This lesson is based on California's Middle School Integrated Model of NGSS.
NGSS Performance Expectation (PE): (MSETS12) Evaluate competing design solutions using a systematic process to determine how well they meet the criteria and constraints of the problem.
Disciplinary Core Ideas (DCI): ETS1.B Developing Possible Solutions  A solution need to be tested, and then modified on the basis of test results, in order to improve it.
Science and Engineering Practice SP4 Analyzing and Interpreting Data  for this activity students will build a mousetrap power car that is able to travel a minimum of eight meters. The students can either close to build a car based on provided instructions or they may opt to build their own design. As the car build proceeds, students will have an opportunity to modify their design to maximize the distance it is able to travel.
Influence of Science, Engineering, and Technology on Society and the Natural World  The uses of technologies and any limitations on their use are driven by individual or societal needs, desires, and values; by the findings of scientific research; and by differences in such factors as climate, natural resources, and economic conditions.
The performance requirements of this project require a mousetrap powered car to travel a minimum of eight meters. Students will use what they have learned of Newton's Three Laws to design and built a car that maximizes the effect of the mousetrap spring, minimize the overall mass to increase force, and reduce as much internal frication as possible.
This project provides challenge to two diverse sets of students. Inexperienced students can close to build a mousetrap car based on provided instruction, thereby guaranteeing success while still engaging students in the engineering process. Experienced students are able to challenge themselves by building any design that uses a mousetrap spring. Both populations of students can test their designs and make improvements to its performance.
I give my students two choices to build. (1) They may choose to build a mousetrap racer from preplanned directions and use a kit or (2) they can build whatever they can engineer with a mousetrap spring. This project requires my students to work with a partner. I give students several weeks notice so they can choose a partner of their liking and they are allowed to pick a student from a different class period (which they find very exciting).
TIP: To make the process transparent I have students give me the names of their partner and I record their choices. Often I have students partner with someone without first checking if that student wants to be their partner. I'm pretty flexible about the process and I often have partnerships dissolve halfway through the project. My only hard requirement is that a partnership can only exist between TWO students. I don't allow partnership of three or more. I do have a lot of students who would rather work by themselves, which I don't encourage but do allow.
The kit is from Kelvin and is called Economical BuildABetter Mousetrap Racer Bulk Parts. I buy the kits in bulk for 100 individual kits and supply old CDs as the racer's wheels. Front start to finish the kids get about six weeks to build their racers.
The difference between options 1 (Kelvin kit) and 2 (student engineered) is in the scoring. I place more value on the student engineering option. This project is worth 75 points, but if the students choose to build from the kit the highest score they can achieve is 67 points (B+). If they want to earn an A grade they will have to engineer their own car (option 2). I do offer extra credit for superior performance (+1 point for every meter after 8 meters) that may in fact push their grade past a B+. If they so wish my students are allowed to modify the kits. My only rule is that is must be powered by a mousetrap spring (no rat traps) and no model rocket engines, which I think is self explanatory but I always have to enforce.
Alternate Designs
The kits come with directions, but I prefer the directions I made and hand out to the students.
How to build a Mousetrap Racer
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On the day of the race I set down a length of masking tape and mark off 10 meters with additional marks every 10 centimeters.
The students are allowed to make steering adjustments as the racer is heading down the track. If a student partnership is split between two class periods, then both students get to race the car and the best distance is recorded.
Along with racing their cars the students have to document their experience in their Science Interactive Notebooks. Their writeups must include a diagram of their car and ten sentences describing how they built their car, how it functions, how well it performed, and what design changes they would make to improve their car's performance. I have my students number each sentence after the period, so as not to have the students make a direct list, but rather a free flowing paragraph.
Pass out Mousetrap Racers Grade Sheet to each set of partners (one/partnership). If the partnership is split between class periods then the car is raced twice and the best distance is recorded.
The cars are scored based on the following three categories:
Overall, I grade pretty easily. I figure that after all the time the students have invested they deserve high scores. I also award extra points if the car is able to travel over 8 meters. Typically the lowest score I give out for a functioning car that has some obvious effort put into it and was turned in on time is a B.
If the car is of an original design (did not follow any directions) they are able to receive more points in the 'appearance' category then a student who may have followed the provided directions.
I score the cars at the moment they are racing and notify the student of their score. I keep the Mousetrap Racers Grade Sheet until I have had a chance to record their scores in my grade book.