Kenna and I built a mousetrap car that traveled 5 meters in 2.84 seconds (our classes fastest!). In the picture below, you can see our mousetrap car. It is not very clear in the picture, but the body is made of hanger rods with a thin circular metal bar as the axis. We used the hanger rods because they were light weight and were easily bendable making working with them easier. We used the thin metal circular bar because it fit perfectly in the drilled holes on the hanger and also was small enough that we could wrap the string around it and attach it to the mousetrap and it wouldn't fail to unravel. To make the mousetrap roll, we
drilled holes in the bottle caps and put the rod through, letting it rotate easier. We didn't use a lever arm really because our model worked great without one, instead we attached the string to the rod and the mousetrap and when it set off, it worked great and produced enough force to send the mousetrap on its way for more than 5 meters and fast too!
Physics of the Mousetrap Car:
For this project, Newtons first law was seen when we used balloons to give friction to help the mousetrap car move faster and longer. The balloons were meant to add friction because there was not enough with just the CD's slippery sides. There was more inertia because there was friction to add traction so that the car moved farther without slowing down. We saw Newton's second law during this lab as well. Newton's second law says that acceleration is inversely proportional to mass, and is directly proportional to force. So, since we made the mousetrap car lightweight, we needed to have a large force so that the car would accelerate fast. Because we had a large force divided by a small mass, the acceleration was large, making our car move fast. Newton's third law was of course applied because what would physics be without all three of Newton's laws? For every action there is a reaction. When the mousetrap went off, the string started unraveling, and this caused the wheels to push the ground backwards, ultimately letting the ground push the car forward.
To solve the problem of too much or too little friction, we wanted the perfect amount, so we used balloons to add friction because the CD alone would not work with its thin and slippery sides. This gave the wheels more traction to help stick to the ground. We also made sure to avoid anything dragging because that would only create more resistance. There was static friction with the wheels and the ground and there was kinetic friction when the mousetrap car was in motion and is stopping.
We used three wheels because we didn't need more than that. We used two wheels in the back because we needed the axel to be attached to two wheels. This was necessary because we needed an axel to wind up the string, one wheel wouldn't have given us room to do this. In the front we didn't need an axel, so we only used one wheel. We decided to use CDs because they were thin and sliced though the air, and also, because they had a medium diameter, and because they were this size, they had very little rotational inertia.
The law of conservation of energy tells us that energy can not be lost or created, only transferred. The mousetrap car did this because when we transferred our energy to the car by snapping the mousetrap, the potential energy was converted into kinetic energy that made the car go, the energy was not lost, but simply transferred to the form of something like heat, and to the ground with friction.
For our car, we didn't use a lever arm, or you could say that we did but it was short because we only used the string attached to the trap. We decided to do this because we thought that that lever arm would only slow it down because it it took a long time to move and unravel the string, and thus slow down the wheels. Our lever arm was short and fast because as soon as the trap snapped, the axel was put into motion making the force so large that it continued to move the wheels week after the trap had gone.
Rotational inertia is the number of rotations in a unit of time. We wanted all of our wheels to have the same rotational inertia because it made it easier. Then, another factor in deciding the kinds of wheels we wanted, was the tangential speed, how much distance is covered. We knew that because we had a small axel, the string unraveling would be quick and since the wheels were attached to the axel, we wanted larger wheels so that for every time the axel rotated once, the wheels would rotate, and this would make the car go fast. There was a lot of force from the trap snapping, so when the axel started to rotate, it continued to and the is caused the wheels to just as fast. Rotational inertia is when an object resists changes in the spin of an object. We chose to have out axel close to our wheels so that there would no be a lag time in which the trap shaped but we had to wait for the wheels to start turning because they were so far away. This proved to be helpful when the wheel immediately started rotating and moving rather quickly
We cannot calculate the amount of work on the spring because the spring is not parallel to the other parts of the car, and we cannot calculate the potential energy because we know that PE = mgh and we don't know the height, and would have to find the height. We can't calculate the kinetic energy because we don't know the velocity and once again would have to find the mass. Finally, we cannot calculate the acceleration of the spring because we don't know the distance the spring traveled or the speed it traveled.
Reflection:
In our first design, we were going to use a wooden plank, but instead found that the hanger rods were more useful because they were light weight and we could attach an axel to them easier. We were also going to use a smaller wheel in the front, but decided that if all the wheels had the same rotational inertia it would be easier.
One thing we really had trouble with when working on our mousetrap car was getting the axel straight so that the wheels both went straight and didn't curve. We had to red rill many times because the car would veer in one direction. Eventually we made the hole that the metal rod fit through smaller so that it wouldn't have room to move around. We also re-drilled and made the holes that the axel fit in straighter and this helped to get the car to move straighter.
One thing that I wish I had done differently with this project was make everything perfect the first time so that I won't have to come back and redo it when it doesn't work in the end. If we had drilled the holes in line with each other in the first place, then we wouldn't have run into that problem in the end that forced us to come in outside of class.
