Physics Principles of a Mousetrap Car

Before Christmas Break, a fun project we were assigned was to design and build a mousetrap powered car. To begin with, my partner and I researched some simple mousetrap car designs, that we could build off of to maximize the distance our car could go. We decided that we would use a thin foam board as the body of our car, as it is a lightweight, and easily attainable material. For the wheels of our car, we initially thought that old CDs would be the best option, and for the axles we thought that we should use wooden skewers. Lastly, we were unsure of the best type of string/line to use to power the car, so we decided that we would experiment with different types of string.

The most difficult part to this project, was just getting the car to move. The propulsion of the car had to be solely from the spring of the mousetrap, therefore we had to find a way for the spring to spin the back wheels of the car. To power our car, we attached the mousetrap to the front of the body of our car, with the hammer of the mousetrap being furthest away from the back of the back wheels.  Then we took fishing line, and tied it to the hammer. With the fishing line, we stetted the line to the back end of the car, and tied the line around the  axle of the back wheels, which spin freely, being supported in drinking straws. I also glued a small piece of toothpick to the back axle, so that the string will wind around the axle. So to make the car move, one person has to hold the hammer back to tighten the spring, and the other person has to start winding the slack of the line around the back axle. Then when both people let go, the spring will begin to pull back into its original position, and the spring force will spin the back wheels to make the car move.

http://www.millerandlevine.com/km/evol/DI/Mousetrap.html

We thought that to maximize the distance the car could go, we would need to use large back wheels, so that one turn of the axle, would make the car  go further. In addition, we thought that making the car longer, and attaching the mousetrap further away from the back wheels of the car, would allow more slack in the string to wind around the wheels, and therefore make the car go further.

When I went shopping for supplies, I found different sizes of foam circles, that are usually used for floral arrangements, but would work well as wheels because they are 1 inch thick, which are much more stable than CDs. I also decided to buy 2 different sizes because the back wheels are the only ones that need to be large as the car is powered by the back axle, therefore for the front wheels I decided to buy smaller pieces of foam as it is lighter. As I also mentioned before, I decided to use fishing line because it is strong, yet a little bit stretchy which will allow us to wind the string/wheels as much as possible.

The car eventually looked as seen above. You might notice in the picture, that all four wheels are covered in masking tape. This is to deal with the issue of friction.  We did this to create more grip on the wheels, and to maximize the distance of each spin of the axle. Although a lever arm would have been beneficial to make the car go further, as it increases the pulling  force on the fishing line (which makes the wheels spin more and make the car go further), I decided not to put a lever arm on the hammer of the mousetrap as I found it very difficult to find a material that would be able to stick to the hammer, be stiff enough to withstand the pull from the spring, and be light enough to not throw off the balance of the car.

On the last day of school before winter break, we performed trial runs to see how far our cars could go. I wasn't expecting much, however our car was actually able to travel approximately 8.3 m which is much further than I thought it could go. An observation I made was that our car moved very slowly when it started to move, but actually went fairly far. With regarding power, this shows that our car is not very efficient because it moves so slowly, which means that lots of power is being wasted. One adjustment that I would make would be potentially adding a lever arm, however I need to find the right material to use to make the car run more effectively, and travel a longer distance.

Newton's Law and Momentum Application

Common practice problems we do when dealing with momentum involve car collisions, but not only does this involve physics theoretically, but also explains some safety features on cars. Newton's first law states that an object in motion will stay in motion, and an object at rest will stay at rest unless acted on by an unbalanced force. For cars, this is true as cars will continue moving unless the brake is applied, or it collides with an object. Newton's third law states that for every action, there is an opposite and equal reaction. For example, if a car and a truck collide, they will experience the same magnitude of force, but the driver of the car will be worse off because of Newton's second law, which means that the car experiences a larger acceleration due to the truck having a greater mass (F=ma).

Since we know that momentum is conserved, this means that it can only be transferred from one object to another. The law of conservation of momentum states that in a closed system, the total momentum stays the same. In a car collision, this means that if one car loses x amount of momentum, the other car will gain that same amount, as it is being transferred.

This explains the reason for seat belts in vehicles. Going back to Newton's First Law, and object in motion will stay in motion. This means that when a car hits an object, the car will be acted on by an unbalanced force and stop, however the driver will continue forward. We wear seat belts to hold the driver or passengers back from flying forward. If we don't wear seat belts, a force will act on the person to slow them down, however this force will come from the windshield causing much more damage, and possibly throwing the person out of the car. The seat belt provides a softer, stopping force to minimize the damage in car accidents.

http://hyperphysics.phy-astr.gsu.edu/hbase/imgmec/carcr5.gif

Airbags are placed in cars to reduce the speed of the passenger in a car during a collision. The passenger of the car has momentum because of the passenger's mass and velocity. In order to stop this momentum, another force must act on the passenger. Instead of the passenger hitting the dashboard or windshield of the car, airbags are used to soften the blow and minimize the injuries of the person. The airbag provides a change in momentum, also known as impulse. The more time a force has to act on an object, the less damage to the passenger. 

http://www.carsp.ca/hitech/airbags.jpg

In addition to having airbags and seat belts, car also have crumple zones to reduce the impact of collisions. The front and back ends of cars have what are called crumple zones. These increase the amount of time that the force acts on the car during a collision. With an increased amount of time, the force acts over a longer period of time, which reduces the affects of the force when it reaches the middle of the car, where the passengers are. Crumple zones help minimize the injuries sustained in car collisions. 

http://tristanmac.tripod.com/sitebuildercontent/sitebuilderpictures/crumples.gif

Lastly, another safety feature of a car are the headrests. Not only do headrests provide comfort, but they also prevent neck injuries during collisions. In some collisions, when a car hits another object, or hit the brakes, the car stops moving, but from what we know from Newton's first law, the driver's body will continue moving. In this case, the driver's head could be thrown either forwards or backwards, and once the head is thrown one way, it will naturally be thrown the other way due to the way our neck muscles react. Headrests help prevent neck injuries by providing a softer way to stop the head and neck of a driver. 

http://tristanmac.tripod.com/sitebuildercontent/sitebuilderpictures/headrest.gif