.

While the engineering team tests the product, they pay close attention to all the variables associated with their design. Two important types of variables are: independent and dependent variables. An independent variable is something that an engineer decides to change during each test to see how the results differ. A dependent variable is something that changes as a result of varying the independent variable. In testing, we also have what is called a control. A control is the part of the experiment that is neither changed by the engineer nor is affected by the independent variable. Engineers are careful to only change one independent variable at a time while testing the product. In this activity, students will investigate how mass affects the motion of their mint-mobiles. They will explore how forces, friction, and energy transfer (potential energy converting to kinetic energy) affect the distance traveled, applying Newton’s Laws of Motion to interpret their results.

Today, you will act as engineers to design, build, and test mint-mobiles. Working within a budget, you’ll need to consider tradeoffs such as cost versus performance or safety versus speed. You will test your designs multiple times, iterating to improve performance, and then investigate how changing the mass of your mint-mobile affects its motion down a ramp. You’ll analyze your results, make predictions, and apply physics concepts to explain the outcomes. Are you ready to brainstorm? Great! Let's get started! 

Background

Engineers use the engineering design process when they are creating new devices. The main seven steps of the engineering design process are: ask, imagine, research, plan, create, test, and improve The improving step involves starting over again at the beginning of the design process with "ask," that is, "Where does this device need improvement? How can it be made to perform better?" Once a device has been built the first time, it is never complete! Engineers are always improving what they design. Imagine if the cellular phone had never been improved. Or cars, or computers.

Understanding forces and motion is critical in engineering. Newton’s Second Law of Motion states that the force applied to an object is equal to its mass multiplied by its acceleration (F = ma). When designing a vehicle, changes in mass affect acceleration and, therefore, distance traveled. Heavier objects have more potential energy at a given height, which converts to kinetic energy as they move. Engineers also consider friction, which resists motion and reduces distance traveled.

Designing a device often requires tradeoffs. Adding safety features, for instance, increases mass, which may improve stability but reduce speed or increase material costs. Engineers weigh performance, safety, and cost when making design decisions.

To evaluate performance, engineers test different variables. The independent variable is the factor changed deliberately, the dependent variable is the outcome measured, and control variables are factors kept constant to ensure a fair test. Changing only one independent variable at a time allows engineers to isolate its effect on performance.

Engineers analyze results using graphical methods. A scatterplot shows the relationship between variables, and a line of best fit summarizes trends. The slope (m) represents the rate of change, and the y-intercept (b) is the starting value when the independent variable is zero. Using the linear equation y = mx + b, engineers can predict performance under new conditions.

By combining the engineering design process, forces and motion, tradeoffs, and data analysis, students gain a foundation in how engineers make decisions and predict real-world outcomes.

Before the Activity

• Gather materials; place them in a central location in the room.
• Cut masking or duct tape into 15 cm strips.
• Set up the testing ramps by placing sturdy poster boards or a thin wooden board at an angle over a smooth surface (such as a tabletop). Use tape to secure the ramp. Tape a meter stick from the bottom of the ramp along the table, so students can measure the distances traveled.
• Divide the class into teams of two students each.

With the Students

1. Distribute copies of the Design Worksheet (PDF) to students. Make sure students see that they have a $20 budget and they must purchase materials and safety features; the costs of these are given on the Design Worksheet.
2. Give students the criteria that their designs must include at least 2 safety features in addition to the required seatbelts and airbags. Ask students to define other criteria and constraints of their mint-mobile design problem. Record student ideas on the board, and have students record their ideas in the Design Worksheet. Ensure that students mention the budget constraint. Student-generated criteria may include: must be roll down the ramp smoothly, must travel in a straight line (not fall off the ramp), mustn’t fall apart, etc. Student-generated constraints may include: can only use given materials, car must be small enough to fit on the ramp, etc. Remind them that physics principles like energy transfer, mass, and friction will influence performance.
3. To begin planning their mint-mobile designs, give the class three minutes for students to individually brainstorm what their designs might look like on their Design Worksheets. Point out the list of available materials and safety features on their worksheets and remind students of the criteria and constraints of their design.
4. Next, working with their partners, direct students to share their ideas and make drawings of their final, agreed-upon designs on their Design Worksheets. Require the drawings to list the materials and amounts needed to build their mint-mobiles, included safety features, and the total cost of their designs. Remind students that when they are selecting a material over a safety feature, or one safety feature over another, they are making a trade-off between two things that are useful to their designs.
5. Once teams have finished their designs, allow them to “buy” materials and safety features. Give students up to 20 minutes to build their initial design.
6. Have students test their mint-mobiles by rolling them down the ramp three times and recording the distance traveled.  Allow students time (up to half an hour) to record data and modifications on their Design Worksheet and to modify their designs to ensure they meet the criteria recorded on the board. Students may modify their designs after each test. As students test their designs, walk around the room and ask them to explain how/why they are (or are not) changing their designs. Encourage students to defend their design decisions and for each team member to answer your questions. Encourage students to explain how mass, ramp angle, friction, and energy transfer influence design changes.

