1. Mousetrap Cars
Unit 11

2. What is a mousetrap car?
A mousetrap-powered racer is a vehicle that is powered by the energy of a wound-up mousetrap’s spring.
A mousetrap car’s basic design implies many of the basic laws of physics.

3. How does it work?
The most basic design is to tie one end of a string to the tip of a mousetrap’s snapper arm and then the other end of the string has a loop that is designed to “catch” a hook that is glued to a drive axle. Once the loop is placed over the axle hook, the string is wound around the drive axle by turning the wheels in the opposite direction to the vehicle’s intended motion. As the string is wound around the axle by the turning of the wheels, the snapper’s lever arm is pulled closer to the drive axle causing the mousetrap’s spring to “wind up” and store energy. When the drive wheels are released, the string is pulled off the drive axle by the mousetrap causing the wheels to rotate.

4. Basically, for a mouse traps to be effective, it must store a sufficient amount of potential energy which should be translated efficiently to kinetic energy. This is important because the kinetic energy must produce enough torque to create a rotational inertia that will move the wheel and axle of the vehicle.

5. All mousetrap cars follow the same basic principles regardless of their design. Whether designing a car built for speed or one made to travel long distances, the following physics concepts will be used:

6. Energy
Energy is what moves your vehicle. The energy of the mousetrap car is originally stored in the potential energy of the wound-up mouse trap’s spring. The spring releases its energy and the potential energy is changed into kinetic energy of motion. Along the way energy is lost to the surroundings in the form of work (heat and sound).

7. Power output
Power output is how quickly the energy stored in the mouse trap is released. There are really only two approaches to consider when building a vehicle:
Build a fast moving car that releases its energy quickly and then coasts as far as possible.
Build a slow moving car that releases its energy slowly over the entire pulling distance.

8. Inertia
Inertia is the resistance that an object has to a change in its state of motion. The more inertia an object has, the more force that will be required to change its state of motion. In theory, a heavy car will require more pulling force than a lighter car for equal acceleration. Lighter cars will be easier to accelerate but ideally will have less coasting distance than a heavy car at the same speed.

9. Rotational inertia
Rotational inertia is the resistance that a wheel has to changing its state of motion, similar to inertia but dealing with a rotating object. The less rotational inertia an object has, the less the torque that will be needed to change its state of rotation or the easier it will be to accelerate.

10. Friction
Surface friction is caused by the rubbing of two surfaces in contact with one another. Where your axle connects to the frame of your vehicle is one place you will find surface friction.
Traction is a wanted surface friction that is between your car wheels and the floor. Increasing your traction will allow for greater accelerations because it will take more torque to make the wheels spin out or break loose.
Fluid friction is caused by an object trying to move the air out of the way as it is moving.

11. Torque
Torque depends on the length of your cars lever arm and the strength of the mousetrap’s spring. A long lever arm has the same torque as a shorter arm. The difference between a long arm and a short arm is that you get more pulling force with a short arm than a long arm.

12. As you begin building your mousetrap car, you will need to make adjustments along the way to better the performance of your car. Here are some adjustments to consider:

13. Extension of the lever arm
By extending the lever arm, the force required to turn the drive axle decreases. By doing so, the stored mechanical energy in the snapper’s lever is conserved while constantly turning rotating the drive axle. This adjustment would guarantee your car to travel longer distances because no energy is wasted in moving the vehicle.
Inversely, if the lever arm is short, the speed of the car will be faster. Remember that a shorter lever arm will require a greater amount of force, and speed is relative to the amount of force applied.

14. Increasing the drive wheel’s diameter
A drive wheel with a bigger drive wheel will result to traveling farther. The reason for this is that a wheel with a bigger diameter will cover a longer distance before it could make a complete turn. But there is a downside to this: your vehicle will travel slower.
A bigger tire will require more force for it to start moving from rest, more power to accelerate but at the same time it will also take more force for it to stop as compared to a smaller one.
Inversely, a smaller drive wheel diameter will produce greater speeds.

15. Increasing the diameter of the drive axle
Increasing the diameter of the drive axle will variably increase the speed of the car. The increase in the axle’s diameter will increase the torque applied by the same amount of force. In effect, you will increase the power used to turn the wheel and since you will increase the power used to turn the wheel, and since speed is correlated to power, this adjustment will generate higher speed.

16. Reducing the mouse trap car’s weight
By reducing the weight of the car, you are efficiently converting the potential energy of the snapper’s spring into kinetic energy. A lighter vehicle will have less inertia and therefore will require less force to accelerate it. This also implies that since it will only require less force to accelerate it, the time required for the vehicle to reach its maximum speed will also be shorter.

17. Using thinner and lighter drive wheels
This will increase the maximum distance that your vehicle could travel and increase the acceleration of the vehicle. A lighter set of wheels will mean that the rotational inertia of the particular part is also less. By using a thinner and less massive drive wheel, less energy is displaced for the rotational inertia and more energy is displaced in the forward movement of the whole car.

18. Reduce friction
Friction is another type of force that is acting on your car, particularly the parts of the car. This will affect both the speed and distance output. If friction is allowed to act on the vehicle, the kinetic energy that moves the car will be turned into heat energy because of the chemical reaction between the contacting surfaces. This transformation of energy to heat will consume the energy that could be used for the desired motion of the vehicle.

19. Increased traction
Increased traction will require an increase in surface area that will be subject to static friction. Static friction is the type of force that prevents two bodies to slide when in contact. This means that the rotational force coming from the spinning drive wheel requires traction so that it would be able to apply a pushing force on the ground to make the vehicle move forward. Without traction, the wheels of the car would slip and cause an energy expenditure without any work output.