Mousetrap(ped) Car!

This past week in Physics, we, in pairs, built cars that were powered by mousetraps! The purpose of the project was see how all of the concepts we learned over the course of the year could be applied to achieve our goal of building a car that would go at least 5 meters. The group with the fastest car would win a milkshake and a question omitted from the final exam.

Final mousetrap car design made by me and Annie.

So... How does physics apply to this project?

Newton's 1st Law
The car in motion will continue to be in motion unless a force pushes it backwards. In the case of the mousetrap car, this force could come from friction between the axles and the frame, friction on the mousetrap spring or friction between the wheels and the ground.

The only friction on the car that is actually helpful is the friction between the wheels and the ground. As you'll see explained below, the larger the force between the wheels and the ground are, the more the car will be propelled. In order to create more friction there, we added electrical tape to the wheels. The tape had the same gripping effect on the ground that rubber tires have on roads. Contrarily, too much friction between the axles and the frame would cause the wheels to stop turning entirely.

Newton's 2nd Law
Acceleration is equal to force over mass. The force causes the car's acceleration but if there is too much mass, the car will not be able to accelerate as much as it would be able to with a smaller mass. Therefore, we wanted our frame to be as light as possible.

Newton's 3rd Law
In this situation the action-reaction pairs are: car pushes ground back and ground pushes car forward. This causes a reaction from the axle torque which causes wheel torque. Therefore, the bigger the wheels are, the bigger the lever arm must be and the bigger the torque will be. It is important to remember that torque is what causes rotation. Since we wanted our car to follow a straight course, we wanted our car to have the smallest torque possible.

My partner and I built our car using:

• Small wooden discs for front wheels-- The size of the front wheels is not as important as the size of the back wheels. They mostly serve the purpose of keeping the car moving in a straight line.
• Wooden dowels for axles-- We used a narrow Poplar dowel for the front axle and a thicker Oak axle for the back axle.
• Balsa wood for the frame-- We drilled holes with circumferences a little larger than those of our dowels to allow for easy rotation. Balsa wood worked well for our frame because it is light, so the wheels don't have to support very much weight.
• CDs for back wheels-- CDs are pretty commonly used for wheels on mousetrap cars because they are thin, light and easy to find. However, we needed to wrap our wheels with electrical tape in order to create friction between the ground and our wheels.
• Poplar dowel for the lever arm-- We attached a 2-foot long Poplar wood dowel to the arm of the mousetrap to act as our lever arm. Our car was powered by our back wheels, so we attached a string to the top of our lever with a glue gun and wound it around the back axle. Therefore, when the trap was released, the string would cause the back wheels to turn. This works because the Elastic Potential Energy stored in the spring is converted to Kinetic Energy when it is attached to the spinning axle. In reality, the lever of the mousetrap doesn't actually do work on the car, it just applies force to the axle. We used the torque of the spring to cause the force. We also increased the length of our lever arm from 1.5 feet to 2 feet because we knew that it would increase the distance and time that the force would act on the axle, not actually increasing the force at all.
• Rubber bands and hot glue-- We made sure that our wheels were stable by putting rubber bands on either side of the wheels and gluing the rubber bands down.

Remember:

• We can't calculate the amount of work the spring does on the car because the force of the spring isn't parallel to the axle and we know that work = force multiplied by parallel distance.
• We also can't calculate the amount of PE stored in the spring and the amount of KE the car used because we can only find the average velocity, not the instantaneous speed of the car, because it accelerates at different intervals over the course. Additionally, PE depends on vertical position, and the car only covered a horizontal distance.
• We can't calculate the force of the spring exerted on the car to accelerate it because we don't have a constant acceleration of the car.
• Rotational inertia is the property of an object to resist changes in spin. Rotational velocity is the amount of rotations made over a certain amount of time. Tangential velocity is the linear speed of something moving along a circular path. This means that we needed to find wheels that had a low rotational inertia but a high tangential velocity in order to have wheels that would spin quickly and easily. Our first wheels had a rotational inertia that was too high, so we downsized to CDs to maintain a higher tangential velocity than our small front wheels and with a low rotational inertia.
• The Law of Conservation of Energy states that the input amount of energy is equal to the output. Therefore, the amount of potential energy stored in the spring to be converted to kinetic energy when it started to move was the maximum amount of energy that the car could have. In order to maximize the amount of energy stored in the spring, we pulled the lever to its tightest position and wound the string attached to the lever arm completely taut. Therefore, we were able to house the most potential energy in the spring's release as possible.

Here is a video of the car in action! We came in 3rd place, covering 5m in 3.77s.






Originally, we intended to use wooden discs (larger than CDs) for our back wheels because we knew that they would have a high rotational velocity. However, the wheels were so big that their rotational inertia was high enough to make them ineffective. We also increased the length of our lever arm to compensate for the rotational inertia of our CD wheels. Since the wheels were large, our lever arm needed to apply more force to the axle in order make sure that the car would travel 5m. Additionally, our original design had axles underneath the body of the car, attached with hooks. This was not as efficient because the hooks were much larger than the axles, which allowed them to wobble around within the hooks. We fixed this by drilling holes into our new frame that were marginally wider than the axles. The use of the frame also made the car's body more narrow and less subject to air resistance.

My partner and I ended up restarting our project about three times. Initially, I thought that this would be easy-peasy because I made a mousetrap car for another science class about 4 years ago. However, that time I had my science/math geek dad to help me. This time, my partner and I were on our own. Almost all of the issues we encountered resulted from our lack of true preparation. Unlike some of our classmates, we didn't have a specific set of instructions that we were following. Essentially, we were just putting parts together and hoping for the best. The first major goal we had was for our car to go to the finish line after we pushed it. This took until the day before it was due because we didn't think through our entire plan before beginning to build. Since we didn't have a concrete idea of how our car would work and how we would execute its construction, we ran into a lot of issues like gluing things on before considering what that would keep us from being able to do. Another issue we had was that once we got our car running off of the mousetrap, it would stop just before the 5m mark. We didn't stop to think about the physics as much as we should have. I think that had we considered that a little more during each step of the process, we would have realized that our lever arm needed to be about six inches longer than it originally was in order to have a greater force on our axle. The most frustrating mistakes, however, were just results of us rushing through the process. We were each frustrated, stressed and tired of burning ourselves with the glue gun and it was easy to stop ourselves from taking the time to do things correctly. One vivid memory of an example of this is when, after starting our frame over for the second time, the wood cracked. It was the afternoon before the car was due and I felt like the world had come to an end. I had been confident that finishing the frame would mean success for us. Unfortunately, we were back to square one. Again. After that, my partner and I measured everything meticulously and devised a methodical plan for drilling holes for the axles. As a team, we moved up drill-bit size by drill-bit size until we reached our goal. That experience helped me realize that doing things carefully would keep us from having to do things more than once.

In the future, I would follow clear instructions and take my time. I truly believe that my partner, Annie, and I would have finished much sooner had we been methodical from the start. I think that taking the time to make sure that everything was perfectly measured and even may have increased our speed. Additionally, I would have used CDs for the front wheels as well and made the frame a little bit smaller. I hope that the next time we have a building project, I will make sure that I have laid out all of my steps and measurements before I actually begin to build.