Two possible designs with safety features listed on the mint-mobiles.CopyrightCopyright 2020, Sabina Schill, ITL Program, College of Engineering and Applied Sciences, University of Colorado Boulder

7. After students have tested their designs, tell them, “Engineers are also interested in how different variables affect their design’s performance. Now that we’ve created some mint-mobiles, we’ll investigate how changing the mass affects how far the mint-mobiles can travel.”
8. Have students begin testing the independent variable of mint-mobile mass, following the procedure and recording data in their Design Worksheets. Students will test their mint-mobiles at various masses three more times. Ensure students are carefully recording data on their worksheets, using the scale to measure mass and the meter stick to measure distance traveled.

Testing the independent variable of mint-mobile mass.CopyrightCopyright 2020, Sabina Schill, ITL Program, College of Engineering and Applied Sciences, University of Colorado Boulder

9. After testing, students will graph their data, draw a line of best fit, and determine the equation of the line. Assist students as needed, making sure they clearly label the graph and helping them determine the line equation.
10. Tell students that they will now use their equations to predict the performance of their mint-mobiles at a new mass. Make sure students choose a mass they did not test earlier.
11. Have students test their predictions by adding (or removing) mass from their mint-mobiles to match the new mass chosen. Students will send their mint-mobiles down the ramp again and record the distance traveled. Ensure that students compare their prediction to the actual distance traveled by writing their reflections on their worksheets.
12. Have students set their mint-mobiles aside and complete the ‘Reflection’ section of their worksheets. Give students up to 10 minutes to reflect within their groups, then bring the class together and ask each group to share their reflections on the engineering design process. Students should describe how mass, materials, and ramp conditions influenced motion, how potential energy converted to kinetic energy, and how tradeoffs affected outcomes.

Pre-Activity Assessment

Identifying Criteria & Constraints: Tell students that engineers often work with criteria and constraints. Have students think-pair-share about what these are in the context of an engineer designing a roller coaster. (Possible criteria: include a large drop, lots of loops, long stretch so the cart can go fast, etc. Possible constraints: cost, amount of space available to build the coaster, other similar coasters nearby, safety requirements, etc.) Encourage students to consider how factors like energy, motion, and forces might influence design choices.

Activity Embedded Assessment

Recording Design & Data: Have students complete the Design Worksheet (PDF) as they work through the activity. Ensure each student has a chance to test their mint-mobiles and record data individually. Students should document observations about how changes in mass, ramp angle, or other design choices influence the distance traveled, connecting these to energy conversion, friction, and Newton’s laws of motion.

Post-Activity Assessment

Class Discussion: Ask students the following questions and discuss as a class.

• What were the independent, dependent and control variables in the Variables Test part of the activity? (Answer: The independent variable was the mass of the mint-mobile, the dependent variable was the distance the cart travels, and the controls included the length and angle of the ramp.)
• How did your groups’ predicted distance compare to the actual distance? Why were your predictions close/far from the actual value? (Answers may vary. Possible answer: The predicted distance was shorter than the actual distance. Our prediction didn’t take into account how bumpy the trip down the ramp was, which slowed down our mint-mobile and caused it to not go as far.)
• How could you design a mint-mobile to travel a longer distance than any of the mint-mobiles you made or saw today? (Have students brainstorm and share ideas; remind students that engineers always look for ways to improve their designs, including collaborating with other teams.)
• What steps did you follow in the engineering design process? (Have students share their own experiences, in their own words, that describe the repeating seven-step-loop of the engineering design process: ask, imagine, research, plan, create, test, and improve.)

If students use the straws as axles and find that the straws are bigger than the candy hole diameter, they can use scissors to slit the straws lengthwise, then curl and tape them tighter (so they are narrower) to make straws that spin better in the candy holes.

The mint-mobiles roll best on smooth surfaces rather than the textured surfaces of carpets, so build the testing ramp on a tabletop and place a thin poster board at the bottom of the ramp.

If students' mint-mobiles are quite small, they may not be able to hold many pennies. If so, tape on a few pennies at a time to determine a reasonable maximum amount.