Yep, it’s the name of a children’s book, but it’s also how Kat and I will fondly remember our mousetrap car.
First: A few nifty things about our little engine - She came in last place with a time of 19.98 (We’re not really sure how she managed to go so slow and still make it 5 meters but you go girl!)
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And now we get down to the real reason Mrs. Lawrence had use build cars with only a mousetrap and 5 dollars. Nope, it wasn’t to torture us or teach us the meaning of “planning beforehand”. It was all about PHYSICS! Naturally anything in physics has got to begin with Newton’s 3 laws of motion. Here’s how they each apply to the car.
Newton’s 1st Law- this is the law of inertia which states that an object at rest will remain at rest until a force acts to move it and an object in motion will stay in motion until a force acts to stop it. Our car wanted to stay moving so all we had to do to keep it that way was decrease the forces acting to stop it, such as friction.
Newton’s 2nd Law- this is the law of acceleration which states that acceleration is equal to fnet over mass or a= fnet/m. By making our car lighter we could increase its acceleration because mass is inversely proportional to acceleration. Much the same way if we could increase the force we could also increase the acceleration.
Newton’s 3rd Law- this is the law of action and reaction which states that for every action there as an equal but opposite reaction. By rotating our axle backwards and winding the string tighter we knew the car would rotate faster forwards (although we didn’t demonstrate this one too well).
Oh yeah, remember that thing called friction? It came back. We faced two different instances of friction in our building of the car, the friction the wheels had with the ground and the friction the axles had with the frame of the car. We needed to make wheels that would have enough friction with the ground to move the car forward but not so much that it caused the car to slow down. We used CD’s like many of the other groups because they seemed to have a perfect balance. To enhance this friction some groups used balloons which seemed like a great idea. The other type of friction, the one the frame had with the axles determined how easily the axle could turn within the frame. Here we wanted as little friction as possible, something we learned was not too easy to do with cardboard.
When we were choosing wheels we first thought about using three wheels, but for the sake of balance we upgraded to 4 wheels by the end of the project. We used CD’s on every axle Having larger wheels would move the car a longer distance but would take more time to rotate, inversely, using smaller wheels would take less time to rotate but would go a smaller distance.
When the car was at rest it had potential energy and as it traveled that potential energy was converted into kinetic energy. Because of the law of conservation of momentum we knew that work in would equal work out. But we knew our car wasn’t as efficient as it could be because the distance it traveled did not equal the force we put in. This means we converted a lot of energy into heat.
Our lever arm was around 7 inches. We had a lot of problems figuring out how long we wanted our lever arm to be. We wanted the lever arm to be long so the power output of our car would be higher because it would travel a longer distance. However, we found that if the lever arm was too long it wouldn’t effectively pull the string. We chose 7 inches because it pulled the car without stopping the string.
Rotational Inertia, Rotational Velocity, and Tangential Velocity were all big factors relating to the wheels, we wanted them to have a low rotational inertia so they would spin easily but a higher rotational velocity so they would spin faster.
Work= f x d. But the catch is that those two forces have to be parallel. We cannot calculate many of the forces in the mousetrap car because they are not parallel.
REFLECTION
Our final design differed only in the sense that we ended up using 4 wheels instead of three. We had originally planned on using two CD’s and a record yet when we attempted to build the car it wasn’t stable enough to balance on its own so we changed the wheel structure to include four wheels.
The major problems we encountered with our little engine were caused by the lever arm. We could not get the lever arm to attach in a solid way and it kept ending and did not, therefore, have enough force to accelerate the rope and turn the axel. We used a lot of hot glue, tape, and determination to get our lever arm stable and then reinforced it with a second metal rod to keep it straight.
The main thing I would do to make this project more successful would be plan. Kat and I worked well as a team, but neither of us took the time to plan out when we were going to get materials and come in to work. Because of this we found ourselves working all day on Sunday to complete the project. The biggest thing I learned was the value of writing out a schedule and then sticking to it.
