Mousetrap Car

Our Mousetrap car looked like this:

Our car came in 2nd place out of all the cars in our class with a time of 3.72 seconds (this is how long it took the car to go five meters.)

Physics of a Mousetrap Car:

Newton’s First Law- an object in motion tends to stay in motion and an object at rest will stay at rest unless acted upon by an outside force

Newton’s Second Law – a=f/m

The greater the force and the less the mass of the car, the greater the acceleration it will have; this is according to Newton’s 2^nd law because acceleration is directly proportional to force and inversely proportional to mass. We took this into account when constructing our car. Although we did not do much to increase the force, mainly relying on the force generated by the mousetrap being triggered, we considered the mass in every aspect of building our car. We tried to use all lightweight materials. We hollowed out Bic pens because they are very light plastic, we used CD’s because they are thin and lightweight, and we did not add anything to the base and used only the mass of the mousetrap itself because this was as light as we could possibly build the base.

Newton’s Third Law – every action has an equal and opposite reaction. When the wheels of the car (formed by CD’s in this case) push the ground backwards, the ground pushes the wheels forward so the car moves.

The two types of friction present are static and kinetic. The problems we encountered with friction occurred when we put balloons on all four of the wheels. This slowed the car down a lot and made it very hard for the car to move because there was too much friction with the ground. We used friction to our advantage by putting balloons around the back wheels to stabilize the CD’s so that they do not slide around on the ground. This allows us to have a better force to propel the car coming form the back rather than the front, which would be less effective.

We based our design off another homemade design we found on the Internet. This design used four wheels which we thought made sense as the car would be stable and not wobbly as we suspected it would be with less wheels. We had two axles with two wheels on each axle. Our wheels were CD’s. As I said before, we figured that lightweight materials would be best so CD’s made sense to us to use. Also they were larger than a different kind of wheel, such as a bottle cap, would have been. The larger something is in diameter, the further its edges are from the center or axis of rotation. This means the object will have a greater tangential velocity and thus cover more distance in the same amount of time. Our CD’s definitely had a greater tangential velocity than most of the other cars’ wheels, as they were much smaller. I think this added greatly to our car’s speed.

Rotational inertia is an object’s resistance to spin. The more rotational inertia an object has, the harder it is for the object to spin. We wanted to decrease the rotational inertia of our car’s wheels and axles as much as we could. We put balloons around the edge of the back two wheels so they would have better traction but we did not put them around the front wheels because we found that this added to the rotational inertia of the CD’s and prevented the car from making any measurable progress. The rotational velocity of both the front and back sets of wheels was made to be the same by using identical axles and wheels. We wanted the wheels to move together as we would be able to better predict results.

Conservation of energy is the principal that the change in potential energy is equal to the change in kinetic energy as an object goes from being at rest to being in movement. The maximum amount of kinetic energy that our car could possibly have would be the amount of potential energy it had when it was at rest. Potential energy is equal to mass x gravity x height. In order to have potential energy, the object must have some height to it. Also, the maximum amount of potential energy that our car would have would be when it was at rest. Right before we triggered the mousetrap would be a great moment to determine the potential energy from. As the car launches into movement after being triggered, the potential energy decreases and the kinetic energy increases. Energy is the car’s ability to do work so as the kinetic energy of the car increased, the work hat the car was doing increased as well (∆KE = work). Work is equal to force x distance, so the car was exerting more force and going a further distance with the increase of the work it was doing. Following this logic, it would be best to have a car with a lot of potential energy to start off so that it will have more kinetic energy, do more work, and go further.

The lever arm on our car was the mousetrap itself. The metal part that physically snaps was what rotated on our car. Instead of using the torque from this lever arm to lead the car the entire race, as many of our peers did, we used the lever arm to spin the axle at its moment of triggering. To do so we attached a string to the lever arm which was wound around the back axle. The axle spun as the string unraveled and set the wheels into motion. The lever arm was our main pulling force even though it only acted for a short amount of time. Power is work over time. The power output of our car was not very much as it did not go very fast. This may have been due to the fact that our lever arm did not continue to pull the car throughout the entirety of its run. However, it did set the car into motion and thus initiated any power generated.

To calculate the exact amount of work that the spring did on the car, we would need to know the distance it took to stop the spring. We would also need to know the exact force it exerted. Since we do not know the distance, we cannot calculate the work the spring did or if it even did any work. To calculate the amount of potential energy stored in the spring, we would need to know the height and to find this we would need to find a solid starting point for measuring this distance. If we do not know the potential energy, we cannot calculate the kinetic energy easily because ∆KE = ∆PE. Also ∆KE = work, so without knowing the work or the potential energy of the spring, it would be difficult to find the kinetic energy.

Reflection:

Our final design only differed slightly from our original design. We replaced the soda caps with zip ties because the caps were not the right shape and did not fit the axle or properly keep the axle from moving. Their original purpose was to stabilize the axles so that the CD’s would move in one line. We put zip ties on because they stayed in place so that the axles did not move too much (they were on the inside of the eye hooks that attached the axles to the mousetrap so that the axle could not move much horizontally as the zip ties would catch at the eye hooks). Also, zip ties were small enough that they did not hit the mousetrap base as they rotated with the axle, which was a problem we encountered with the soda caps. I would not recommend using soda caps for this purpose as they were difficult to work with.

The major problem with our car was that it was unreliable and did not follow a straight path. The sources of this problem were the wheels as they were only slightly off center and turned a bit to the side. To solve this, we tried to create tape barriers to correct the curving of the CD’s. This did not work very well but it got the job done. I would have liked to reconstruct the way the CD’s were attached to the axles however this problem did not arise until the final day of the project and time was of the essence.

If I were to do this project again, I would like to try a design similar to that of most of my peers. Most of the other groups created a lever arm that rotated as the car covered distance and gradually unraveled the string to make the axle spin. I am not sure how much faster this would make our car but I think if it was well built it would be a great design. However, I liked my group’s design a lot as it was unique and worked very well in theory. The main things I would change are that I would make the base longer so that the CD wheels had no possibility of touching. This was a slight problem for us that we adjusted by placing the CD’s at different intervals along the axles but it did cause a slight road bump. I would also find a more reliable way to attach the CD’s to the Bic pen axles. We used masking tape wound thickly around the pens to make a thick thing that the CD could fit snugly around as it would rotate with the axles. The tape bunched and snagged when we tried to put the CD on it. I would like to find a way to perfect this and make it more practical and easier to make.

Here are some videos of my group's mousetrap car in action!