Mousetrap Car Reflection

Mousetrap Car

In this blog-post I will be explaining the Physics behind my mousetrap car and how the final product came out!

1. It took my car 4.65 seconds to go 5m. It ended up in 5th place in my class

2 and 3. Below is a picture of Dylan and I holding our car. I have added labels to the vital and most visible parts of our car.

4. If you want to see our car race in action, watch the video below!! 

5. I will now go into more detail about the actual creation of our car and how each step had physics properties involved in it.

Physics Behind the Mousetrap Car

A) Each of Newton's three laws can be applied to the mousetrap car. I will list each one.

NEWTONS 1st LAW : Newton's first law states that "An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an outside force." Therefore, in this case, when we set the mousetrap and let it release, the car will want to continue to move until something else stops it. This 'something else' could be air particles, the frictions between the wheel and the ground, or even the wall (if you car does not go perfectly straight like mine). Although in this experiment, friction is the real and most common enemy. 

NEWTONS 2nd LAW : Newton's second law states that "Acceleration is produced when a force acts on a mass." This means that as the mass increases, you will need a greater amount of force to accelerate the object. If you remember, the formula for acceleration is acceleration = force / mass. This means that for every extra piece that we added on to our car, we were going to need a greater force to accelerate it. We tried to avoid having such a large mass by using the knitting needles as the main part of the car instead of a block of wood or something heavy. We knew that by having a light car, we wouldn't need such a strong force to give us an ample acceleration. 

NEWTONS 3rd LAW : Newton's third law states that "For every action, there is an equal and opposite re-action". For every action that occurs (lets say you pushing your friend) there is an equal and opposite re-action (meaning theoretically when you push your friend, they are pushing back against you with the exact amount of force that you exerted on them.) A perfect example to explain equal and opposite forces is the example of the horse and buggy. If you do not remember this example, I will quickly explain it again.

Horse and buggy- First of all, there is the force of the horse pulling on the buggy and the buggy pulling back on the horse. This creates an action and reaction pair and the forces and both equal and opposite. Then there is the horse pushing the ground back and the ground pushing the horse forward. This is also an action & reaction pair of equal and opposite forces. The last set of action and reaction pairs is between the buggy pushing the earth forward and the earth pushing the buggy backwards. There is one thing you have to remember about the horse/ground reaction and the buggy/ground reaction. In order for the buggy to be moving (which in this case, let's say that it is), the arrows that you draw between the horse and the ground when making a diagram must be bigger than the set of arrows between the buggy and the ground.

There are multiple action/reaction pairs in my mousetrap car, but the main one that I think is most important to mention is the forces between the wheels and the ground. The wheels push the ground backwards while the ground pushes the car forward. 

B) This is the main reason why we need friction between the ground and the wheels of our car. We attempted to create more friction but wrapping the plastic lining of a balloon around each of the wheels. This is one of the ways to increase friction. The other way, which I did not consider until out class discussion, is that you can increase friction by simply increasing the weight of the car. Although, you have to be careful with how much mass you add on to the car because if it becomes too heavy, like I said earlier, you will need a much larger force to accelerate it which may not be able to be achieved when the weight gets to a certain intensity. 

C) I think that the size of the wheels on my mousetrap car were a great advantage to me. We used four, standard CD's for our wheels. Like many parts to this project, you had to find the right balance with the size of the wheels. This all ties in with torque, force and lever arm. If you put HUGE wheels on your car (like the size of records) you would have a HUGE torque, but you would need so much more rotational inertia in order to get the wheels to move. Oppositely, if you had very small wheels (like ones taken off of a toy car) you would barely need any rotational inertia to get them to move, but the torque would be super small. You have to find the right balance between all of these challenges. There were a few surprises that I learned from discussing the mousetrap car in class, after we raced them. I learned that the car will always have the same torque regardless of the size of the lever arm. This can be very misleading because we know that when you increase the lever arm, you require a smaller force and when you shorten the level arm, you need a larger force. So, while the lever arm does not affect the torque of the car, it DOES make the distance larger. Just like in previous cases, you have to find a balance between the length of the lever arm and the amount of force used to maintain that constant torque. 

D) There is a lot of Potential and Kinetic energy in our mousetrap cars. When you wrap the string around the axle generating potential energy. Whatever amount of potential energy the string has will be the maximum amount of kinetic energy it can turn into. Once you release the mouse trap and the string pulls the axle forward, that potential energy is transformed into kinetic energy. Some things that you DO NOT want in your car is wobbly wheels. This WASTES ENERGY!! It was funny to hear other groups exclaim that they could only get their car to go in a circle (although i'm sure this was very frustrating for the students). We learned that this is because when the wheels are slanting inwards, there is a centripetal force, JUST LIKE what we learned in that unit when a race car is attracted to the center of the track when it goes on the ramps. If you are trying to win a race in the straight, forward direction, you do NOT want a centripetal force. 
E) Rotational inertia, rotational velocity and tangential velocity played a large role in the wheels of our mousetrap car. The larger that your wheels are, the lower the rotational velocity will be. Meaning that oppositely, if you have very small wheels, you will have a great rotational velocity. The key was to find the perfect size of a wheel that the rotational inertia and rotational velocity coincided well and gave you a fast car. Larger wheels are much harder to spin and take more effort to complete a full rotation, but smaller wheels (while quicker to rotate) have to spin many more times which can result in a loss of energy. The tangential velocity is just the actual speed that the wheels cause the car to turn at. If you were lucky, you had a steady tangential velocity because you balanced out your rotational inertia and rotational velocity. 
F) When looking at our mousetrap cars, we realize something extremely surprising! We cannot calculate the amount of work that the string does when it pulls on the axle and consequently, causes the wheels to rotate and propels the car forward. This is because the force and distance are not parallel and we just recently learned that if the force and distance in an object are in perpendicular directions, we will not be able to calculate the work done. 

Reflection

A) The final design of our mousetrap car was extremely similar to our primary sketches. I think that we were very practical when deciding what materials we would use so there were not a bunch of drastic changes to our original thoughts.
B) The one main problem we had with our car was trying to get it to go straight. When attaching the wheels I suppose we should have been more precise about gluing them on securely so that they were facing directly forward. We attempted to overcome this issue by rearranging the already glued-in-place wheels, which was extremely difficult. Finally, even though we were not able to get the car to go perfectly straight, the wheels were fixed enough and we started the car at the right angle so that we were able to make it over the 5m mark.
C) If I was to do this project again, I would overall be more precise with my cutting and gluing because I think that every extra moment of care helps. One of the main things I would try to change it the friction of my wheels and the lever arm. heil I do need a certain length of lever arm to get the car to cross the set distance, I would experiment with the length and weight of it to try and have it go as fast as possible. 
D) If I were to do ANOTHER building process, I would remind myself that hard work always pays off. When planning, I would come up with a backup plan for when things did not go the way you had expected them to. I would plan out my goal and my plan if the goal was not completed because it was very stressful as the due date approached and we were still struggling to put the final touches on our car when I assumed that we would be done with ample time. Overall, I would try to maintain a good attitude like I did in this building project because it was genuinely a lot of fun.