Olivia's Physics Blog: Mousetrap Cars

Mousetrap Cars

Mousetrap cars can vary in their designs. Although each design has their similarities, the two main designs for mousetrap cars are for long distance or speed.

How It Works

Mousetrap cars are powered by the spring of the mousetrap. When the mousetrap snaps back, the lever causes a release of energy. The lever is attached to a string that is wound around the back axle of the car. Therefore, when the lever snaps with the mousetrap, the string unwinds, which spins the axle and makes the car move.

Lever

One of the main deciding factors between distance cars and speed cars is the lever. Levers control the car's acceleration and travel distance. The different lengths of levers influence the pulling force that is applied to the drive axle and change how much string can be pulled off of the axle to make the car go far. For distance cars, long levers are best because more string can be pulled off of the axle, to make the wheels turn more and the car travel farther. On the other hand, a shorter lever is better for speed cars because it will provide more force and power.

Wheels

The size of the wheels is an important part of mousetrap cars. Large wheels travel farther with each rotation than a smaller wheel. However, large wheels also require more force to initiate motion. Therefore, really large wheels will be ineffective, because the force from the mousetrap will not be enough to keep the wheels in motion.

On the other hand, smaller wheels will cover a shorter distance per rotation, but will be significantly easier to accelerate than larger wheels. It will require less force to achieve the same acceleration of a large wheel.

Speed

Long Distance

Axles

The axles of the mousetrap car influence a number of factors and are very important. They can cause the car to go faster or slower, a short distance or long distance, and can decrease or increase friction in the mousetrap car.

Another significant factor of mousetrap cars is the diameter of the axle compared to the diameter of the wheel. A small diameter of the axle in comparison to the wheel will require more force to get started but will cover more ground per rotation. For long distance cars, a large diameter wheel with a small diameter axel is best. A small diameter wheel with a large diameter axel is the best for speed cars.

Speed

Long Distance

Friction

Friction is the force that is opposite of the relative motion of two surfaces that are in contact with each other. Friction would make the mousetrap car slow down, so it is best to reduce friction as much as possible. Moving parts will cause friction. Since friction will increase with the amount of moving components, the goal is to make the design of the mousetrap car as simple as possible (but still making it functional).

Gearing

The gearing of the mousetrap car can affect its travel distance and acceleration. It is controlled by the length of the lever and the ratio between wheel diameter and axel diameter. Extremely long distance mousetrap cars are geared to use the smallest amount of energy to go very far with a lot of efficiency.

Traction

The traction, or the grip of the car's wheels on the floor, will determine the mousetrap car's maximum acceleration. It is important for the wheels to have friction so they do not spin out when the mousetrap is triggered. If the wheels don't have enough traction, the car will not be able to accelerate like it was supposed to. Tires will naturally have traction because of their rubber edges, but CDs or other options may not.To add traction to CD wheels, rubber or strips of balloon can be added to the outside edge of the wheels.

Inertia

Inertia is also significant in the building of a mousetrap car. Inertia is the resistance that an object has to a change in motion. The more inertia an object has, the more force will be required to change its state of motion. A heavy mousetrap will need more force to cause motion that a light mousetrap to achieve equal acceleration. Light mousetrap cars can use less energy at the beginning and have longer levers for increasing the travel distance.

Rotational Inertia

Rotational inertia is similar to inertia, and is an object's resistance to a change in rotation. The greater distance between the mass of the wheel and the point of rotation, the greater the rotation inertia is. That means that large wheels have more rotational inertia than smaller wheels. The more rotational inertia the wheels have, the more force will be required the change the state of rotation. Wheels with large amounts of rotational inertia are helpful in some ways, because once the wheels are moving they will be difficult to stop. These types of wheels are helpful for long distance mousetrap cars. However, wheels with large amounts of rotational inertia will be a disadvantage for speed mousetrap cars because they will be hard to get in motion.